Effects of f/stop when using central ND filters?

[Greg]

Finally acquired a central ND filter for my 65mm f/4 Nikkor-SW. So yesterday to test it out, shot one sheet of 8x10 film with it on the lens at f/5.6, and another with it on the lens at f/45. Used 8x10 film cause I wanted to see the coverage of the lens. When comparing the prints made with the 2 negatives, I swear that there is a difference in the overall effect of the central ND filter. Are central ND filter made and optimized to be used at certain apertures? Or am I seeing something that is not really there?

[Bob Salomon]

They have to be used at least 2 stops down from wide angle + the filter factor, or they won’t work.

At 32, 45, etc you will be in diffraction and lose center sharpness. Your lens should be optimized for 16 or 22.

[Dan Fromm]

Greg, I've explained this many times. Our beloved wide angle lenses suffer from mechanical vignetting. Central filters can't fix this, they fix (somewhat) optical vignetting (cos^4). Stopping down greatly reduces mechanical vignetting. This is why w/a lenses gain coverage when stopped down.

The manufacturers' rule of thumb is that mechanical vignetting is gone two stops down from wide open. Sometimes, but for some of my lenses three stops is better. But it really depends on how far off axis you want image illuminated as well as possible.

[John Layton]

Greg...could you be more specific about the affects you are seeing?

[Dan Fromm]

John, pardon me for jumping in. The effect Greg saw is a larger circle of acceptable illumination at f/45 than at f/5.6. This is due to mechanical vignetting, which center filters can't reduce.

He may even have seen a sharp cut off, as I see with my little 38/4.5 Biogon. It illuminates an 84 mm circle, outside that is, well, the outer darkness. I think this is by design, to prevent lens abuse.

[Greg]

John, pardon me for jumping in. The effect Greg saw is a larger circle of acceptable illumination at f/45 than at f/5.6. This is due to mechanical vignetting, which center filters can't reduce.

Dan, that's exactly what I thought I saw.