Various TTL definitions

Hello All,

I'm new (universal sign for "here comes the same question again). I'm trying to learn enough to pick a dSLR camera, housing, strobes etc. In that quest I've found that I'm bewildered by the various TTL terms. Can someone point me to a good set of definitions of all the various types of TTL, and other strobe variations?

As I understand it:

TTL = Through The Lens light metering. It is used to control the strobe, via a cable (or the built in flash for that matter) by sensing the amount of light coming back through the lenses.

eTTL = Evaluative TTL, which means a lower power pre-flash happens before the flash, and the built in meter uses the first flash to predict the proper power of the second flash.

iTTL = ?

aTTL = ?

I also see references to strobes that are slaves, which as I understand it means they use a sensor to sense when another strobe goes off, then fire quickly in succession.

I'm trying to understand the strobe types, so I can figure out which camera will or won't work. For instance, brands or types of dSLRs that use one type of TTL or another, or are compatible with more than one type.

I hope I can be pointed in the right direction to understand some of this. My goal is to purchase a "complete" setup, which I can upgrade over time with lenses, etc. I dive in the NW part of the US much of the time, where I'll take relatively close up pictures with flash most of the time. I also travel to warm water destinations, and I love cave diving. I'd like to be able to take good pictures in the caves as well. I don't know if any of that is pertinent to the strobe discussion though.

Thank you for any and all help/direction,

Greg Cooley

You are not alone in being confused, Greg!

I am too. And it is not entirely our fault. Sometimes I think that the camera manufacturers purposely use confusing terminology to hide the basic deficiency of nearly every digital camera to do proper TTL.

Proper TTL is that the camera monitors the flash power during the exosure (through the lens) and quenches the flash when the correct exposure has been achieved.

With the exception of the Fuji S2, no digital SLRs offer true during the exposure TTL flash control. Instead they monitor the flash exposure during pre-flashes and use these to estimate the amount of light required for the main exposure. This happens very fast and usually this estimate is fine.

Canon call their system eTTL and upgraded it to eTTL2. Nikon's was called dTTL and this was upgraded to iTTL.

Where this gets even more confusing is when a company like Nikon says that both the film camera the F6 and the digital camera the D70 both use iTTL. The difference is that the Film camera can still monitor flash exposure during the main exposure while the digital relies on pre-flash only. IMO, I think this is very misleading to the customer - who could be misled that both have the same TTL capabilities - because they have the same name - when they do not.

Alex

Alex gave a good summary of the sorry state of TTL, but I suggest you ignor all xTTL altogether. I think it's more trouble than it's worth. It's easy to do without TTL. Have a look at the TTL Anonymous thread

http://wetpixel.com/forums/index.php?showtopic=6199&hl=

You'll get a good mix of opinions.

You also don't need to worry much about slave strobes. All housed DSLR's use electrical cables to connect to the strobes with the exception of the Inon setup that uses an optical fiber. With few exceptions you can get any of the current strobes working with any DSLR.

Thank you both! The link was fairly informative. I say fairly because now I realize that the answer to my questions are not really as straight forward as I had hoped. But this gets me one step closer to purchasing my first underwater camera.

Greg