When I set my camera iso to 'auto', the exif data reports 'auto'. How can I find out what iso the camera actually took my picture at?
Not sure about other brands, but for Canon cameras, if ISO is set to Auto the actual ISO used by the camera is not recorded. The ISO setting is only recorded if a manal ISO setting is being used.
My question is why do you need to know the ISO number?
If you can define what your trying to figure out we may be able to offer you a much simpler method
"My question is why do you need to know the ISO number? "
I don't know John's reason for wanting to know the ISO that the camera set itself to (it could be just natural curiosity), but if the picture showed some noise, you might specifically want to know whether the ISO was excessive and the cause of the noise. Knowing the ISO that caused the problem could help you decide what ISO to set the camera at for a future similar situation. I use Topaz's DeNoise 5, so when I get a camera that will let me set the ISO aggressively, I will do so with the expectation that I will be able to deal with the resulting noise in post processing with DeNoise.
(not associated with any product or vendor mentioned or linked)