Flash is bad!

Photonrate

Time

End of exposure

“Available light”Rate of incoming light is constant

Photonrate

Time

End of exposure

“Low light”Very few photons collected

“Direct Flash”Flash shoots light at subject, photons bounce back to camera

Photonrate

Time

End of exposure

Problems• Intensity drops off with distance squared• And distance disparities are doubled! (light has to bounce back)

• Light comes from an unnatural direction• Very intense specular (mirror-like) reflections• Usually, on-camera light sources are small (lighting is harsh)

Advantages• Natural lighting direction• Less intensity disparity with distance• Softer “diffuse” reflections

“Bounce flash”Flash points to a surface, light reflects off surface onto subject

Exposure with flash

• Flash exposure:– Flash power– Aperture– ISO

• Ambient Exposure:– Shutter Speed– Aperture– ISO

Natural light

Direct flash

Direct flash – slow synchro

Direct flash (external) – slow synchro

Reflected flash

Ceiling bounce

Wall (left) bounce

Camera-mounted FlashCamera-mounted Flash• Direction of flash-to-subject similar to subject-to-Direction of flash-to-subject similar to subject-to-sensor (optical axis)sensor (optical axis)• Harsh specular (mirror-like) reflections are returnedHarsh specular (mirror-like) reflections are returned• Red-eye is an example of thisRed-eye is an example of this

• Direction of flash-to-subject different from subject-to-Direction of flash-to-subject different from subject-to-sensorsensor• Even, “diffuse” reflections are returnedEven, “diffuse” reflections are returned

External, bounce FlashExternal, bounce Flash

Light falloff with distance

Bouncing off a surface results in less disparity in light intensity vs. distance from cameraIn this example:

Direct: 1.82x distance, 3.3x light drop-offBounce: 1.4x distance, 2.0x light drop-off

Tradeoff: Much more power needed (greater distance, losses from bounce off surface)

3.843.84

77

66 8.8.4242

Surface area of light source

• Small-area light sources emit rays from a similar direction• Light rays move in straight lines – easily blocked, creating shadows• A light source from at a different position can “fill in” shadows• Large-area light sources are like an infinite number of lights over a range of positions

Shadow!

Important Distinction! Physical vs. Angular area

• The sun is a HUGE light source (radius 7e8 meters, area of 1.5e18 meters2!)… physically• But angle-wise, it is pretty small from the Earth• Direct sunlight causes extremely harsh shadows• Solutions: shaded area, overcast days (clouds), backlighting• Large angular area means a large range of directions that light hits the subject from

Because the flash duration is very short (1/10,000s), flash is also effective at freezing action