I've read, on this forum, that you can control the light ratio between two off-camera flashes that are in TTL or E-TTL mode by CHANGING THEIR RELATIVE DISTANCE FROM THE SUBJECT.

I've carefully re-read the posting but I'm not clear on why this works. Specifically: Why wouldn't each flash simply provide higher output, at a greater distance, according to your aperture?

I understand how this works using the Guide Number system with flashes set in Manual mode, but not in ttl / e-ttl.

Could someone kindly explain this?

Thanks

-- Roy Kekewich (roykekewich@yahoo.com), September 28, 2000

Answers

Flash intensity is determined by the flash duration. In the Canon TTL system, all speedlites fire for the same duration**. Therefore, the way to control lighting ratios is a combination of flash distance and/or flash power adjustments. Flash power can be varied by changing the "zoom" settings or by using different strength speedlite units. (**I'm not sure about E-TTL ratio flash....)

Guide numbers have limited use in the realm of TTL (or E-TTL) flash exposure control.... You could use guide numbers to determine aperture based on flash power and distance, for each unit, as if each was the only unit. The resulting apertures should give you an idea of the lighting ratios.

-- Dave Herzstein (herzstein@juno.com), September 28, 2000.

Thanks for the above answer. I hate to say it but though I understand how zoom settings on the flash heads or distance would determine aperture (and flash ratios) with flashes on Manual mode, I still dont' grasp WHY DISTANCE WOULD EFFECT TTL FLASH.

Considering that flash output is measured OTF (off the flim) in TTL, why wouldn't, say, two 380EX flashes, both in contact with the camera via off-shoe cords or Ikelite Litelinks, both output according to the camera aperture AND METER READING OTF?

Any further explanation would be greatly appreciated.

Thank you

-- Roy Kekewich (roykekewich@yahoo.com), September 30, 2000.

The camera doesn't measure the flashes individually, it measures them together and shuts them off, both at the same time (when the camera sensor determines the correct lighting has been achieved). So if you have two identical flash units, they will both output the same amount of light before they both get shut off together, but if one flash is closer it will light the subject more than the other one.

-- Jim Strutz (jimstrutz@juno.com), October 01, 2000.