Understanding ETTL

I just purchase a low budget flash with ETTL (Opteka EF 600) and was wanting some links on reading material. I somewhat understand what it does. The thing that I am not understand is when u take it off camera using cord or wireless. The strobe is at a fixed distance from the subjest. While the camera is traveling around the subject. How does the strobe know how much light the subject needs?

I could be wrong but if the flash is stationary it doesn't matter where the camera is. The light output from the flash reaching the subject is the same no matter where the camera is.

This should get you started. Written by Chuck Gardner.

http://super.nova.org/​DPR/Canon/

In ETTL, the camera sends a signal to the flash (on- or off-camera, if it is connected via off-camera cord or via a wireless that supports ETTL signaling). The flash emits the pre-flash, the camera meters the emitted light reflected back from the subject, and then the camera tells the flash how much light to send out for the main exposure. As RJC1 said, the camera distance does not matter, it is the flash-to-subject distance which is subject to Inverse Square Law of falloff of light intensity.

E-TTL II requires that you have a flash mounted on-camera, or on a compliant off-camera E-TTL cord. The camera asks the flash to emit a low-power pre-flash so that the camera can determine the difference between an ambient exposure reading and the flashed exposure reading. The required settings are then given to the flash, and it is told to fire. It is the camera which determines the exposure required, in accordance with your settings on both camera and flash.

For Canon Wireless, a second E-TTL-compliant flash is required as Master, or a 7D camera body, or the latest Pocket Wizards. RadioPoppers require two flashes to provide the coding stream.

Simple radio triggers cannot handle the complex E-TTL communications between camera and flash.

Pixel have the promise of E-TTL at a mid-price, and do not require a Master flash, but they are overdue for releasing the TR-332. Read first assessments when they are published, before buying.

When using a ettl flash with a modifier do you have to compensate for the light you will lose? If so how?

Flash exposure compensation.

Here is a nice Canon article on flash:

http://www.usa.canon.c​om …ArticleAct&arti​cleID=2930

extrememc wrote in post #9657644
When using a ettl flash with a modifier do you have to compensate for the light you will lose? If so how?

In ETTL mode the theoretical answer is no - YMMV. The pre-flash measured by the camera is traveling the same route as the main flash will travel so it should already be compensated for.

That said, the use of FEC is always quick and easy.

I agree with Dave. E-TTL will take care of that automatically. But flash exposure compensation may still be needed, as you may come to a different conclusion than the E-TTL logic does.
Like in this picture, where I had to use a FEC of 1 2/3 steps, due to the white snow fooling the E-TTL metering into severe underexposure. Fortunately, I've been around long enough to understand that before I took the picture, so I took some test shots before the action started, to understand how much to change the flash power. Once that action started, it didn't last long...

Watch this and become enlightened

http://cpn.canon-europe.com …asterclass/cano​n_flash.do

E-TTL does not know that you have a modifier installed on the flash head, ergo, flash exposure compensation.

Savas K wrote in post #9660442
E-TTL does not know that you have a modifier installed on the flash head, ergo, flash exposure compensation.

ETTL doesn't care that you have a modifier installed, ergo, no adjustment needed.

ettl measures the preflash at the sensor, so the effect of any modifier is accounted for at the sensor. Thats what ettl means, through the lens...

Titus213 wrote in post #9661460
ETTL doesn't care that you have a modifier installed, ergo, no adjustment needed.

Bingo.
Any light loss by a modifier is basically the same as the flash being further away. It automatically adjusts the power of the flash to compensate.

ootsk wrote in post #9661659
Bingo.
Any light loss by a modifier is basically the same as the flash being further away. It automatically adjusts the power of the flash to compensate.

Within the limits of the maximum available flash output.