D80 - FP sync exposure problem

Discussion in 'Nikon DX DSLR' started by DougD50, Jul 5, 2007.

  1. DougD50

    DougD50

    97
    Aug 29, 2006
    I have my AUTO FP setting turned on on my D80. I took the following 4 pictures in M mode (f/5.6. varying only shutter speed each time: 1/100, 1/200, 1/400, 1/800), with my SB-600 set on manual at 1/64. I expected my exposure to be about the same for the 4 pictures because the flash should happen much faster than 1/800 S. I tried the same test on my friend's D200 and got 4 shots (varying only shutter speed) to have the same exposure . Is something wrong with my D80? Do other D80 owners have this problem?

    #1: iso 200, f/5.6, 1/100, SB-600 set to 1/64
    723834790_b11a29051e.

    #2: iso 200, f/5.6, 1/200, SB-600 set to 1/64
    View attachment 104183

    #3: iso 200, f/5.6, 1/400, SB-600 set to 1/64
    View attachment 104184

    #4: iso 200, f/5.6, 1/800, SB-600 set to 1/64
    View attachment 104185
     

  2. You probably know this, but at shutter speeds shorter than your normal mechanical sync speed (1/200 sec) the shutter is actually a moving slit. Therefore, to achieve even illumination across the sensor, you need a flash duration longer than normal, which is to say it must last at least as long as your shutter speed.

    That's the whole idea of FP flash. The flash duration, which normally would be very short (much shorter than your shutter speed), is extended by firing the flash very quickly many times during the exposure. Because a graph of the light output is not square edged, the SB-800 can do a fair job of simulating a continuous light source for a brief period of time using FP sync. In the old days (really old) I used to do it with special flash bulbs which burned for a longer time than normal flash bulbs.

    So if you have set a manual power setting on your flash, that's what you'll get. As you shorten the shutter speed, you get less and less exposure. In FP manual, think of your SB-800 as something like a powerful flashlight and you'll get the idea.

    I don't use FP a lot in manual mode, but I do have a D80 and an SB 800, so I reproduced your test and got the same results, which is what I expected. That is to say there's nothing wrong with your D80.:biggrin:

    As far as the other camera, I can't say 'cause I don't own one. Perhaps the flash was in TTL rather than manual.:confused:
     
  3. DougD50

    DougD50

    97
    Aug 29, 2006
    Bob,

    Thanks for the answer, it almost makes sense to me. Following your explanation and assuming that manual flash is a short burst (really fast), then I should get some part of the frame completely dark when shooting with shutter speed faster than 1/200s. This is because at any time during the exposure, the sensor is partially blocked by either the front curtain, the rear curtain, or both.
    If on the other hand FP flash means that the flash is trying to mimic the square wave (with many small flashes), lasting the length of the exposure, then the flash should be able to output an appropriate amount of power such that the area under the flash power vs. time curve is equivalent (sorry for the geek speak).
    If the D200 does not have an issue with this, and our D80 does, I'd say that Nikon has a design defect and hope that not too many will explore this feature and figure it out.
    I'll post in the flash technique section to see if some one else has experienced this problem.

    Doug
     
  4. It certainly can do that. Just take the flash out of manual. MANUAL MEANS MANUAL (sorry for the plain speak). Just like in regular sync, if you tell the flash to output 1/4 of it's max power by dialing in manual 1/4 then it will do so. If that's appropriate for a proper exposure, fine. If it's too much or too little, then the flash figures it's your problem. You told it to output a certain amount of power and what you do with that amount of power is your business.

    Now, if you want the flash/camera combination to figure out the exposure for you, then it is happy to do so, but you must put it into one of the automatic modes. Just select TTL rather than manual and what you refer to as a problem will go away. That is to say, your exposure will be correct because the flash/camera combination will make it so.

    Think about it. Why should this equipment work any other way? If, as you suggest, you should be able to set the flash to manual and dial in some random fractional power and then somehow insist that the flash/camera combination will produce it's own idea of a proper exposure wouldn't that defeat the whole idea of manual? If you want automatic exposure you should (and must) select one of the automatic modes. If you don't, then Nikon has provided a way for YOU to decide on the amount of flash power which will be produced and they have programed the flash to obey without question. I like it that way, don't you?:Dizzy:

    Again, I don't know for sure how the D-200 works when it comes to various FP modes, but I suspect it works the same.
     
  5. In manual mode it will work exactly the same.
     
  6. The D80 has 1/200th synch speed while the D200 is slightly better at 1/250th so should give slightly better results . As mentioned manual mode will not give auto fp , that has to be set in the ttl mode . I gave up on high speed fp mode because as the speeds get faster the flash gets so much " weaker " [ actually less time for exposing the flash which has to fire a few times and so can't do full power] . In bright sunlight at a few metres it is almost useless at small apertures and 1/500th sec . I suggest staying within 1/200th and relying on normal flash mode unless you have really perfected it and know how much flash you will have avilable ....
     
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