the way TTL was explained to me was that the flash goes off and shuts down DURING exposure of the image. But i-ttl and e-ttll both use preflashes now a days to set the power before the the shutter actuation. is tru ttl even and option anymore these days?

TTL just stands for "Through The Lens". Yes, current Nikon and Canon cameras (iTTL and E-TTL II respectively) do use the camera's exposure system to calculate and set both flash output and camera exposure.

You are correct in that "some" film camera TTL used a reflection of the scene from the film at the film plane in order to calculate the flash component. As such the flash exposure was terminated when the "proper" amount of flash illumination was sensed and the flash itself was cut using thyrister circuitry.

Modern dSLRs also use TTL in that the camera's exposure unit handles both ambient and flash, and that the pre-flash generally does occur before the main mirror is raised. The current versions of camera flash control also incorporate distance information supplied by the lens when the flash head is oriented as direct flash. When indirect flash illumination is detected, i.e. when the flash head is moved from direct forward, then the flash illumination is calculated without the distance component.

Originally Posted by ziggy53 TTL just stands for "Through The Lens". Yes, current Nikon and Canon cameras (iTTL and E-TTL II respectively) do use the camera's exposure system to calculate and set both flash output and camera exposure. You are correct in that "some" film camera TTL used a reflection of the scene from the film at the film plane in order to calculate the flash component. As such the flash exposure was terminated when the "proper" amount of flash illumination was sensed and the flash itself was cut using thyrister circuitry. Modern dSLRs also use TTL in that the camera's exposure unit handles both ambient and flash, and that the pre-flash generally does occur before the main mirror is raised. The current versions of camera flash control also incorporate distance information supplied by the lens when the flash head is oriented as direct flash. When indirect flash illumination is detected, i.e. when the flash head is moved from direct forward, then the flash illumination is calculated without the distance component.

Why I ask is if you are using off camera manual flashes with off camera ttl flashes then the TTL flash will think there is no manual flash there since it usses preflashes to determine exposure. Thus depending on how you uses the manula flash you may risk over exposure.

Originally Posted by Qarik Why I ask is if you are using off camera manual flashes with off camera ttl flashes then the TTL flash will think there is no manual flash there since it usses preflashes to determine exposure. Thus depending on how you uses the manula flash you may risk over exposure.

That is correct if the manual flash is triggered to go off during primary exposure. If you use simple optical triggers they will set off the manual flash during pre-flash and that "would" be measured by the camera with unpredictable results since the camera cannot moderate the manual flash output.

Originally Posted by ziggy53 That is correct if the manual flash is triggered to go off during primary exposure. If you use simple optical triggers they will set off the manual flash during pre-flash and that "would" be measured by the camera with unpredictable results since the camera cannot moderate the manual flash output.

Of course, if this happens, you're screwed anyway since the flash most likely won't be able to recycle fast enough to fire correctly during the actual exposure. So even if the manual flashes were read correctly by the camera's metering system (I suppose it would be registered as ambient light) and the camera was able to accurately adjust the TTL output to compensate, your final exposure would still be sabotaged since the manual flash wouldn't be there when the camera expects it to be. So it might actually underexpose the TTL flash... but who knows.

Originally Posted by Tim Kamppinen Of course, if this happens, you're screwed anyway since the flash most likely won't be able to recycle fast enough to fire correctly during the actual exposure. ...

Most manual flash units have some sort of selectable power output. Depending on your power needs the recycle rate can vary widely. If recycle speed is paramount then yes, I agree that most inexpensive manual power flash units do not have terribly fast recycle rates compared to the best from Nikon and Canon, especially since those top line units can also use external high-voltage power supplies to provide "much" faster recycle rates at high output.

Edit: OK, I get what you are referring to now. You mean 2 flash outputs; the first for the pre-flash and the second for the capture output. You're right, I don't know of a flash that will recycle that quickly.

Originally Posted by Qarik Why I ask is if you are using off camera manual flashes with off camera ttl flashes then the TTL flash will think there is no manual flash there since it usses preflashes to determine exposure. Thus depending on how you uses the manula flash you may risk over exposure.

This why you should just eliminate the manual flashes out of the equation OR better yet turn the newer flashes to manual and meter with a flash meter......if I am putting my SB900's into a manual flash mix that is the what I do and then it works like a charm...............

There is a lot of flash info in the online Nikon CLS Practical Guide. Maybe answer some of your questions.