playing with light - digital slr guide · recycle to full power in a matter of seconds. but as the...

Playing With Light Lesson Three: Introduction to Flash

Table of Contents Introduction........................................................................................................................ 1 Flash Terminology................................................................................................................ 2

Guide Numbers................................................................................................................. 2 Flash Power Levels ............................................................................................................ 3 Recycle Time .................................................................................................................... 4 Swivel and Bounce ............................................................................................................ 5 Zoom .............................................................................................................................. 6 Wireless Flash .................................................................................................................. 6

Flash Exposure .................................................................................................................... 7 Aperture and ISO.............................................................................................................. 7 Auto vs. Manual Flash........................................................................................................ 8 Adjusting Exposure With Manual Flash ................................................................................. 9 Automatic Flash and Camera Modes................................................................................... 11 Flash Exposure Compensation........................................................................................... 14 Shutter Speed ................................................................................................................ 15 Sync Speed.................................................................................................................... 17

Summary ......................................................................................................................... 18 Exercises .......................................................................................................................... 19

Introduction

Now that you have a better handle on the different types of natural light, we're going to start talking about artificial light. When I say "artificial light" what I really mean is light from a flash. Now, there are a wide variety of different flash options available to you: the built-in flash on your camera, a small external flash unit, powerful studio strobes, etc. But regardless of which type of flash you're using, there's some common terminology associated with all flashes that you have to understand first. This first half of this lesson seeks to explain some of that flash terminology, for two main reasons:

1. If you decide that you want to upgrade your current flash to something more sophisticated, understanding these terms will help you make the best choice for your flash needs

2. Flash has some limitations, and you need to have a clear grasp of these limitations if you want to maximize the quality of the images you take with flash

All text and photos copyright © 2010 The Digital SLR Guide This document may not be reproduced without permission.

The second half of the lesson will explain how you expose photos when you're using flash - because it's not exactly the same as how you set exposure for natural light. With this in mind, set down your camera for a moment - you won't need it all that much for this lesson - and come on an exploration of some of the obscure terminology associated with digital camera flash.

Flash Terminology

Guide Numbers Curious about just how much light your flash puts out? Then take a look at the guide number. But as you do, keep this in mind: in the specifications for a flash, the guide number listed by the manufacturer is captured under ideal conditions. Put another way, if one flash has a guide number of 50 and another's is 60, don't just instantly assume that the one with 60 is better. Used in a real-world environment rather than a testing lab, you might be hard-pressed to tell the difference between the two. With the accuracy disclaimer out of the way, let's take a closer look at how you can compute a guide number and what the number really tells you about the light your flash can put out. A guide number is made up of three separate variables:

1. Flash to subject distance

2. Lens aperture

3. ISO value

Lenses with higher guide numbers allow you to do two different things:

1. If you use a wide aperture, you can light subjects that are farther away (greater flash-to-

subject distance)

2. If you stay close to your subject, you can use smaller apertures for increased depth of field

Let's take a closer look at how the guide number is computed and why higher guide numbers give you more flexibility when it comes to your flash photography.

Guide Number (GN) = Flash to subject distance (in feet) x Aperture (in f-stops)

You'll note that at the beginning of this section I mentioned that there are three variables associated with the guide number calculation…but ISO is missing from the equation above. ISO does play a role, but it's a constant one: for all guide number calculations, it is assumed that the ISO is set to 100.


With ISO fixed at 100, the two main variables are:

1. Lens aperture

2. Flash-to-subject distance

First let's keep the lens aperture constant. The following calculations show that flashes with higher guide numbers are capable of illuminating subjects that are farther away.

GUIDE NUMBER APERTURE DISTANCE CALCULATION

40 f/4 10ft. / 3m 40 (GN) = (f)4 x 10(ft)

80 f/4 20ft. / 6m 80 (GN) = (f)4 x 20(ft)

120 f/4 30ft. / 9.1m 120 (GN) = (f)4 x 30(ft)

160 f/4 40ft. / 12.2m 160 (GN) = (f)4 x 40(ft)

Now we'll keep the flash-to-subject distance constant. In this case, the higher guide numbers let you use narrower apertures for increased depth of field.

GUIDE NUMBER APERTURE DISTANCE CALCULATION

40 f/4 10ft. / 3m 40 (GN) = (f)4 x 10(ft)

80 f/8 10ft. / 3m 80 (GN) = (f)8 x 10(ft)

110 f/11 10ft. / 3m 120 (GN) = (f)11 x 10(ft)

160 f/16 10ft. / 3m 160 (GN) = (f)16 x 10(ft)

While you often don't need to keep track of all this information when you're right in the middle of taking pictures, it does help to be aware of just how much your flash can illuminate, and what aperture settings you can use. For example, if it's your intention to light subjects that are 10ft. / 3m away and you want to use apertures between f/8 and f/16, then you'll need a flash with a Guide Number between 80 and 160. However, if you like using wide apertures to minimize depth of field and often take pictures close to your subject, then a flash with a Guide Number of 40 should be just fine.

Flash Power Levels Every flash unit has a variable power level, even the built-in one on your camera. The power level regulates how much light the flash puts out, and how much time it takes for the flash to be ready for the next shot.


When you're using the built-in flash on your camera or an external flash unit set to full auto mode, the power level is determined automatically based on two variables that should be pretty familiar by now: flash-to-subject distance and aperture setting. This makes sense when you consider that a Guide Number represents the MAXIMUM power of a particular flash unit.

Example: you're using a flash with a Guide Number of 40. This means that with your aperture set to f/4 you can stand 10ft. (3m) away from your subject and still light them with the flash. But what if you're only standing 5 ft. (1.5m) away from your subject OR you're using an aperture of f/2.8 (which lets in TWICE as much light as f/4)? In this case, the flash can operate at HALF power and still illuminate your subject.

While your built-in flash selects its power level automatically (with a few exceptions), all external flash units let you select a power level manually. For these flashes, the power levels can be set to all or a subset of the following (where 1 represents FULL power):

1 1/2 1/4 1/8 1/16 1/32 1/64 1/128 1/256

There are two main reasons to have an awareness of the power level of your flash:

1. Knowing the maximum power of your flash lets you know what you can expect it to illuminate

2. When a flash uses less than its maximum power, it can fire more rapidly

This second point brings us to another term for flash photography - recycle time - and that's what we'll talk about next.

Recycle Time Unlike taking pictures in natural light where you can blaze through photos at your camera's highest speed, taking photos with flash requires you to pause to let your flashes "recycle". A flash - to use a somewhat silly bathroom analogy - is a lot like a toilet. When the tank of a toilet is full of water, flushing releases all the water - this leverages the maximum power of the toilet. Once flushed, you have to wait for a period of time for the tank to fill all the way up again before you can get another full-power flush. If you try to flush when the tank is only half filled, the power of the toilet is reduced. In a similar fashion, electronic flashes need some time to build up a charge to emit a full-power flash. When you fire the flash at full power, this completely depletes the charge, and the flash needs time to get back to full power.


If you happen to fire the flash before it has reached full power, it will still go off - but the light that it releases is reduced. If your camera's exposure settings require a full power flash, the image that you take at reduced power will be under exposed. The amount of time that a flash needs to get back to full power is called the recycle time, and it varies from flash to flash (although most only take a couple of seconds). Recycle times are affected by source of the power. For example, a built-in flash often recycles pretty fast because it is using your camera's battery to build up a charge (it is also small, which helps speed up recycle time). A large, powerful external flash requires batteries. When the batteries are brand new, the flash will recycle to full power in a matter of seconds. But as the batteries get used up, you'll find that the recycle time on the flash gets longer. Recycle times are also affected by the power setting on the flash. A full-power flash might take several seconds to recycle, but a 1/16th power flash will only take a fraction of a second. What recycle time means for shutter-happy photographers is that once you start working with flash you have to pause briefly in between shots. If you just blaze through image after image, you'll find a lot of variation in your exposures.

Swivel and Bounce Although it sounds like it, this is not the latest nightclub dance move. Instead, swivel and bounce refers to a feature that is only available on external flash units. On some external flash heads, the head can be angled to point straight up to the ceiling, or you can swivel the head to point either to the left or right. Some flash heads can even point straight back if you so desire. The point of all this rotation is to prevent the flash from being aimed squarely at your subject. When the light from the flash comes from the direction of the camera, your subject is blasted with light. As you now know from lesson one, direct light - the type you get in the middle of a sunny day - is not all that flattering. To get soft light from a flash that is locked on top of the cyou need to REFLECT the light off a nearby surface.

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That nearby surface can either be a wall or a ceiling. The ability to angle a flash so that it points straight up lets you bounce loff the ceiling while the ability to swivel it lets you bounce light off a wall or other flat surface. Heck, you can even bounce the flash off the floor if you so desire - but this creates a "horror picture" look to portrait subjects that's not appealing…unless you intentionally want that type of look.


Note Virtually all external flash units let you angle the flash head, but not all of them let you to swivel it. Every flash with a swivel also includes the ability to angle.

Zoom The zoom feature of a flash works a bit like the zoom feature of your lens. When you zoom your lens from a wide angle setting to telephoto (by increasing the focal length) you are able to capture close-up images of subjects that are far away. In a similar sense, when you zoom a flash head from wide angle to telephoto you're able to illuminate subjects that are far away. When a flash is set to spread a wide-angle beam of light, it illuminates a lot of the background but also means that you have to be closer to your subject to get a correct exposure. When you set a flash to its telephoto setting, it concentrates the beam of light. This means that your background appears darker but it also means that you can be farther from your subject. When your flash unit is made by the same manufacturer as your camera, it can often tell what focal length you have the lens set to (the camera acts as a bridge between the electronics in the lens and the electronics in the flash). When the zoom of the flash is set automatically, you'll notice that it changes to match the focal length of your lens as you zoom in and out. Like the power setting, the zoom of external flash units can often be set manually.

Wireless Flash If you're sick and tired of your flash always coming from the same direction, or can never seem to find a good surface to use for reflected light, then wireless flash is for you. Before we talk about wireless flash, it's good to talk about its cheaper relative: wired flash. With wired flash, you're able to remove the flash from the camera so that you can light your subject from a variety of different directions. However, the flash is still tethered to the camera (via the hot shoe on top of the camera or via a special port called a PC port) by means of a trigger cord or cable. While removing the flash from the top of the camera does allow you to change the direction of the light falling on your subject, the limitation lies in the length of the cable. For example, if you'd like to place the flash directly behind your subject for some dramatic impact, they will be standing right on top of the cord that connects the flash to the camera. If you really want versatility, go wireless.


With wireless flash, you can set up the flash anywhere you want and the camera is "virtually" connected to the flash via a wireless trigger. When you press the shutter release button, the flash detects that the camera is taking a picture and it fires. Each camera manufacturer has its own way of handling wireless flash, and there are a variety of third-party triggering accessories that will enable wireless flash for any digital SLR camera.

Flash Exposure

Now that we've covered the "tech" of flashes, let's talk more about how to get good exposures when using flash. Working with flash can be tricky at first, especially if you're used to using your camera in natural light. Probably the biggest issue is that taking pictures with flash places LIMITS on your camera that don't exist when you're not using flash. To deal with these limits, you have to adjust your shooting style and it's this adjustment that can take some time to master. In the sections that follow I'll talk more about the limitations you have to work with when using flash, and how you can compensate to improve the photos you take.

Aperture and ISO When you're taking photos in natural light, there are three main camera settings that you can change to capture a correct exposure: aperture, shutter speed and ISO. A quick refresher:

• Aperture is the width of the opening in the lens, and it controls the apparent depth of field in your photos

• Shutter Speed is how long the shutter stays open and it determines whether moving subjects appear sharp or blurry

• ISO determines how quickly the sensor absorbs light

For natural light photography, you use all three of these settings in conjunction with each other to create a certain "look".


• If you're taking photos in dim light without a flash and want them to appear sharp, you increase the ISO setting to get a fast enough shutter speed to eliminate motion blur

• If you want to blur out the background for a portrait image, you open the lens aperture wide to limit depth of field

• If you're taking pictures of a sunset, you narrow the aperture for increased depth of field AND increase the ISO to ensure a shutter speed that's fast enough for a sharp image

For every natural light photo you take, you can use shutter speed, aperture and ISO together to ensure that the exposure is correct (not too bright, not too dark), to eliminate motion blur and to get the right amount of depth of field. However, when you turn on your flash, it's a slightly different situation. Let's start with the big one:

For flash photography the MAIN settings that affect the exposure are aperture and ISO.

Changing shutter speed with flash will impact the brightness of the background, but not the exposure of your primary subject - I'll explain more about this in just a moment. Back to aperture and ISO - the effect that these two settings have on flash exposure depend on whether you are using a flash that is automatic or manual.

Auto vs. Manual Flash In the section on flash power settings, I mentioned that the power of all external flashes can be set automatically or manually. While the names for automatic flash do vary slightly, the common reference term used for automatic flash is TTL Flash (note: built-in flash units are all TTL). When you use TTL flash, the flash communicates with the camera via the contacts on the hot shoe. All the information that the flash needs to output the correct amount of light is transmitted from the camera to the flash, including the focal length of the lens, the lens aperture and the ISO value. When the camera communicates this data to the flash, the flash is able to adjust its power levels dynamically to kick out just the right amount of light. The image at right was taken with TTL flash. The camera communicated all the settings to the flash and the flash emitted just the right amount of light to illuminate the subject. Although this sample photo doesn't really demonstrate it, TTL flash is best if you're trying to capture a subject that's moving all over the place.


Since the flash-to-subject distance is constantly changing, it helps to have the camera working out the right power settings for the flash on the fly for every photo you take. The main limitation of TTL flash is that the flash must be physically attached to the camera (either directly to the hot shoe or to a hot shoe extension cord). Some new wireless triggering systems that leverage TTL flash do exist, but they are expensive and aimed at the pro photographer market. This means that if you want to use a flash OFF the camera you typically have to set the power output of the flash manually. This is much less intimidating in practice than it sounds, and is made hundreds of times easier by the digital format. In the days of film, you had to use a light meter to determine if the manual flash setting was putting out enough light to illuminate your subject. The flash meter would also tell you waperture setting you could use to ensure a properly exposeimage.

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With digital, this is no longer necessary. Setting the power of a

flash manually is much simpler:

1. Set your camera to manual (M) mode

2. Pick a shutter speed and aperture

3. Ballpark the power setting on your flash based on the flash-to-subject distance - if the flash is quite close to your subject, pick a power setting of 1/4 or 1/8 to start with

4. Fire a test shot and check the results on the LCD screen

5. Check the histogram to see if the image exposure is correct, too dark or too bright

If you've nailed the settings for a good exposure right out of the gate, congrats! You don't need to fiddle around with your setup at all. However, if the image is too dark or too bright then you've got to make some changes.

Adjusting Exposure With Manual Flash When you set the power of a flash manually, the flash emits the same amount of light for every photo you take. If you want the flash to produce more or less light, then you have to change the flash power yourself. As I mentioned above, shutter speed plays virtually no role in a flash photo, so long as the aperture and ISO remain constant. Take a look at the two sample photos below: the ISO is 200, the aperture is f/11 and the flash is manually set to ¼ power. Even though both are taken with very different shutter speeds, the subject illumination looks pretty much the same in both shots.


Shutter Speed: 1/30 Shutter Speed: 1/125

If we change aperture instead of shutter speed, there is a visible change in the exposure. For the two following shots, the shutter speed is constant at 1/60 and the ISO is 200. The flash is still manually set to ¼ power.

Aperture: f/11 Aperture: f/16

Right now, you might be thinking "Fantastic! One less thing I have to worry about to get a decent exposure!" While it's true that shutter speed does not have the same impact as it does in natural light images, there are two new exposure variables that you have to consider:

1. The flash-to-subject distance

2. The power level of the flash

When you add these two options to aperture and ISO, you wind up with four ways to control exposure when you're lighting a subject with a manual flash:


To Over Expose To Under Expose

1. Widen the lens aperture 2. Increase the ISO 3. Move the flash closer to the subject 4. Increase the flash power

1. Narrow the lens aperture 2. Decrease the ISO 3. Move the flash farther from the subject 4. Decrease the flash power

Which one of the four settings should you change to adjust your exposure? The answer depends entirely on the limitations of the gear you're using and the environment you're working in. Let's take a closer look.

SCENARIO SOLUTIONS

You're taking photos in a very small room so your flash cannot be moved. Your lens is open to its maximum aperture and your ISO is set to 100. The subject is under exposed.

In this case, you can't alter the flash-to-subject distance but you can increase the power of your flashes. You also can't open your lens aperture any wider, but you can increase the ISO setting.

You're taking pictures in an open field with a flash set to full power. Your aperture is set to f/5.6 and your ISO is 800. The subject is under exposed.

You can't increase the power of the flash, but you can move the flash closer to your subject. You can also open the aperture wider. While you can increase the ISO value, you're going to see some noise in your final image. If you want to keep noise at a minimum then move the flash or open the aperture.

You're taking pictures of a small object on a table top to sell on eBay. Your place your flash right next to the object and set it to its lowest power setting. You pick an aperture of f/6.3 at ISO 100. The image is too bright.

There are really only two options here: move the flash farther away from the object or reduce the aperture setting. The flash is already at its lowest power so you can't reduce it further and you can't set ISO lower than 100.

Simply put, if there's a limitation on one or more of the variables, then your only option is to adjust the remaining variables until you get the exposure right. Do you have to keep in mind all of these options when you're just taking pictures with your camera's built-in flash? No, you don't. But I wanted to explain manual flash exposure control FIRST, since it will help you to understand why your automatic flash behaves the way it does.

Automatic Flash and Camera Modes When taking pictures with your camera's built-in flash or with an external flash unit attached to the camera's hot shoe, you can choose between the shooting modes that you use for natural light:

• AUTO

• Program

• Aperture Priority

• Shutter Priority


• Manual

AUTO and Program mode work fine, but they offer you very little control over other camera settings like aperture and ISO. Aperture is the more important of the two, since changing aperture allows you to control depth of field in your photos with flash. While Aperture and Shutter Priority modes are just fine for taking photos in natural light, neither one fares well when you're trying to take pictures with flash. Let's start with Aperture Priority: in this mode, you pick a lens aperture and the camera automatically selects a shutter speed for you. The available light is probably quite dim otherwise you wouldn't be using flash at all. First, imagine what would happen if your flash was off. You'd set your lens to its widest aperture and your camera would pick a relatively slow shutter speed to compensate for the overall lack of light. Here's the catch: even when you activate your flash in Aperture Priority mode, the shutter speed doesn't increase. The camera sets the shutter speed just the same as if the flash were not enabled. End result: while the flash fires to light your subject, you'll still see blur thanks to the slow shutter speed.

Aperture: f/7.1 Shutter: 1/15 No Flash Aperture: f/7.1 Shutter: 1/15 Flash Enabled

OK, fine. Aperture Priority is out. Let's try Shutter Priority. The problem in Shutter Priority mode is similar to what you'll run into in Aperture Priority mode, only in reverse. Let's say that you're trying to take pictures of a moving subject with your flash and you set the shutter speed to 1/125 in Shutter Priority mode. Well, your camera is STILL acting as though the flash isn't turned on. It will attempt to select a nice wide aperture to match the shutter speed that you've selected. But here's the snag: if your lens can't open wide enough, one of two things is going to happen:

1. Your camera will flash the aperture number in the viewfinder and won't take a picture at all


2. Your camera will flash the aperture number in the viewfinder and will take a picture that's very under exposed

Neither one of these options is very good. Now we're left with good old Manual mode. Now, remember what I explained about exposure settings with flash: shutter speed really doesn't play a role. This means that you can set it to just about anything you want. I often will pick a shutter speed of 1/60 or 1/125 when shooting with flash and will just leave it there. With shutter speed set, the only two remaining settings you can fiddle with are aperture and ISO. It's at this point that you'll quickly see the main difference between manual flash and TTL flash: you can adjust aperture and ISO as much as you want with TTL flash and you won't see ANY change in the exposure. This is because your flash is automatically adjusting its power output based on your camera settings to ensure that your photo is correctly exposed.

WIDE APERTURE ISO: 100 Shutter: 1/100 Aperture: f/3.5 This first shot was taken with a relatively wide aperture and an average shutter speed. The shutter speed that I selected was just fast to eliminate subject blur (even though my daughter was zipping around on her trike).

NARROW APERTURE ISO: 100 Shutter: 1/100 Aperture: f/7.1 For this second shot I narrowed the aperture of the lens substantially. If I were using a manual flash with a constant power output, this image would be under exposed relative to the previous one. Since I am using TTL flash, the flash knows that I have narrowed the aperture so it increases the power level accordingly to put out more light for a good exposure.


Here's the neat trick: TTL flash is also capable of adjusting its power setting based on the flash-to-subject distance. The flash will fire with more power when the subject is far away and less power when the subject is close by. So the real reason for using manual mode with TTL flash is not so you can adjust the exposure of your subject. Instead, the primary reason to use manual mode is so you can pick a shutter speed that's fast enough to prevent image blur. This brings us to an interesting question: how DO you adjust exposure when using an automatic TTL flash?

Flash Exposure Compensation When you take pictures in natural light, one camera feature that you can use to control exposure is called exposure compensation. Exposure compensation lets you tell your camera to intentionally over expose or under expose every photo you take. There's a similar setting for flash, and it's aptly named flash exposure compensation. When you're using a built-in flash, you adjust the flash exposure compensation using a setting on the camera body. If you're using an external flash connected to the hot shoe, you can adjust flash exposure compensation either

using the camera body or by changing the setting on the flash unit. Just like with regular exposure compensation, you'll be presented with an exposure scale:

• If you change the setting so that the meter is POSITIVE, then all the photos you take will be over exposed

• If you set it to a NEGATIVE value, then all the photos you take will be under exposed

It's all pretty straightforward - until you ask yourself: why would I ever use this? After all, the TTL flash is doing all of this complex computation so that it puts out just the right amount of light to expose your subject evenly. Why fuss with a good thing? The answer to this question depends on how much available light you are working with and whether or not you're using any light modifiers with your flash. For example, let's say that you're using your flash to brighten shadows on a sunny day (a technique called "fill flash" which I'll describe in a later lesson). Your first several test shots with the flash at full power look very washed out. Since you just want a small pop of flash to brighten the shadows, you can set your flash exposure compensation to a negative value to reduce the light output from the flash.


NO FLASH EXPOSURE COMPENSATION ISO: 100 Shutter: 1/100 Aperture: f/13 This first image uses no flash exposure compensation – I just let the flash fire at the power that it felt appropriate given my aperture and ISO setting. I checked the image on the LCD and felt that it was just too bright and washed out.

-1 FLASH EXPOSURE COMPENSATION ISO: 100 Shutter: 1/100 Aperture: f/13 Keeping my other camera settings exactly the same, I dialed down the power of the flash by using a negative flash exposure compensation setting. With all other variables unchanged, this reduces the light from the flash, creating a more natural-looking image.

Now imagine that you're using an external flash unit that is pointed straight up to bounce light off of a white ceiling. You take a few test shots and notice that your subject looks too dark. This is because a lot of the power of the flash is lost when the light is reflected off another surface. To compensate (and make your subject appear well-lit) you have to set the flash exposure compensation to a positive value so the flash puts out more light.

Shutter Speed We now finally come to shutter speed, the orphan child when it comes to flash photography. Here's the good news for all you shutter speed enthusiasts: it does play a role, it's just hard to see. Here's what it does in a nutshell:

Adjusting shutter speed when using flash will either lighten or darken the background.

It makes sense when you consider it further:


• When you're not using flash and the available light is low, you have to use a slow shutter speed in conjunction with a wide aperture (and even a high ISO setting) to ensure that your photo is correctly exposed

• If you increase the shutter speed, the image will be under exposed

• When you activate the flash, it only puts out enough light to evenly expose the subjects closest to the camera

• Anything that is beyond the range of the flash (a.k.a. the background) is still subject to the same exposure rules as if no flash were being used

The relationship winds up looking like this:

• Faster shutter speed Darker background (more under exposed)

• Slower shutter speed Lighter background (less under exposed)

Let's take a look at a concrete example of this in action to help it sink in.

NATURAL LIGHT ISO: 100 Shutter: 1/160 Aperture: f/3.5 This first image is taken purely with natural light. To get the exposure correct, I had to use a wide aperture which results in shallow depth of field. I'd like to get more depth of field in this shot, so I add some light from a flash.

FLASH – MODERATE SHUTTER SPEED ISO: 100 Shutter: 1/160 Aperture: f/9 Since I have added more light to the scene using a flash, I can reduce the aperture to f/9 and still keep the orange evenly exposed. While I'm now happy with the depth of field, I don't like how dark the background has become and I'd like it to be a bit brighter.


FLASH – SLOW SHUTTER SPEED ISO: 100 Shutter: 1/50 Aperture: f/9 Keeping all other settings constant, the only change I make is to shutter speed: slowing it from 1/160 to 1/50. The slower shutter speed brightens up the background and balances the background exposure more with the exposure of the subject lit with the flash.

So shutter speed does have an effect on flash photos after all. However, unlike the aperture and ISO value which you can set to your heart's content when using flash, an upper limit is placed on shutter speed. This upper limit is called the sync speed.

Sync Speed The sync speed of your digital SLR camera is the fastest shutter speed that you can use when the flash is enabled. The sync speed applies whether you are using the built-in flash, an external flash unit attached to the hot shoe, or a wireless flash. Without getting too much into the mechanics of it, just take a look in your camera's manual to figure out the value of the sync speed. While it varies from camera to camera, it is typically 1/160th of a second or 1/250th of a second. This will have a different impact depending on the type of flash you're using and the settings on the flash.

• Built-in or attached external flash: when the flash is connected directly to the camera, the camera will BLOCK you from selecting shutter speeds that are faster than the sync speed (since the flash is attached to the camera, the camera is aware when the flash is active)

• Off-camera wired or wireless flash: with the flash off-camera, the camera is not aware when the flash is turned on and will allow you to select shutter speeds faster than the sync speed. However, you'll wind up with a band of darkness over half of your photo

The first one is straightforward: you simply won't be able to select a shutter speed that's too fast. But the second is more problematic. If you do eventually decide to take photos with off-camera flash, you just have to get in the habit of keeping an eye on your shutter speed.


Since the best way to change exposure settings for flash is in full manual mode (as described above) it's quite easy to set a shutter speed by accident that's faster than the sync speed. The good news is that the minute you do, you'll be able to see a black bar across your photo when you review it on the LCD screen. To eliminate the black bar, just reduce the shutter speed below the sync speed and you'll be fine.

NOTE Some camera / flash combinations do allow you to shoot faster than the sync speed with no ill effect. For example, Nikon external flash units have a setting called "Auto FP High-speed Sync" that lets you use very fast shutter speeds with the flash enabled.

The ability to "outsmart" sync speed depends on both the exact model of your camera and the particular flash that you are using. Check your manuals or check online to see if this is possible with the camera you use.

Summary

Having completed lesson three, you should now have a pretty good handle on just what your flash can and cannot do. Check the manual for either your camera (for built-in flash) or for the flash unit to determine the guide number: this will give you a general sense of just how far you can stand from your subject and what sort of an aperture you can use before you exceed the power capabilities of your flash. If you've been thinking about upgrading to an external flash unit from your built-in one, then just be aware of the limitations of flash: things like recycle time and flash sync speed. Perhaps you've been using your built-in flash for awhile now, and are wondering whether or not you should upgrade to an external flash unit. There are several reasons for doing so:

• Eliminate red-eye: glowing eyes are caused when the flash is too close to the camera lens - since external flashes increase the distance from flash to lens, they also eliminate red eyes

• Swivel and bounce: if you don't want to always light your subject head-on, then you can bounce light off of white cards or nearby walls

• Higher guide numbers: external flashes are more powerful and can light subjects that are farther away

• Manual power: with an external flash you can either set it to full auto mode or you can control the power output manually

• Off-camera flash: the only way to light a subject from different angles is to get the flash off the camera - this can only be done with an external flash


Exercises

Exercise 1 - Guide Numbers To complete this exercise, you're going to need one additional accessory in addition to your camera and flash: a tape measure. Find a portrait subject or inanimate object that you can place at different distances from the camera. If you have a flash that lets you set the power level automatically, then set it to full power. If you cannot manually set the power level of the flash the exercise will still work, but not quite as effectively.

1. In your camera or flash manual, find the guide number of your flash

2. If you're using an external flash, place it on the camera's hot shoe (so the light comes from the top of the camera)

3. Set your ISO to 100

4. Lay out the tape measure on the floor, and set your subject and aperture to leverage the guide number of your flash (example: if your guide number is 110, set your aperture to 11 and place your subject 10 ft. / 3m from the camera since aperture x distance = guide number)

While your subject might not be perfectly exposed at the very limit of your flash's power, the subject should still be moderately well lit

5. Halve the distance between the camera/flash and the subject and reduce your aperture to

compensate (example: a subject 10 ft. away at f/11 can be captured at f/22 from 5 ft. away)

Note the increased depth of field you're able to capture the closer your subject is to the flash.

Exercise 2 - Reflected Light This exercise will definitely be easier for you if you have an external flash with a tilt and swivel head. If you only have a built-in flash, you'll just have to get creative with pieces of white cardboard (or mirrors) to create the same effect.

1. If you have external flash, place it on the camera's hot shoe

2. Pick a subject that will stay in the same place for all of the photos you take

3. Keep the subject-to-flash distance a constant for all the images

4. Select a moderate aperture (anything between f/5.6 and f/8 is fine) and a moderate shutter speed (between 1/60 and 1/125)

5. Picture #1: with your camera held horizontally, point your flash straight at your subject and take a photo

6. Picture #2: point your flash straight up at the ceiling and take a photo (you might have to adjust aperture to get a good exposure)


7. Picture #3: point your flash to the right and take a photo

8. Picture #4: point your flash to the left and take a photo

9. Picture #5: rotate your camera vertically, point the flash straight down and take a photo

Exercise 3 - Flash Exposure How you approach this lesson depends a lot on the type of flash you are using: automatic (TTL) or manual. I'll start off with the TTL approach to adjusting flash exposure.

1. Place your camera in Manual mode

2. Activate the built-in flash or attach your external flash to the camera's hot shoe

3. Set your shutter speed between 1/60 and 1/125

4. Set your aperture to something relatively wide: between f/4 and f/5.6

5. Leave ISO at 100 or 200

6. Take a few test shots to see how much light the flash puts out

7. Narrow your aperture - between f/8 and f/11 - and take some more test shots

While you probably won't see a real difference in your photos, you might hear your flash taking longer to recycle since it has to use more power with the narrower aperture. Now that you've played around a bit with manual controls and your automatic flash, let's adjust the exposure.

1. Still in Manual mode, set your flash exposure compensation to +1

2. Take some test shots - they should all be over exposed

3. Now set your flash exposure compensation to -1

4. Take some more shots - they should all be under exposed

If you really want to get fancy and are using an external flash unit with swivel and bounce, try the following:

1. Take one photo with the flash pointed straight at your subject and with zero flash exposure compensation

2. Now, angle the flash head so that it is pointing at a reflective surface - either a light-colored wall or the ceiling

3. Take another photo with zero flash exposure compensation

4. Review the image on your LCD - it should be under exposed since the light on your subject is indirect rather than direct

5. Increase your flash exposure compensation to +1 and take another shot using the reflective surface to bounce light

6. The image on the LCD should now show a subject that is correctly exposed


Phew! That covers everyone using TTL. Now for all you people with manual flash.

1. Set your camera to manual mode, and check that the ISO is set to 200

2. Pick a shutter speed between 1/60 and 1/125 and pick moderate aperture between f/5.6 and f/8

3. Determine your flash-to-subject distance and guess the power level of your flash: you can often start with a setting around 1/2 or 1/4 power

4. Take a photo and check your exposure

5. If the photo is over exposed, try one of four different solutions: a) move the flash further from the subject or b) reduce the flash power or c) narrow the aperture or d) decrease the ISO

6. If the photo us under exposed, try the same solutions in the opposite direction: a) move the flash closer to the subject or b) increase the flash power or c) widen the aperture or d) increase the ISO

7. Play around with all four variables to get a better sense how each one impacts your flash exposure

Exercise 4 - Shutter Speed and Flash For this exercise, you'll get to see how changing the shutter speed either brightens or darkens the background. This exercise only works in somewhat dim light - you won't really see the effect if you try it on a bright sunny day or in the middle of the night. A shady spot in the early evening is optimal.

1. Set your camera to manual mode and make sure the ISO is set to 100 or 200

2. Pick a relatively wide aperture between f/3.5 and f/5.6

3. Select a shutter speed of 1/160th of a second (or 1/250 if your camera allows that fast a sync speed)

4. Place a subject relatively close to the camera, and compose your photo so that a fair amount of the background is visible behind your subject

5. Activate the flash (either TTL or manual) and take a test shot to make sure that the subject close to the camera is correctly exposed – note how dark the background appears relative to the subject

6. Reduce the shutter speed anywhere between 1/15 and 1/30 and take another photo

In the second image, you should be able to see that the background is much brighter but that the exposure of the primary subject has not changed very much.


playing with light - digital slr guide · recycle to full power in a matter of seconds. but as the...

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