Southswede wrote in post #17478624
Why would the flash cause the auto ISO to go above 25,000? Taking the flash off the camera all together and shooting with ambient light gave me settings in the area of 1/60, f5.6 and an auto ISO of 4,000. Why would adding the flash move the ISO up to more than 25,000?
The following is based on how my D700 works, but I believe there isn't any real difference with the newer Nikon cameras.
Is there any chance you have the ISO set above 25,000? If you do, the camera will display that number in the viewfinder when the flash is mounted, although when you take a shot the ISO required will be calculated after the preflash and be much, much lower ... probably 400 to 1000'ish. To calculate the right background exposure the contribution of the light from flash (used for subject exposure) needs to be subtracted.
Until the pre-flash is fired, the camera doesn't know exactly what ISO will be required to get the background/ambient exposure right so it "displays" the ISO the camera is set at (not the auto-ISO value). If you take a shot and look at the actual values that were used, the ISO is not the same as what the viewfinder displayed. This is the correct behaviour.
However, when you remove the flash, auto-ISO displays the ISO it's going to use because you no longer have to worry about the extra light added by the flash. This number should be slightly higher than the value ACTUALLY USED (not displayed) when you shot using flash.