As photographers we know we want to use the lowest ISO possible to capture our imagesand keep the noise level to a minimum. This is even more so when we are shooting HDR’s because the noise can get compounded when during tone mapping some tones are boosted and with that the noise
In most instances we want to keep the ISO at our camera’s minimum, 100 or 200 as the case may be. Just one note, don’t use any of the ‘Extended” ISO such as ISO 50 available on some Canon Cameras or ISO 100 on some Nikons. Because of the way these “Interpolated” ISO work, the images while lower in sensitivity, really don’t give lower noise and in fact give lower Dynamic Range per image.
So since we use tripods for good HDR’s or at least we should. Normally we don’t have a problem using low ISO and longer shutter speeds because we, as a practice, don’t have moving objects in our HDRs, although, I did show you how to do that in this blog post. But sometimes we may not have obvious moving objects because to our eyes they aren’t.
Some of those moving objects may be Clouds (especially low clouds on windy days or close to sunset). The Moon ( It moves about 15 degrees per hour). Boats at harbor ( even with soft swells they move) And I’m sure we could come up with a few more.
So in cases like this we may want to boost our ISO for two reasons, WE want our longest exposure of our series to have a fast enough shutter speed to stop the motion of anything so you don’t get blurring of a single frame. And we also want to be able to shoot the entire series of shots without any object moving in the total time it takes to shoot ( where you most likely would see movement of clouds or the moon. )