Does lens speed still increase meaningfully when the physical aperture is larger than the sensor size?

Question

This question came to me this morning, thinking about 50mm lenses. I have the Pentax 50mm f/1.7 (manual) and it's a very nice lens, but would be considered (relatively) slow on 35mm film, and many recommendations would be for a 50mm f/1.4.

However, doing the math, the diameter of the f/1.7 aperture wide open is 29.4mm, which is greater than the diagonal size of an APS-C sensor — effectively meaning that no part of the sensor is "hidden" by the aperture. So, the questions is, does this actually have any meaning, or is the f/1.4 lens still going to be distinctly faster for APS-C?

Answer

f1.4 will always be 2/3rds stops faster than f1.8.

The diameter has nothing to do with whether or not part of the sensor is hidden. That is a separate measurement referred to as vignetting, and not the image circle's light level. The image circle's light level/brightness is directly affected by the aperture of the lens design.

FF lens simply means the image circle is designed to cover a full frame sensor (which can be film). Using it on a APS-C will be using the inner part of the image circlce.

An APS-C lens of the same focal length and speed could have been created at a smaller size the image circle does not need to be as large, but the lens would need to be redesigned.

Also, note that the Pentax 50mm f1.7 (if this is what you have) is generally regarded as sharper and/or more contrasty than the Pentax 50mm f1.4 at common apertures up until f2.8 or so.

Pentax 50mm f1.4 advantages include one third f-stop faster, rounder aperture blades for rounder highlight bokeh only when stopped down. It may or may not have "better" and smoother bokeh as that is not simply a function of aperture blades and I have not seen any other comparisons.

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