Flash and auto ISO

Discussion in 'Nikon' started by BeBu Lamar, May 5, 2020.

  1. I never had a compatible Nikon flash before so of course I always shoot flash in manual and no auto ISO. Just bought a SB-800 yesterday and I will try to find out how the camera/flash behave when auto ISO is on. Anyone tried that before?
  2. It works in auto ISO on my D750, but

    1) The camera will boost the ISO higher than necessary to illuminate the main subject which introduces noise.
    2) Due to the increased ISO the background will bee properly illuminated. When I take flash images, I usually want the subject properly illuminated and the background subdued to enhance subject separation.

    I usually set the ISO to between 200 and 400 and let the flash do the illumination. But it depends upon the image I am taking and whether the background is important. Of course with the SB-800 you can set the flash to BL, balanced, limit the ISO, and still illuminate the background.

    The SB-800 is a great flash, and as a bonus, it also works on my F100 film body.
  3. The D850 has separate control (menu e4) over whether the auto-ISO is trying to expose the background assuming the flash won't illuminate it, or whether it's exposing the foreground with the flash's benefit. I don't think my previous bodies had this as an option.

    Generally I turn auto-ISO off when I'm using flash (and shoot at base ISO if I can). If the camera doesn't get confused, I do. But then I'm not necessarily trying to do a lot of dynamic lighting with balanced flash and natural lighting.
  4. If it were to ignore the background then which ISO would it pick?
  5. Auto ISO is set according to the illumination of the scene sans flash. The flash is set according to the pre-flash measurements, after the ISO, shutter and aperture are set. Auto ISO would have the effect of rendering fill flash (slow flash).
    bgelfand likes this.
  6. So it's not usable for me. Thanks.
  7. In most cameras you can adjust exposure compensation for flash and ambient light independently. You could reduce the exposure for ambient light, but your flash exposure would be based on the ISO (and aperture) at the time of exposure. You can also set a compensation value in the flash itself.
    Last edited: May 5, 2020
  8. To quote the manual entry:

    The camera works out what the "main subject" is (I've no idea whether it does this relying solely on the selected metering point or matrix identification of the scene, whether it works out what the flash is actually illuminating, or some combination). If the flash can illuminate it at base ISO, you get that. If you're trying to illuminate a distant subject with a small flash (or if your flash power is reduced by auto-FP HSS, or if you manually set the flash to a low output) I'd expect the camera to boost the ISO to get the right foreground exposure.

    I could believe this would be useful behaviour. It's effectively the difference you get from slow sync, except choosing whether the exposure varies by ISO instead of extending the shutter speed. If you want a sharp foreground subject that's flashlit and grain free, you probably want slow sync off and auto-ISO affected by the subject only (or just trust that you have enough flash power). If you want a reasonably-exposed background as well, you probably want some combination of slow sync and auto-ISO allowing for the background turned on - but you'll get some noise and/or blur in your foreground subject as a consequence.

    I think, anyway.
  9. I personally have tired of the "natural" look of fill and bounced flash. Give me the old "cup of flour in the face" look of the Brownie days ;)
    stuart_pratt likes this.
  10. With the Auto ISO range of 100 to 12800 when the flash is ready to fire the camera switch to ISO400 regardless of aperture chosen or the amount of ambient light. This is on the Df.
  11. Interesting. I just tried my D810 (with the integrated flash). Auto ISO, manual mode. Pointing the camera out of the window, it stays at ISO 64 and the flash does a reasonable job of exposing the window frame. Pointing into the darkest corner of my study, it drops to ISO 4000 (and still fires the flash). Auto ISO seems to be vaguely working as I'd expect under those circumstances.

    I do recall there being something odd about ISO400, though. Apparently we did discuss this a couple of years ago.

    Ah, there's this. It might be specific to program mode - on a D810 in manual, the ISO behaviour definitely matches with both the integrated and a hotshoe flash; I'd not registered that the post was talking about program mode until I'd put the camera away again, and forgive me if I don't get everything out again. Also I think the description is backwards: according to the images, it's the integrated flash which uses a higher ISO, which I assume might be some combination of not overloading the integrated flash's cooling ability and wanting to get better CIPA numbers for battery life when flash photos are taken (cynical? moi?) Even so, since the Df can't possibly be using an internal flash, it wouldn't explain a jump to ISO 400.

    There's a note in the Df manual about the auto-ISO minimum shutter speed being ignored in flash photography in favour of the flash sync speed. It definitely couldn't be that?

    My D810 memory cards are still full of some not very aesthetic images of a trip to the Tutankhamun exhibition in London shortly before the lockdown. I was grumbling to myself about how much longer it takes to scroll through all the images to delete both the SD and CF versions (JPEG and RAW respectively in my case) of my test images (the D850 is both much faster at showing them and lets you scroll faster because of the touch screen) when I discovered that you can jump between them if you go to the minimum zoom (lots of images on screen - which I never normally do because it takes several seconds to populate) and then zoom out one more time. Apparently I failed to RTFM on that one. Learn something new every day...
  12. First it does switch to flash sync speed. Second I didn't point the camera at a bright subject like the windows so that the ambient exposure for the aperture chosen doesn't need to be ISO 400 or higher. When I point the camera at various scene that would need higher ISO than 400 to properly expose for the ambient light it switched to ISO400 and exposed correctly.
    One thing it bothers me is that even in M and auto ISO off it switched to flash sync speed when the flash is ready. I normally shoot flash with the Df at 1/125. I have to set the flash sync speed to 1/125 now instead of the default 1/200. Even then it wouldn't let me chose the shutter speed at will in M.
    Any way ISO 400 is OK to me for flash. I actually use ISO800 for flash with the Df quite often as the noise isn't a problem at that ISO and it gives me assurance that I have enough flash power to bounce and the recycling time not too long.
  13. You've confused me a bit, BeBu (it sounds like when you need more than ISO400 it gives you ISO400, and you didn't point the camera at a bright scene but it still didn't need a high ISO?) - but so long as it's doing what you want, I'm glad! If there's a Nikon camera for which unnecessarily high flash is acceptable, it's the Df. Well, it's the D5 or any of the D3 generation (which gain a lot less from lower ISO than other sensors), but the Df certainly holds up well to moderate ISOs.
  14. Yes when the ambient light is low and ISO higher than 400 is needed to give correct exposure without flash then it will give me ISO400 and the flash would expose for ISO 400.
  15. Ah, so it doesn't expose for the background but it boosts to ISO400 anyway if the foreground needs it for the flash?

    I barely understand flash on a D810 (with auto ISO, arguably anyway). I think I'm going to say my brain is too full of other useless information to learn how to do it on a Df. :) (Unless someone donates me a Df, obviously. But there are more worthy recipients.)
  16. First thought I think there are a lot more worthy recipients than you. Second thought you have 1 thing that makes you a bit worthy. You want to use one to be sure why you hate it.
    Andrew Garrard likes this.
  17. Since it costs nowt to shoot digital, maybe the best way to find out how your camera and SB-800 'play' together is just to shoot some flash scenarios that are typical for you.

    Nikon's i-TTL flash defaults to 'BL' (Balanced Lighting) mode when the camera's using matrix metering. So it'll try to balance the flash, and presumably the chosen ISO, with ambient unless the ambient lighting falls below some undisclosed threshold, or the shutter speed gets too long.

    Personally, I gave up on i-TTL flash quite some time ago. It's vaguaries were quite beyond my fathoming. So good luck trying to make sense of what the camera and flashgun decide between them.

    I suspect you'll revert to manual mode, or AA mode if you want some understandable and reliable flash automation. Although the camera will still choose whatever ISO speed pops into its mechanical little brain.

    BTW. Attaching a charged flash to a Nikon camera has automatically switched the shutter speed to X synch since the days of the FE. In fact it's not even the maximum synch speed that's automatically set on modern Nikon DSLRs, but some lunatic default speed like 1/30th, unless you dig into the menu and set a sensible option.
    Last edited: May 6, 2020
    Ed_Ingold and stuart_pratt like this.
  18. I can't test TTL performance with my F5 unless I put film in it. If I try to measure the flash output with the flash meter the pre flash triggers the meter.
  19. Cancel the pre-flash by either tilting the flash head down 7 degrees or selecting rear curtain sync..
  20. Well when the camera is in M and no auto ISO it doesn't switch to flash sync speed which is good for me. TTL exposure is good. Checking the power output at 35mm it came out to about GN110 which is as expected lower than the rated GN125 but better than my Metz 60CT4 which is rated at GN197 but actually is only GN160. So overall I do like it.
    Andrew Garrard likes this.

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