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Bouncing Flash

BriansepBriansep Member
edited March 2011 in flash & lighting
This is my first post...... I have a d700 and SB800. I use TTL on camera flash most of the time. I bounce flash off of anything I can to soften the light, ride the flash to get a decent exposure and have just started to "play" with yr BFT.....

My question is when u bounce flash with the flash head rotated away from straight ahead, how does the the flash metering work? if you bounce off a wall to yr left say, the pre flashes and have to travel further to get to the subject, be reflected off that subject and then back to the camera for metering........ does the metering not confused with all this bouncing and reflecting?

Briansep
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Comments

  • Neil vNNeil vN Administrator
    the camera measures the brightness of the pre-flash. Since it emitted a known quantity, it can calculate how much light to emit for the actual flash burst, to give correct exposure.
  • Neil thanks, so the extra distance bouncing the flash then makes little difference to the correct exposure.......
  • It shouldn't but in practice you might find that you need some extra flash exposure compensation, up or down, depending on your system and whether you are bouncing a long way or using direct flash or whatever.

    I'm not sure about Cannons system, but Nikon's system is supposed to work slightly differenty depending on whether you are using TTL or TTL-BL. My understanding is that in TTL-BL the flash metering incorporates distance information from where the lens is focused, but it doesn't use distance information if you swivel the flash head off centre. Straight TTL doesn't use distance information regardless of where the flash head is pointed.

    Another difference with TTL-BL is that it meters for flash off the focus points rather than the matrix or spot metering that TTL uses. What this means is that it meters for the brightest part of the scene so long as it is under a focus point.

    This guy has some very good videos demonstrating how this works, watch how the flash output changes when he changes the focus distance and the difference between TTL and TTL-BL when you introduce something white into the scene.



    Now this is interesting and can be useful to know, but it can also get bloody confusing, that's why as Neil always points out it's best to use the method that works for you and get used to how your system reacts. They are pretty consistent for a given situation, If you always use a particular mode for a given type of situation, you get used to how it responds and you can predict what it will do without needing to know 'how' it is doing it.




  • Neil vNNeil vN Administrator
    Like Graham says - in theory the change in distance should not affect TTL exposure metering. But practice might show some variation. Then you just ride your FEC a bit.
  • K31koK31ko Member
    Neil, I am still learning from your website and book. I am struggling with the lack of white walls in certain places like a dimly lit restaurant with dark/brown walls and ceilings and red carpet. How can I use bounce flash in such places? I want to catch some ambient light as well.
  • Besides what Neil suggest, perhaps shoot in RAW and correct the WB in post-process?
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