Auto-ISO with fill-in: problem with D5100/SB-400

Discussion in 'Nikon' started by jcartwright, Jul 26, 2015.

  1. Hi, I have just acquired an SB-400 external flash to pair with my D5100, and am encountering a problem when using it for fill-in flash.
    I normally use the camera in aperture-priority mode with ISO set to AUTO and the shutter set to a minimum of 1/60th. For fill-in, I usually like to compensate the flash minus 1 or 2 stops so that it is just taking some of the darkness out of the shadows, but not overpowering the ambient light. When using the camera's built-in flash this works fine: the image is exposed exactly the same as it would be with no flash; the flash simply "fills in" the shadows a bit. Great!
    However, when using the external SB-400 the compensation does not appear to work. Instead of the camera exposing correctly for ambient light and the flash filling-in the shadows by minus 1 or 2 stops (whatever I have set it to), the overall picture is underexposed by minus 1 or 2 stops. The problem seems to be related to the auto-ISO function, because when I set the ISO manually the compensation works again.
    As an example, I took a photo indoors with no flash at f/3.5. The shutter automatically selected 1/60th and the ISO automatically selected 1600. The exposure was correct.
    I did the same using the built-in flash. Whether I set it to zero compensation or -2 stops compensation, the camera exposed at 1/60th, ISO 1600. The exposure was correct.
    I did the same with the SB-400 external flash. I set it to zero compensation, and the camera exposed at 1/60th, ISO 500. The exposure was correct. (I presume lower ISO because the flash is more powerful.)
    Again using the SB400, I set it to -2 stops compensation, and the camera selected 1/60th, ISO 110. The picture was underexposed.
    As you can see, the flash compensation setting, when used in tandem with auto ISO, reduces the OVERALL exposure, not the fill-in exposure.
    Am I doing something wrong?
    I borrowed an SB-700 flash, and the same thing occurred, so it is not a fault with the flash. As I said before, if I turn off auto-ISO, the problem goes away: the flash compensation setting returns to adjusting flash compensation, not overall exposure.
    Any advice would be most welcome.
    Jon
     
  2. if I turn off auto-ISO, the problem goes away​
    dont use auto-ISO with flash. it's kind of a pain to keep changing this setting, especially on a camera without a dedicated ISO button, but that's how it is. FYI, auto-ISO can overexpose in dark settings with flash, too. if you have an assignable button which can be set to Auto-ISO, you can at least make the process somewhat less tedious. not sure if the D5100 has this, but the D300s does. or if you have custom settings you can set one profile to base ISO and one to auto ISO.
     
  3. Nikon's implementation of auto-ISO combined with flash is known to be weird. I don't know whether someone at Nikon thinks this behaviour is useful (in the way they tried to claim that breaking trap focus was a good thing, until they brought it back) of whether it's just broken but they haven't had enough complaints to spend time fixing it.
    The upshot is that I reached the same conclusion as Eric: although I normally shoot in auto-ISO pretty much all the time (usually in manual mode), I turn auto-ISO off when using flash, because I don't trust it. The good news for me, as a D810 user, is that I can turn auto-ISO off quickly using the second dial when holding down the movie record button (although arguably I'd rather the second dial controlled minimum shutter speed or shift rather than just being a toggle). Unfortunately, it's a little slower to do this on a D5100.
     
  4. i got tired of auto ISO settings where the camera would select 1600 with flash. auto-ISO doesnt consider the flash value, so you really have to set it manually when using flash. the sb-400 doesnt offer manual settings so its pretty much full power, all the time. that said, it's pretty good at fill flash. but i always set base ISO when using flash (usually 200). i'll bump it to 400 if i want faster recycle, but 800 and 1600 are reserved for super-dark interiors. flash and ISO 1600 just dont mix, except in extreme low light.
     
  5. Thanks for your responses, everyone. I suppose the answer is: no-one really knows what auto ISO does! I might contact Nikon to see if they can explain the behaviour – for although I can happily use the flash without auto-ISO, it would be reassuring to know why the camera selects the settings it does.
    If Nikon gives me a good answer, I'll post it here, of course.
     
  6. Thanks, Jon - I'd like to see a definitive statement as well. (Experimentally, I believe the behaviour recorded in the post I linked to, although I've not personally confirmed it - I just think it's unhelpful!)
     
  7. essentially, auto-ISO introduces a variable into the exposure equation, which doesnt help when you are trying to control lighting using flash. the camera takes a blind guess at correct exposure, which may underexpose in bright settings or overexpose in dim settings, depending on what limit you set. but since we dont know the algorithm being used, and the camera doesn't know exactly what we are trying to emphasize in the shot, it throws everything off. i suspect that even testing all the possible variables, including metering and setting manual flash (which you can't do on an sb-400) would result in non-replicable results, because it's just quirky. while auto-ISO can be useful, especially when shooting in manual, it seems to sometimes default toward the highest possible value.
     
  8. Eric: I've not personally confirmed the reports in the article I linked above, but I can understand the old behaviour: keep the ISO as low as possible except in slow sync, and in slow sync raise the ISO as needed for the background exposure (and fill the rest with flash). Exactly what's going on I agree depends on Nikon's secret matrix metering algorithms (although I wish they had a predictable ETTR mode). If the recent behaviour is to run at higher ISO than strictly necessary, I'm unimpressed. But whatever they're doing, I trust it little enough that I just turn it off when I'm shooting with flash. If Nikon come back to Jon and actually tell him what's going on, that might help me enough that I'd use it!

    I'd really like it if it were possible to set the bounds on the exposure parameters, and the priority of breaking those bounds. It would be a pain to do for any one shot, but for an entire session it would probably be worth it. So, for example, set aperture to a range between "acceptable aberrations" and "diffraction" (for my f/2.8 70-200, that might be f/4-f/11, and that's a bit more either side than I'd like), set shutter speed to somewhere between "I want a bit of motion blur" and "I can't hold the camera steady", and ISO to "I hate anything above 6400". Then say which of those rules you'd like to break first, or even how much worse one change is than the others (I hate ISO 12800 more than f/2.8...) Alternatively, maybe set a centre value (I really wanted f/5.6 and 1/60s at ISO 64) and let things progressively shift away from it (it's dark, I need f/4; it's darker, I need 1/30s; it's darker, I need ISO 100; it's darker, I need f/2.8...) but with some fine tuning. As I say, a pain to set up, but I'd use it for "every dance at the wedding", set-and-forget. SPAM modes are so last decade. Then add the flash behaviour to those schemes. We can make the manuals longer these days, they're digital. :) (Canon had "safety shift" a long time ago; this is just an extension.)
     
  9. This is an excellent question and I'm relieved to see that other people who know how to use their cameras are mystified when it comes to auto iso and flash.
     

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