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Sb-800 forcing the camera to work at the highest ISO?

Discussion in 'General flash photography, lighting, and technique' started by Rui Lopes, Aug 27, 2008.

  1. I am not by all means an expert in flash photography.
    Today, taking some interior shots I noticed something weird (to me) and would like to ask wether it is normal or if I need to review my settings:
    Using the D300 in manual I adjust the correct exposure according to the meter, getting f/5.6, 1/30s, ISO200. Then, I turn the Sb-800 on, TTL mode, bounced to the ceiling to fill shadows. Immediately, both the flash and the camera adjust the ISO to 1600 (the maximum sensibility the camera is allowed to go)
    Then, I noticed that if set the camera to a different maximum ISO, the Sb-800 also goes to that limit, forcing the camera to adjust the ISO nr. accordingly.
    Is it normal or do I need to adjust my settings?
    Thanks for the help.
    Rui

    ________________________________________________
    D300, D200, MB-D10 and MB-200
    Primes: 10,5 2.8 fisheye, 50D 1.4, Tokina 100 AT-X
    Zoom : 12-24 (Nikkor),17-55 2.8, 18-200VR and 70-200VR
    SB-800, TC Kenko Pro300 1.4, Manfrotto 190MF4+804RC Head

    www.pbase.com/ruilopes
     
  2. Johnny Yuma

    Johnny Yuma

    372
    Jun 27, 2007
    SE MI
    Turn off Auto ISO.
     
  3. Thanks Drew.
    So there's no way to avoid ISO raising while Auto ISO is on?:confused: 
    Why is that? Is it logical?
    Thanks again
    Rui

    ________________________________________________
    D300, D200, MB-D10 and MB-200
    Primes: 10,5 2.8 fisheye, 50D 1.4, Tokina 100 AT-X
    Zoom : 12-24 (Nikkor),17-55 2.8, 18-200VR and 70-200VR
    SB-800, TC Kenko Pro300 1.4, Manfrotto 190MF4+804RC Head

    www.pbase.com/ruilopes
     
  4. WayneF

    WayneF

    Apr 3, 2006
    Texas
    I would not swear this was the best answer for Auto ISO, but in general it is true.

    Regardless how dim it may be, the camera meter always only reads the continuous ambient light in the room, and sets an exposure accordingly, say for example 1/60 second at f/4. Whatever the ambient continuous light actually meters (or whatever you set in M Manual mode) - this is the camera exposure settings. The flash has not fired, so its light is not yet present, and the flash is not metered or taken into account. Continuous ambient is all the camera meter can read. This meter reading surely affects Auto ISO too, but the resulting exposure value is only about the ambient light. If the ambient is so low to need high ISO, then Auto ISO will provide it. Flash is not a factor for that exposure setting, which is only about the continuous ambient exposure. The camera will use this same exposure setting regardless if the flash is present or not (see 1/60 second exception below).

    Say the aperture ends up at f/4. Then the TTL flash automation is metered through that f/4 aperture, and at that resulting ISO value, and the automatic TTL flash mode will simply provide the appropriate flash power to match that resulting f/4 aperture which is specified for it to use. If it were some other aperture and ISO value, it would try to match that too, if it has sufficient power to be able to do it. Shutter speed does not affect the exposure from the flash, the flash is much faster than the shutter. Shutter speed only affects the ambient continuous light, and a slower shutter possibly could cause motion blurring from ambient...

    The only small way the flash might influence this exposure setting is that the camera has a menu (D300 menu e2 - Flash Shutter Speed) to not allow any slower shutter speed than 1/60 second (the default) if the flash is present and Ready. Indoors at night, simply watch the exposure shutter number as you open and shut the internal flash door. If the shutter meters slower, it changes to 1/60 when the flash is present (however Slow Sync mode ignores the e2 menu to allow slow shutters which are metered). So 1/60 is as slow as we normally can see if the flash is turned on. But we will see a faster shutter speed if the ambient is brighter to meter it, for example outdoors in the sun.

    The most fundamental fact is that every flash picture is sort of a double exposure. Basically, this is the way the camera tries to match the ambient continuous exposure with the flash instantaneous exposure, to make both exposures be right, if possible. Ambient may be low and insignificant, or aperture may be tiny requiring more flash power than is available, but it tries to get both right if within the limits of what can be done. If you want to shoot at specific values, some ISO and some aperture, then you must set those values, and then the TTL flash automation will react to them.
    Just how it works.
     
    Last edited by a moderator: Aug 27, 2008
  5. Thanks for your detailed explanation, Wayne. I think that I perfectly understood it.
    However, I still don't understand why turning on the the flash raises up ISO to the limit of 1600 since the (auto) ISO value calculated to a given exposure was previously 200...
    According to your explanation : quote: "Say the aperture ends up at f/4. Then the TTL flash automation is metered through that f/4 aperture, and at that resulting ISO value, and the automatic TTL flash mode will simply provide the appropriate flash power to match that resulting f/4 aperture which is specified for it to use. " unquote, I think that the flash should only go matching the settings previously calculated, respecting it (including the previous ISO nr.) and not doing any adjustments. Furthermore, I also noticed that regardless any adjustments done either to the aperture or the sspeed after turning flash on, the use of it always raises ISO to the allowed limit .
    Please tell me if I am missing something or misunderstanding the way this works.
    Thanks again
    Rui

    ________________________________________________
    D300, D200, MB-D10 and MB-200
    Primes: 10,5 2.8 fisheye, 50D 1.4, Tokina 100 AT-X
    Zoom : 12-24 (Nikkor),17-55 2.8, 18-200VR and 70-200VR
    SB-800, TC Kenko Pro300 1.4, Manfrotto 190MF4+804RC Head

    www.pbase.com/ruilopes
     
  6. WayneF

    WayneF

    Apr 3, 2006
    Texas
    If so, I dont know, and it must be weak hole in my theory.

    But forgive me, are you really certain that you actually see that happen?

    I never use Auto ISO (because knowing ISO seems much more important that letting it run free - seems better to deal with it other ways),
    but when I try it, I see this:

    D300, Auto ISO, ISO set to 200.
    a room indoors, not bright, P Mode meters f/2.8 at 1/80 on wall at 6 feet.
    But am not using P mode, using M mode, with f/16 and sync speed 1/250.
    Viewfinder still shows ISO 200.
    Using SB-800 flash (TTL mode, bounced), the EXIF in picture shows ISO 1600.
    Turning off flash, the EXIF in picture shows ISO 1600. Black picture of course.
    Viewfinder still shows ISO 200.
    All EXIF shows f/16 and 1/250 and ISO 1600.
    Cannot see ISO 200 in the EXIF values, yet the viewfinder shows ISO 200.
    Could be misleading?
     
  7. Wayne, I am sure that it happens. A test done 5 minutes ago:
    D300, Auto ISO set to 1600 maximum.
    Also a room indoors, ceiling lamp lightened. Using M mode, f/2.8, 1/13, ISO 400, no flash, I got an average picture.
    Then, turning the SB-800 on, I immediately got the ISO shown both in camera and flash display "1600". If I proceed and take the picture with those adjustments I'll get an overexposed picture. Exif confirms all the chosen settings and the ISO 1600.
    However, turning auto ISO off and adjusting it to 400 (manually) I got a well-exposed image even maintaining the initial settings (f/2.8, 1/13). Exif confirms all the chosen settings and ISO 400.
    Something I also noticed was that altough the min. sspeed set was 1/15, the flash fired even at 1/8 or slower.
    What do you think?
    Rui
     
  8. auto-ISO should always be turned off when shooting with flash

    afterall... almost all of us shoot "MANUAL" when using flash...
    and auto-ISO makes that NEARLY impossible

    when shooting with flash, it is VERY critical to control the exposure
    don't stop with setting the aperture and shutter speed
    you MUST "lock" the ISO
     
  9. Thanks for the input, Greg. Yes, it's something that I'm learning from my self-experience and also from your words... Now I see the reason why sometimes I got overexposed pics using fill flash under daylight...
    I just wonder why Nikon engineers didn't yet find a safe way allowing the use of auto ISO together with flash.
    Rui

    ________________________________________________
    D300, D200, MB-D10 and MB-200
    Primes: 10,5 2.8 fisheye, 50D 1.4, Tokina 100 AT-X
    Zoom : 12-24 (Nikkor),17-55 2.8, 18-200VR and 70-200VR
    SB-800, TC Kenko Pro300 1.4, Manfrotto 190MF4+804RC Head

    www.pbase.com/ruilopes
     
  10. WayneF

    WayneF

    Apr 3, 2006
    Texas
    Thanks, that helps me. I looked again, and I am now getting different results now too, more similar to yours. But it seems pretty strange, not an effect that I want happening.

    Auto ISO set to 400. In M mode now, no flash, viewfinder and EXIF shows ISO 1250 at 1/80 f/2.8 setting, which pretty much matches P mode. I have no clue why I was always seeing 200 before in viewfinder. I cannot duplicate that now. The error was mine, but I cannot identify what error must have caused it.

    Then turning the flash on drops from 1250 to ISO 400, viewfinder and in EXIF, if bounced. Some smartness is saying we dont need so much ISO if using flash. M mode is acting more like P mode, regarding Auto ISO. And EXIF shows it drop to 200 if direct flash (but shows 400 in viewfinder). Direct flash would be bright on a close wall at f/2.8 and ISO 400, so it reduced it anyway.

    Seems to be several stories here, but I must stand corrected. Obviously a processor is recomputing need for Auto ISO after all, even in Manual mode, obviously depending on the flash. It must know and compute the limits of the flash, and there is more to it than my notion.


    Not sure which minimum you mean. e2 has a list of what it applies to, and it does not affect some things, like M mode, which always uses any setting you set manually, which is what Manual is. Obviously AutoISO is still running however.

    Minimum speed in that ISO box is not the same as e2 menu. In the ISO box, it is not a setting that will be used for shutter, but appears to be a threshhold regarding how ISO may be affected, depending on shutter.

    The shutter speed itself has no effect on the exposure from the flash.

    Sorry, I am very much less sure now about Auto ISO, and I think I will go back to not using it. I sort of like to know what is going to happen. :) 
     
  11. i fully agree with you, rui
    of course, i only learned what i spoke here though my own trial and error

    i wish that the next firmware upgrade would address this issue, if possible

    NIKON.... are you listening? :smile:
     
  12. Correction! I was wrong!
    After some investigation, I found out a safe way to go:
    Auto ISO: set to maximum 1600 (with the D300 I do not dare a higher value)
    In "MY MENU", at ISO sensitivity settings: first box (ISO sensitivity - adjust to 400); second box - ISO sensitivity auto control: ON; third box - Maximum sensitivity 1600 (my choice); fourth box - Minimum shutter speed 1/15 (again, ny choice).

    This way, I still can benefit from the use of auto ISO till 1600 (no flash) and when I need to turn the Sb-800 on, the ISO auto adjusts to the limit of 400, regardless of the previously metered aperture and speed avoiding the overexposure caused by ISO 1600.
    Hope this helps.
    Rui


    ________________________________________________
    D300, D200, MB-D10 and MB-200
    Primes: 10,5 2.8 fisheye, 50D 1.4, Tokina 100 AT-X
    Zoom : 12-24 (Nikkor),17-55 2.8, 18-200VR and 70-200VR
    SB-800, TC Kenko Pro300 1.4, Manfrotto 190MF4+804RC Head

    www.pbase.com/ruilopes
     
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