Term definition about flashes

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Hello again, I'm sorry if I botter somebody, but I just can't get through with those terms.

What is the differences between E-TTL / A-TTL and TTL flases???. And what does mean that a filtering lens will take you 2 stops of light ???

Thank you .....

-- Diego Garcia (samsagaz07@hotmail.com), April 30, 2001

Answers

TTL means Through The Lens metering, where the meter (in this case the flash meter) is built into the camera and meters the light that is actually hitting the film. This is an improvement over "auto flash" where the flash meter is built into the flash body and looks at the general scene in front of it. Auto flash works fine as long as you don't use a wide lens or long lens that sees a different angle of view. Most TTL flashes use a flash meter that is placed in the bottom of the camera's mirror box that actually looks back and measures the flash light reflecting off the film. This is also called OTF for Off The Film flash metering.

A-TTL (Advanced TTL) is Canon's system that is very similar to TTL except there is a preflash (usually near infrared) fired when you press the shutter button half way, that is primarily used to determine a good lens aperture. Most people agree that it's advantage over TTL is very small.

E-TTL (Evaluative TTL?) uses Canon's regular ambient, multi- segmented, evaluative meter to do double duty as a flash meter too. They do this by using a preflash about 1/5 second before the main flash fires. This preflash fires just before the mirror lifts up and you can see it in the viewfinder just before it blacks out for the exposure. The advantage is that it uses a real evaluative meter system for more accurate flash metering. It also can bias its metering in favor of the auto focus point that is selected, and use flash Exposure Lock (FEL) to lock in the flash exposure before you recompose the scene.

"And what does it mean that a filtering lens will take 2 stops of light ???"

Some/most filters reduce the amount of light that gets through the lens to the film. A polarizer usually reduces the light to 1/4 the original brightness. That's two stops. One stop is equal to 1/2 or twice as much light, depending on if you are increasing or decreasing the light.

-- Jim Strutz (jimstrutz@juno.com), May 01, 2001.


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