The Recent F/stop Controversy

Started 1 month ago | Discussions thread
Michael Fryd
Michael Fryd Forum Pro • Posts: 11,288
Re: Really?

Pixel Pooper wrote:

  • Michael Fryd wrote:

Actually, at the same angle of view the "equivalent aperture" is the one with the same diameter. In other words it really is the same aperture; no math needed.

You only need to do the math if you normalize everything to light per unit area (which is a great idea when shooting film).

Whether you are shooting film or digital makes no difference. The reason we use the f/stop is that it gives us the same exposure regardless of focal length or angle of view. If we used aperture diameter it would be more complicated because we would need to change the diameter every time we changed our focal length.

While shooting I am much more likely to change my field of view than my sensor size and if I do change my sensor size it is to take advantage of the differences between the two formats, not to take the same picture on both.

The fact that the same aperture diameter gives the same total light at the same field of view is good to know, but it's not as useful when taking pictures. Whatever format you use, the way to maximize total light is to maximize exposure and your total light is limited by the need to avoid overexposure.

Equivalence is a way of comparing formats, and it is a great framework to understand how things work, but it is not a replacement for f/stops and exposure.

This is the issue. You have built your workflow around "exposure" (light per unit area). While this makes sense for film it isn't necessary for digital.

When taking a photo we need to balance shutter speed (which affects motion blue), aperture (which affects depth of field) and total light (which affects image noise). When you have ample subject illumination, there will be a range of settings that give you the desired results.

The important situation is what happens when there isn't ample subject illumination? How do you balance the three elements?

If we start by picking one, then we have taken it out of consideration for balancing. If we set the camera to a fixed ISO (which selects a target exposure) and put the camera into shutter priority mode, then we are telling the camera to use aperture for balancing. This means that if there is more than ample illumination, we get more depth of field, and when there isn't enough then we don't get as much depth of field as we need.

If we move the camera to aperture priority, we run into the problem that not enough light give us motion blur.

By basing our workflow on exposure we tend to forget about our third choice. Set the aperture to give us the depth of field we need. Set the shutter speed to give us the motion stopping ability we want, and let the exposure fall where it may (Auto-ISO).

This third choice isn't a practical option with film, and our entire workflow and nomenclature is based on the need to hit the sweet spot on the response curve for film.


If you have ever shot in aperture priority mode, then you have let the camera pick the shutter speed. If you have ever shot in shutter priority mode, then you have let the camera pick the aperture.   What's so special about exposure?  Why do we have to start by picking an exposure?


A typical response is that the photographer needs to pick the exposure as he wants to control the noise in the image.   The fallacy of this is that the photographer should also want to control the depth of field and motion stopping ability.  All three are important.

Building a workflow around exposure (light per unit area) is a holdover from film days.  It was important then, but is an artifically imposed  limitation with digital.

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