Digital Photography

Pixels

• Digital cameras break image up into pixels (picture elements)

Image sensors replace film• Digital vs. Analog

– Film is an analog (essentially continuous) medium

– Image sensors digitize the amount of light• Image is a series of numbers

Amount of light

expo

sure

DigitizationBinary arithmetic

Bits - 0 or 1

8-bit - 256 different levels

00000000 = 011111111 = 25501110111 = 119

10100011 = ?

CCDs• Image sensors - Charge coupled device• each pixel is a miniature

photodiode – converts light into electrons

• stores electrons• readout after exposure• electronics (ADC-analog-digital converter) counts

electrons• string of numbers representing light intensity at

each pixel

How many Pixels?• counted as Megapixels (106 pixels)

• cheap (< $100) cameras - 1-2 Mpixels• good ($200-400) - 4 - 8 Mpixels• professional ($2-8,000) - 10-16 Mpixels

(note nonlinear pricing!)• How much do I need?

• 35 mm film (24X36mm) is about ~15-25 Mpixels• most photoprinters give at best 200 dpi

• 4”X6” photo needs 800X1200 pixels ~ 1 Mpixel!

• why buy 5 Mpixel camera?

Compression• High Mpixel means big image files

– Each pixel has information about 3 colors– Cameras offer different levels (quality) of

compression (to make smaller files)• Lose some information• Depends on image (e.g. blue sky is easy)

If you buy many pixels, but use always use low resolution,you have wasted your money!

Resolution

Low resolution640X480File size: 240 kB

High resolution2048X1536File size:1.8 MB

Low resolution640X480

High resolution2048X1536

Color

• photodiodes are not color sensitive• add color filters on each pixel (RGB)

Bayer filter

note twice as many green as red and blue

Image processingUse information from neighborsto determine color

“de-mosaicing”

RAW file format - straight from camera

Digital camera lenses

• Why don’t digital cameras use std. 35mm lenses?

• 35mm film - 24X36mm• 3 Mpixel sensor - 6X8mm

– Smaller sensor - use shorter focal length lens

Optical vs. digital zoom

• Optical zoom– Move lens/lenses

• Digital zoom– Just use smaller portion of sensor– No different than cropping high resolutionpicture!– Lose resolution - use fewer pixels…– Only benefit is that it saves space on camera.

CMOS - the future• CMOS - Complementary metal oxide semiconductor

– same electronics as most semiconductors– used to be less sensitive than CCD– faster readout– lower power consumption - battery life!

13.3 Mpixel CMOS sensor35mm format!- use std. 35mm lenses