<p>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.</p>
<p>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!</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.)</p>
<p>Again using the SB400, I set it to -2 stops compensation, and the camera selected 1/60th, ISO 110. The picture was underexposed.</p>
<p>As you can see, the flash compensation setting, when used in tandem with auto ISO, reduces the OVERALL exposure, not the fill-in exposure.</p>
<p>Am I doing something wrong?</p>
<p>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.</p>
<p>Any advice would be most welcome.</p>
<p>Jon</p>