Lesson 1:The Art and Physics of

Photography

Digital Photography

MITSAA

IAP 2003

Rob Zehner

General Info

• Instructor: Rob Zehner (zehner@pobox.com)

• Meeting time: 7:00 - 8:30 p.m. every Wednesday

• We will meet in front of the SAA office

• Please bring any borrowed equipment with you to class every week

Outline

• How the camera works– lens– shutter– aperture– film

• How the digital camera works

• Some rules of thumb for good photos

The Camera

imagelens

aperture

virtual image

The Lens

• Purpose: to collect light from the sample and focus it on the film plane

• Focal length: distance from the lens to the film plane (for a single-element lens) when focused at infinity– Longer focal length = more magnification

The Shutter• Prevents light from reaching the film

• Opens to allow light through

• In most cameras, the shutter is actually two pieces of cloth or metal that move across the film plane, one after the other

• Shutter speeds are usually reported as reciprocal seconds, e.g. 1000 = 1/1000th sec.

The Shutter, continued

• Shutter speed determines relative effects of motion– Slow shutter speeds = blurry images– Fast shutter speeds = stopped motion– Rule of thumb: 1/60th or faster will eliminate the

effects of shaky hands

• Also determines film exposure– 2X shutter speed = 1/2 the light to the film– 1/2 the shutter speed = 2x the light to the film

The Aperture

• Placed at the focal point of the lens

• Typically a circular opening

• Diameter of circle determines amount of light that gets to the film

• Reported as f-number - ratio of focal length to aperture– Larger f-number = smaller opening

The Aperture, continued

• Size affects “depth of field” - distance in front of and behind the focal plane that is in focus– Small apertures (large f number) = greater

depth of field

• Size also affects film exposure– Increase of 1 stop (x 1.414) = 1/2 as much light– Decrease of 1 stop (1.414) = 2x as much light

The Film

• The film’s job is to detect light– Photons hit the film and promote a reaction

• Film speed is typically reported in ASA– Higher ASA = “faster” film - needs less light– Doubling ASA = 1 stop less light needed

• Rule of thumb - “sunny f16 rule”– Under bright sun, use f16 and 1/ASA shutter

• Generally, film with a higher ASA is grainier

Digital differences

• Film is replaced by an image sensor– Photons hit the sensor and create charge, which

is converted to voltage, which is converted to bits

– All image sensors are actually analog

• Shutter may be electronic, not mechanical

• Image sensors are subject to thermal noise– Long exposures may not be possible

Digital differences, continued

• For now, image sensors are smaller than film– Image size is a function of focal length vs. sensor

size

– Small sensor = short focal length

– Short focal length means it’s hard to get small apertures

– Short focal length means that depth of field is relatively larger at all apertures

The Rule of Thirds

Other tips for good photos• Use natural light

– Avoid on-camera flash as much as possible

• Find new perspectives– Move around– Take horizontal and vertical shots

• Find converging lines

• Play with negative space

• Take candid portraits

• When in doubt, bracket exposures