Nikon CLS question

Captain IK · Apr 17, 2009

A question regarding NIkon's CLS...
I am using 2 SB-600's remotely fired from a D90 in Commander mode.
One is used as a key light and fired through an umbrella, the other is a backlight to washout the backdrop with no umbrella.

Does the umbrella on the key light affect the TTL metering?

I ask because it seems the key light is underexposed a little. I don't have a flash meter so I cannot check it independently.

Any help would be appreciated.

Thanks

Dave

JerryPH · Apr 17, 2009

No, it just is made more softer. CLS automatically ups the power on the flash behind the umbrella to acheive the aperture and shutter speeds you require or have set at the camera.

This is assuming that your commander settings in the D90 are set to TTL and not a manual remote setting, which is also possible, and in that case, an umbrella WILL affect the light... it will lose about a stop going through it (depending on the quality of the umbrella and whether it is bounce or shoot-through).

If in all iTTL, you focus and aim at your subject... CLS pulses are sent through your on camera's flash to the remote flashes... triggered, measured, set and the picture is taken. The iTTL calculations are made with the umbrella in place. What you perceive as being dimmer may indeed just be softer/diffused. light vs the harsher 2nd flash.

Basically it all depends on what your in-camera settings are. If all iTTL... should be no diff. If you are using remote manual settings... it will make a difference. Another possibility, though I left this one out becuase it is obvious... you have one flash set to channel A and the second one to channel B... and have different power or ratio settings set to be different.

bdavis · Apr 17, 2009

If you have your D90 in commander mode and the 600s set as manual on the camera, then it wont matter. However you ask if it will effect the TTL metering, my thoughts are yes.

JerryPH · Apr 17, 2009

bdavis said:
If you have your D90 in commander mode and the 600s set as manual on the camera, then it wont matter. However you ask if it will effect the TTL metering, my thoughts are yes.

You have it backwards... think about it:

In remote manual, the iTTL does NOT care what the current lighting situaion is.. all it is sending to the off camera remote flashes is what power to set themselves to before making the final flash (ie: 1/4 power, 1/2 power, etc...). If you place one of them behind an umbrella or in the other room, power levels will not change. In effect, there is NO iTTL dynamics!

In iTTL mode, the scene is metered THROUGH the umbrella as the camera sees the scene via the preflashes (with the effect of the umbrella in place), so CLS will tell each flash what power level to adjust to BEFORE the final flash hits and the scene SHOULD be exposed the same if I take away the umbrella or leave it there or move the flash closer to or farther from the subject. The ONLY thing that will change is the softness or harshness of the light... NOT the exposure levels. That is the job of iTTL with CLS!

kundalini · Apr 17, 2009

^^ That's one of the great benefits of CLS and TTL. Thank you Nikon.

If you need more or less power, just make the bump through your Commander menu. That is also a good reason to have your SBs on separate groups.

Captain IK · Apr 18, 2009

Thanks for the responses...Sorry for my delayed reply...
I have everything set to TTL (both camera and flashes) in this case
I suspect Jerry is correct in saying that it is the softer light that is causing the effect. That makes sense to me.
Thank you for the help.

Captain IK · Apr 18, 2009

As a follow up...
If I want the backlight flash to be over exposed, would I set it to Channel B and adjust the exposure compensation.
So the flash in Channel A would meter correctly and the flash in Channel B would over expose as per the compensation adjustment.

Garbz · Apr 18, 2009

Well it does affect TTL but not in any abnormal way. TTL means the flash fires once briefly while the camera meters the scene. Now the difference here is a direct flash would produce an image with far more contrast, whereas the umbrella would soften the overall light.

TTL will see this as different scenes, and adjust output accordingly. But obviously it can't get things right anytime as is with any other automatic exposure control. If you find your soft images are being reproduced a tad too dark, then up the exposure compensation on the flash unit from the commander. That's the power CLS gives you.

JerryPH · Apr 19, 2009

Don't forget that one can also globally affect the exposure using the exposure compensation on the camera, it too has an effect on the CLS power level settings of the flashes.

JerryPH · Apr 19, 2009

Captain IK said:
As a follow up...
If I want the backlight flash to be over exposed, would I set it to Channel B and adjust the exposure compensation.
So the flash in Channel A would meter correctly and the flash in Channel B would over expose as per the compensation adjustment.

Yes... and Bob's your uncle.

But I would set the rear flash to remote manual and just set it to a level that blows out the rear scene, if that is what you want... that way it stays perfectly the same no matter what changes above, below or in front of it. On the front flashes, I would set it to TTL and if clouds passed or the sun came out... the exposure would always be the same on our subjects.

Captain IK · Apr 20, 2009

JerryPH said:
Yes... and Bob's your uncle.

But I would set the rear flash to remote manual and just set it to a level that blows out the rear scene, if that is what you want... that way it stays perfectly the same no matter what changes above, below or in front of it. On the front flashes, I would set it to TTL and if clouds passed or the sun came out... the exposure would always be the same on our subjects.

That's an excellent idea Jerry. I didn't think you could set one on TTL and the other on manual, but that will likely solve my problem.
Don't know why I didn't think you could do it as it does seem obvious now.

Sometimes if you stare at something too long....you stop seeing it!!

jlykins · Apr 21, 2009

Something else that might be happening, when you say that you are trying to blow the background, I'm assuming that you're using a white background and you want it to go all white... I have had issues with this before. The all white plays tricks on the meter. You may be better off going all manual mode and adjusting accordingly. Just a thought though.