Exposure and ISO

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Today’s Question: In an earlier Q&A [from January 23rd] you refer to underexposing by raising the ISO.

There is some confusion here somewhere. If one keeps all other parameters constant and raises the ISO isn’t this equivalent to using a faster film and therefore one would, relative to the earlier ISO, be over exposing? What am I missing?

Tim’s Answer: I would be more than happy to clarify.

When I was referring to the notion of raising the ISO, resulting in an under-exposed image, I wasn’t trying to suggest that raising the ISO setting actually caused the image to be darkened. Rather, I was referring to the impact of ISO on overall exposure and image quality.

I think it will be helpful to talk about specific exposure settings in order to help clarify. So, let’s assume a “sunny 16” exposure with an aperture of f/16, a shutter speed of 1/125th of a second, and an ISO setting of 100.

If I raise the ISO setting by two stops (to 400) and adjust other settings to compensate, I might end up with an aperture still set to f/16 but a shutter speed of 1/500th of a second. So, you could reasonably suggest that the faster shutter speed (the shorter exposure duration) would cause the image to be darkened, but that the higher ISO setting caused the image to be brightened to the same degree, resulting in an exposure that is exactly the same as would be achieved with the prior settings.

The key thing that I think photographers need to understand is how each setting affects the final image, and that is why I refer to the “underexposure” issue when you raise the ISO setting. More on that in just a moment.

The aperture primarily affects, of course, the depth of field in the scene. The shutter speed has primary control over the degree to which motion is frozen (or not) in the photo. And the ISO setting determines (in many respects) the amount of noise in the photo.

When you raise the ISO setting you are making a change that will have a brightening effect on the photo, all other things being equal. But you aren’t doing so by “magically” increasing the sensitivity of the image sensor.

So, to my point about raising the ISO resulting in a reduced exposure, let’s take a look at the exposure settings referenced above. At an ISO setting of 100 I referenced an aperture of f/16 and a shutter speed of 1/125th of a second. Raising the ISO to 400 resulted in a change to a shutter speed of 1/500th of a second at f/16.

But going from a shutter speed of 1/125th of a second to a shutter speed of 1/500th of a second represents two stops of exposure reduction. We’ve caused two stops less light to actually reach our image sensor. The image sensor can’t magically collect more light, or be more sensitive to the light. The result is that we’re actually taking a photo that is two-stops under-exposed, and the camera is then applying amplification to the signal information that was recorded to create the effect of a brighter exposure. In the process, noise will result.

To be fair, today’s digital cameras do a remarkable job of applying amplification through higher ISO settings without creating excessive noise. And there are a variety of ways you can mitigate the noise after the fact. But if you think of a higher ISO setting as representing an underexposed image that needs to later be brightened considerably, I think (and hope) it will provide a useful way for you to evaluate the ISO setting relative to other exposure settings. In other words, I hope this information helps encourage you to avoid raising the ISO setting on your camera unless it is necessary for your other exposure goals, in order to minimize the amount of noise in the final image.