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Can someone explain TTL flash technology?

Discussion in 'Accessories' started by ante, Aug 2, 2012.

  1. ante

    ante Mu-43 Regular

    65
    Jul 15, 2012
    Denmark
    I am rather new to modern digital cameras. I have not been able to find detailed info on how exposure is actually controlled during flash photos. I am interested in both built-in and external slave flash(es). Can someone give some detailed explanation of how it works?

    One reason for my question is that I am old enough to remember when the OM-2 came out and, as I recall it, was the only camera able to close the shutter when enough light had hit the film. Other cameras at the time, as I understood it, had to determine flash/exposure time prior to opening the shutter.

    I assume with modern electronics, this is a lot more sophisticated, but exactly how? For example, if I have two slave flashes and the built-in flash, can they all be controlled by TTL? What happens if, say, during the exposure, there is another light source, not controlled by the camera, adding to the light in the image - will the shutter close, and I have a correctly exposed image?

    Sorry if my question reveals my stupidity, but I may as well admit it. :confused:
     
  2. flash

    flash Mu-43 Hall of Famer

    Apr 29, 2010
    1 hour from Sydney Australia.
    Gordon
    The system for most DSLRs is slightly different to mirrorless cameras. It's a big subject and difficult to condense into a few lines.

    In a DSLR the sensor isn't exposed to light before exposure so there is a small light sensor that calculates ambient exposure and one for TTL flash in the camera body. Just before the mirror moves up the camera gets the flash to send out a small pre-flash. The camera uses that to calculate the flash exposure. Then the shutter opens and the flash is fired for the appropriate duration (1/300 to 1/5000 of a second).

    In a mirrorless camera the main sensor does all the work. The pre flash still fires but it's measured by the CMOS sensor not a separate one. Because of this theres two main drawbacks compared to a DSLR. Firstly there's a longer delay in a mirrorless system as he sensor and processor calculates the exposure. The second one is that the mirrorless system is not as good at balancing mixed light as the main sensor tries to calculate both ambient and flash exposure at the same time.

    Most, but not all, wireless multi TTL fash systems just use a series of pre-flashes to calculate the appropriate exposures for each group of flashes.

    Gordon
     
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  3. ante

    ante Mu-43 Regular

    65
    Jul 15, 2012
    Denmark
    manual flash

    Thanks flash,

    very appropriate nick, I guess. :smile:

    So - if I use one or more manual slave flashes, I can still rely on the shutte/sensor/processor to give me correct exposure?
     
  4. sprocket87

    sprocket87 Mu-43 Veteran

    306
    Jun 29, 2011
    Not if you use manual slave flashes. If your flash is in truly manual mode then it is firing at a set power level as defined by you (1/1 - full, 1/2, 1/4, etc). In this case the camera meters the scene's ambient exposure as normal and your flash adds fill. But you still have to get the manual value right to avoid overexposing the image.
     
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