What factor determines the largest usable sensor size with a certain lens?

Question

Reading this excellent answer about field of view, focal length and sensor size a new question came to mind.

From the drawn diagrams it is clearly visible that for the same focal length an increase in sensor size means an increase in field of view. If, following the diagram, an (imaginary) sensor of 100mm wide would be used with a 50mm lens the field of view would be 90 deg.

However, I have the feeling that something will be limiting the actual field of view. What determines the maximum sensor size a lens can project onto and still increase field of view, i.e. without showing a black border? Is it the size of lens element closest to the sensor? The size of the frontal element? Some other factor?

Answer 1

It depends on the lens design, a wide angle retrofocus lens is mainly limited by the front element size, the lens barrel places a hard limit on the range of angles that can see the entrance pupil. As an example of this mounting a filter on an ultrwide can reduce the size of the image circle. This is why separate designs exist for APS-c DSLRs, but mostly for wide lenses, as the front element can be made smaller, saving weight.

With symmetric designs like most 50mm lenses the determining factor is sharpness - it's easy to make the image circle bigger but corner sharpness drops off very rapidly limiting the sensor you can practically use. Most 50mm ish lenses designed for APS-C mirrorless cameras would cover full frame but be very soft in the corners. The front element of a telephoto lens only has to be big enough to see the entrance pupil from a narrow range of angles so the image circle could be made larger but with a decrease in corner sharpness.

Finally sometimes the image circle is intentionally limited by mechanical vignetting in the rear of the lens. There's no point in casting a larger circle than you plan to use as the wasted light can reflect off surfaces inside the camera and cause flare. This is onl y really an issue with wide angle lenses as you could compose a shot with the sun out of frame (according to the viewfinder) and still have it shining through the lens.