Metering systems Canon EOS flash system

1 metering systems

1.1 a-ttl
1.2 e-ttl
1.3 e-ttl ii

metering systems

canon has introduced several different metering systems flash products: a-ttl, e-ttl, , e-ttl ii. each system represents different approaches achieving proper flash exposure.

a-ttl

a-ttl initialism stands advanced-through lens flash metering. ordinary ttl flash exposure systems, sensor inside camera reads amount of light being reflected off film during exposure. when sensor determines exposure sufficient, terminates flash pulse. a-ttl, first seen on t90 (which predates eos family), flash exposure system adds brief preflash during exposure metering when camera in programmed exposure (p) mode. amount of light returned preflash used determine appropriate tradeoff between aperture , shutter speed in p mode. depending on specific flash unit , whether or not set bounce, preflash might infrared or might normal white flash. in a-ttl system sensor reads preflash return located on flash unit; caused issues when using filters filter cover lens (but not sensor outside lens) causing inaccurate settings. canon eos cameras used a-ttl preflash in non-programmed exposure modes detect out of range conditions; out of range warning feature dropped on later models, reportedly due patent conflicts.

e-ttl

e-ttl (evaluative-through lens) canon eos flash exposure system uses brief pre-flash before main flash in order obtain more correct exposure. unlike ttl , a-ttl metering, use dedicated flash metering sensor mounted in base of mirror box, e-ttl uses same evaluative metering sensor used ambient metering. ttl (and actual flash metering, not pre-flash, of a-ttl), sensor internal camera , takes exposure via lens filters added lens affect e-ttl readings giving more accurate exposure information camera.

the pre-flash occurs before main flash (except when using camera / flash in 2nd curtain synch mode) , barely perceptible, although can seen if watch it. pre-flash may cause undesirable reactions animate objects - e.g. humans may blink reaction pre-flash, , have eyes closed time main flash occurs. when using flash exposure lock (fel), pre-flash fired when fel activated.

e-ttl ii

e-ttl ii software improvement on e-ttl , standard in eos cameras introduced or after canon eos-1d mark ii in 2004. e-ttl ii implemented in body, not flash unit, , therefore can use existing e-ttl flash units , ef lenses.

the main improvement of e-ttl ii gives more natural flash exposure, being able handle tricky scenes old e-ttl system thrown off. such improvements possible because e-ttl ii incorporates lens-to-subject distance information in calculation, available, assist in determining approximate guide number flash output. flash metering system no longer linked af system, whereas in old e-ttl metering, bias given selected af point. rather, e-ttl ii compares ambient , pre-flash light levels of scene determine subject lies, in conjunction subject distance information if available. gives photographer flexibility lock focus , recompose scene without fooling flash metering system. hotspots (areas of high reflectance) throw off flash metering system ignored in calculation.