Off camera flash, TTL mode, through an umbrella

[andrew_andrews1]

<p>Hello,<br>

If I use my SB900, off camera, through an umbrella, using my D300's on camera flash as commander in TTL mode, I assume that TTL won't give me the correct flash exposure because the flash cannot tell how far the subject is from it because its view is blocked by the umbrella.<br>

Am I correct?<br>

Obviously there is light lost due to the umbrella as well, but let's ignore that for now.</p>

[jose_angel]

<p>Being TTL, the exposure is controlled by the camera, through the lens, not by the flash. The exposure should be then correct. Only in Auto Flash mode, the light is measured/controlled by the flash`built-in sensor.</p>

<p>The distance parameter is measured/taken into account with a featured lens on the camera too (e.g. "D" type lens), not by the flash.</p>

<p>If the umbrella scatter too much light, the lightning power of the flash could be insufficient. The result will be then an underexposed shot. </p>

[Matt Laur]

If you're in TTL mode, the flash power is set by a pre-exposure metering burst. Works just fine off-camera and

through modifiers, as Jose says.

[Two23]

<p>As others mentioned, iTTL works by sending a "test" preflash and then calculating an exposure. What I've found is that I should be prepared to tweak exposure though, usually using flash comp.</p>

<p>Kent in SD</p>

[rsands]

<p>Agree with Kent. When using modifiers of any kind (bounce, umbrella reflect, umbrella shoot-through) I always have to flash compensate.</p>

[nigel_farmer]

<p>The iTTL pre-flash will, as I understand it, calculate the flash exposure as if the flash is naked ie no modifier, but also based on the subject distance from the camera. Therefore in addition to compensating for the modifier, you will also need to compensate for the distance from flash to subject, if nearer of farther away from the subject than the camera.</p>

[rodeo_joe1]

<p>"you will also need to compensate for the distance from flash to subject" - Not so.</p>

<p>TTL metering of any sort, be it pre-flash or OTF, should automatically compensate for flash distance and any diffuser or reflector being used. I say "should" because no technology is perfect and TTL flash metering is, after all, only a guess at the correct exposure by a machine. It's no substitute for the human eye and brain.</p>

<p>It's also quite possible that the flash will run out of power by being shot through a brolly, giving underexposure, and i-TTL won't help in this case. I've measured the output of many hotshoe strobes with a number of flashmeters, and it appears that a true GN of 32 (meters/100 ISO) is about all you can hope for, regardless of any stupid numbers claimed by the makers. Take away 2 stops for a shoot-through brolly and you're down to a true GN of 16 with even the most powerful hotshoe strobe - so draw your own conclusions!</p>

[bob_h]

<p>You are dealing with digital here so it should be very easy to try a few shots and evaluate the results on your camera, and make adjustments from there. If you are working in a studio setting with somewhat constant light, you could always use a flashmeter to get dialed in and work in manual mode. If you describe more about your subject and setting, you can get more enlightened advise here.</p>