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SU800 TTL-metering faulty?

Discussion in 'General flash photography, lighting, and technique' started by Vulcan, Aug 24, 2008.

  1. Vulcan


    Jun 26, 2008
    Using my SB900 or my SB800's on camera in TTL-mode (FP but not BL) the resulting picture is well lit and exposed. If I use my SU800 to trigger the flash the pictures gets underexposed (same settings on camera and same distance to subject with flash on/off camera, i.e. manual mode, matrix metering, F7.1, S125. distance to object some 2 meters (6.5 feet), the SU800 set to commander mode and not close up mode).

    Is my SU800 faulty or is there something that I'm missing? There is no doubt that there is enough power to expose correctly (with EV compensation or the flash in manual mode) and I can't understand why the unit causes underexposure. Input is highly appreciated.
  2. I don't know the answer but I have experienced the same. In my case the underexposure is relatively minor - less than a stop. My assumption is that the difference is because the camera does not know the flash to subject distance once the flash is moved off the body. I suspect that the exposure is a little conservative as a result. But I could be wrong. I believe the same situation can occur when you use cords like the SC-17/SC-28.

    As an old film shooter I remember how good flash metering was with film when flash exposure was controlled by a sensor reading the light reflected off the film. In my opinion we are not quite there yet with digital.
  3. chemisti


    May 24, 2007
    McKinney, TX

    I don't own a SU-800, but I do use CLS/AWL a little...

    1. I don't think the SU-800 has anything to do with exposure evaluation in the CLS system. I think that the unit is merely an interface for contol (setting individual exposure changes) and a IR communications transiever to relay settings/instructions to the remote units.

    2. Exposure evaluation is performed by the camera body based upon settings (aperture, shutter speed, metering mode, etc.). The system uses a pre-flash to evaluate exposure, calculate an adjustment to remote power, and then communicates that adjustment to the remote unit(s).

    3. I think that part of the perceived challenge may be related to matrix metering. I have not run any kind of scientific testing to verify this, but sometimes it seems to me that the matrix metering adds another "not quite sure how it works" variable to this equation.
    Without a flash, I have noticed that matrix metering works quite well - usually. However, there are times that I must meter for selected parts of the scene to get the desired exposure.
    With flash on-camera, sometimes I must do the same thing to prevent over/under exposure depending upon the scene. Then - add 2 or 3 flashes to the equation and the vagueries of matrix metering (what exactly in the scene is the camera trying to properly expose?)

    I would suggest that you might consider spot or center-weighted metering on a critical portion of the scene in an attempt to ensure that whatever you deem as critical is properly exposed. FV lock can be helpful here.

    I have found this to be useful - but I must confess that sometimes it is merely trial and error when it comes to off-camera TTL flash use!

    Hope this helps,
    Good luck.
  4. JeffKohn


    Apr 21, 2005
    Houston, TX
    I think if the camera-subject distance is substantially different from the flash-subject distance, there's bound to be some inaccuracy in the flash metering. The camera has no way of knowing how far the flash is from your subject, the only thing it can do is guess based on distance info received from the lens.
  5. Vulcan


    Jun 26, 2008
    Thanks for the replies. I now understand that part of the problem is the difficulties to calculate the distance and that the flash off camera defaults to its wide-angle setting. Once again this proves that there is no such thing as a free lunch. Guess I have to live with this and either add flash ev compensation in TTL or use manual.

  6. smodak


    Jun 11, 2007
    Franklin MA US
    I have the same experience..I mostly shoot manual but when I do use TTL, I alwaya dial in +1EV flash.
  7. Miguel


    May 12, 2006
    Norwalk, CT
    Just got the SU-800 and experiencing the same when firing the SB-900 with it. I've found that +1EV gives me proper exposure.

    I've never experienced this when using the SC-29. Guess it's just the way it is.
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