The Recent F/stop Controversy

Started 1 month ago | Discussions thread
Pixel Pooper Veteran Member • Posts: 3,121
Re: The Recent F/stop Controversy

Michael Fryd wrote:

Well, we started with the question should we get rid of the f/stop. I think there is a good case to be made the we should deemphasize that terminology and emphasize aperture diameter instead.

If there is a good case for that, you haven't presented it yet.

Digital cameras have a lots of computational power and sophisticated displays. I think camera firmware should be helpful enough to display the aperture diameter.

If you want to know the aperture diameter for some reason just divide the focal length by the f number. It's not difficult, and it's usually not necessary either.

The creative advantage is that it reduces the amount of mental calculations one needs to do.

It reduces the amount of calculations needed to compare different format sizes, but it increases the amount of calculations needed to shoot with the camera you have in your hands. What do you do more of, compare formats or shoot pictures? Hopefully it is the latter.

Assume you move closer to your subject and change the focal length to maintain the same framing. If the aperture diameter remains the same, you get the same depth of field. If the f/stop remains the same, the depth of field changes.

You've got that backwards. When you change focal length and distance to keep the same framing, you get the same DOF at the same f/stop, not the same aperture diameter. The same f/stop also gives you the same exposure and the same total light.

If your camera displayed aperture diameter instead of f/stop you would need to divide your focal length by your aperture diameter, and then divide your new focal length by the result to calculate the aperture diameter needed at the new focal length.

If your camera also displayed angle of view instead of focal length you would have to resort to trigonometry to calculate the required aperture diameter at the new angle of view. Now instead of reducing the number of mental calculations, you have made the calculations too difficult to do mentally.

Benefits include:

  • The explanation of how things work is simpler. It leaves out implementation details that are not important to the result.

You can still explain how things work using aperture diameter, but you will end up having to explain exposure and focal length ratios either way.

  • It moves mental models away from exposure being paramount, and onto a more equal footing for shutter speed and aperture.

Exposure is paramount. If you want the best results from any format you need to maximise total light which means maximising exposure. If you do that you should not use the same aperture diameter and shutter speed on a different format because you would overexpose a smaller sensor or handicap a larger sensor. Instead you would use the same exposure on both formats, but possibly not the same f/stop.

  • it is independent of sensor size, which makes it easier to teach or discuss amongst people who may be using different sensor sizes.

It is independent of sensor size, but it is tied to angle of view. As soon as you use a different angle of view, the same aperture diameter no longer gives the same results. This leaves you with a fairly narrow set of circumstances where aperture diameter is useful. In the end you will have simplified equivalence calculations at the cost of complicating everything else.

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