The Physics of Photography

How your camera works

Chloe Kuba and Karin Oxenham

The purpose of the Lens is to collect light from the sample, and focus it on the image sensor.

The Lens

Focal Length

Focal Length:135mm Focal Length: 200mm

The shorter the focal length, the greater the extent of the scene captured by the lens. On the other hand, the longer the focal length, the smaller the extent captured by the lens. If the same subject is photographed from the same distance, its apparent size will decrease as the focal length gets shorter and increase as the focal length gets longer.

Aperture

● Circular opening, diameter of circle determines amount of light that gets to the film.

● If an aperture is narrow, then highly collimated rays are admitted, resulting in a sharp focus at the image plane.

● If an aperture is wide, then uncollimated rays are admitted, resulting in a sharp focus only for rays with a certain focal length. This means that a wide aperture results in an image that is sharp around what the lens is focusing on and blurred otherwise.

Depth of FieldDepth of Field (DOF) is the front-to-back zone of a

photograph in which the image is razor sharp. As soon as an object (person, thing) falls out of this range, it begins to lose focus.

Aperture and Depth of Field

The size of the aperture’s opening is measured in f-stops -

one of two sets of numbers on the lens barrel (the other

being the focusing distance). The f-stops work as inverse

values, such that a small f/number (say f/2.8) corresponds

to a larger or wider aperture size, which results in a shallow

depth of field; conversely a large f/number (say f/16) results

in a smaller or narrower aperture size and therefore a

deeper depth of field.

The Image Sensor● A silicon chip that

converts light into electrons through the photoelectric effect

● Two types○ charged-coupled device

(CCD)○ complementary metal oxide

semiconductor (CMOS)

● Grid of thousands of photosites/pixels

Converting Photons into Electrons● Photodiodes absorb photons and convert them into

electrons● Electrons are stored in potential wells● Electrons are converted into voltage

○ CCD (charge shifts across)○ CMOS (transistors on each photosite)

● Each pixel ends up with an electric charge (voltage) proportional to the number of photons that hit it

Getting a Digital Image● A/D converter converts charge into digital

data● Stored on memory card● What about color??

○ Photodiodes only measure light intensity, not color value

○ Bayer filter: each pixel measures one RGB value○ A processor estimates actual color of each pixel

Image Resolution● The amount of detail that the camera can

capture● Higher resolution means less grainy● The more pixels, the higher the resolution● Examples

○ 1216x912 - megapixel - printing ○ 2240x1680 - 4 megapixel cameras

○ 4064x2704 - 11.1 megapixel cameras - top-of-the-line

○ 18 megapixels - entry-level DSLR

Sources● http://www.shortcourses.com/guide/guide1-3.html

● http://www.olympusmicro.com/primer/digitalimaging/cmo

simagesensors.html● http://fstoppers.com/this-is-how-your-digital-cameras-cc

d-chip-works

● http://electronics.howstuffworks.com/cameras-photography/digital/digital-camera2.htm

● http://electronics.howstuffworks.com/cameras-photography/digital/digital-camera.htm

● http://photographycourse.net/physics-of-depth-of-field ● http://www.astropix.com/HTML/I_ASTROP/HOW.HTM