Flash is bad!

Photon rate Time End of exposure “Available light” Rate of incoming light is constant

Photon rate Time End of exposure “Low light” Very few photons collected

“Direct Flash” Flash shoots light at subject, photons bounce back to camera Photon rate 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 Flash Direction of flash-to-subject similar to subject-to-sensor (optical axis) Direction of flash-to-subject similar to subject-to-sensor (optical axis) Harsh specular (mirror-like) reflections are returned Harsh specular (mirror-like) reflections are returned Red-eye is an example of this Red-eye is an example of this

Direction of flash-to-subject different from subject-to-sensor Direction of flash-to-subject different from subject-to-sensor Even, “diffuse” reflections are returned Even, “diffuse” reflections are returned External, bounce Flash

Light falloff with distance Bouncing off a surface results in less disparity in light intensity vs. distance from camera In this example: Direct: 1.82x distance, 3.3x light drop-off Bounce: 1.4x distance, 2.0x light drop-off Tradeoff: Much more power needed (greater distance, losses from bounce off surface)

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 meters 2 !)… 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