Add aperture blades to old manual lenses

petrochemist wrote:

ProfHankD wrote:

petrochemist wrote:

I've used paper discs up against the front element (with both circular & shaped apertures) and have tried an iris in front of the lens. In a few cases I've ended up with noticeable vignetting (usually when the lens has a long built in hood) but many have been OK.

Vignetting happens if your opening is too large. I know that's counter-intuitive, but it's how optics work. You're then vignetting rays from the lens aperture, which is placed differently, and that explains why the front element is bigger than focal_length/f_number.

I know that's one of the causes but wasn't the case here. The lens I was using for my last experiments was a 50mm/1.2

None of the apertures I used were even remotely close to the limiting size for this lens, typically being around 1/2" in their biggest dimension. For ease of use I was just applying the masks at the end of it's hood about 1.5" from the front element.

Ah, wrong place. That can cause vignetting too.  Most lenses have at least three valid places for a stop -- near the middle, in front, and behind -- but positioning matters a lot and it's not uncommon that any of those spots is mechanically awkward/impossible (e.g., intersecting an element).

However, be warned that most f/1.2 lenses actually vignette a lot. It usually takes more elements to make such a fast lens, and that tends toward a thicker (longer) optic, which tends to increase vignetting. Even some of my f/1.8 lenses vignette less wide open than some f/1.2 lenses stopped down to f/1.8. Try measuring the OOF PSF to see how bad the vignetting is: photograph a point light source (e.g., white LED) placed far away in a dark room with focus set close. When the spot is centered, it should be a bright disc (outside shape matching the aperture); when the spot is near a corner, you'll see vignetting as re-shaping the disc. For example, here's a composite of 5 OOF PSF varying only placement of the OOF PSF within the frame:

You want an aperture that is small enough to not be clipped in the corners -- still have the same shape. For an f/1.2 lens, that could be anywhere from f/1.2-f/8.

A slot in the hood to allow easy inserting of masks & waterhouse stops is something I'm considering...

Again, I'd vote against cutting anything. Better to make your own hood with a filter slot or even with an iris (hard to 3D print an iris, but you can buy them).