Optimal aperture position?

Started 2 months ago | Discussions thread
Alan WF
Alan WF Senior Member • Posts: 1,990
Pupil Stops
1

The stops in photographic lenses are designed to act as pupil stops.

What does an ideal pupil stop do? Loosely, as it closes, it reduces the amount of light reaching the focal plane in a way that is even over the whole focal plane.

To see the importance of the italicized phrase, consider putting a stop right in front of the sensor. As you closed the stop, it would indeed decrease the light reaching the focal plane, but in a way that is very uneven over the focal plane. Technically, it would be acting as a field stop rather than as a pupil stop.

Experimentally, how can you tell if a stop is acting as an ideal or close-to-ideal pupil stop? Look at the lens from the back. Look at what happens as you open and close the stop from different positions in the focal plane. The change in the amount of light reaching the focal plane should be about the same over the whole focal plane. So, you might try placing the aperture at different positions in your system and selecting the one at which it most closely shows this behavior.

Now, the pupil stops in photographic lenses are rarely completely ideal. Often, they have vignetting, so that as you open up the lens you lose some light in the corners of the field compared to the field center. However, they typically aim to smoothly and continuously vary the amount of light reaching each point in the focal plane, even if the variation is not identical in each point.

You can place an aperture stop before the first element. This will often work quite well, but will tend to have more vignetting. This is one of the reasons actual lenses have their stops other than before the first element.

Regards,

Alan

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