Rear Filter Mount for 55mm APO Grandagon

[JPlomley]

I would like to send my 55mm APO Grandagon to SK Grimes to have a filter thread placed on the rear cell. It will probably work out to 58mm. Am I better to mount a 58mm filter directly to this thread, or am I better off having them mount a thread that steps up to 77mm? Is there really any appreciable vignetting on the rear cell?

[Ed Richards]

Consider a gelatin filter holder, they do not screw up focusing, which a glass filter does when it is behind the lens.

[JPlomley]

Ed, I would focus with the rear filter in place. They will strictly be color conversion filters so only 1/3 stop loss in light.

[Bob Salomon]

Vignetting may be a problem, image deterioration is a problem if the filter has any dust, fingerprints, scratches, etc. a focus shift of 1/3rd the thickness of the filter always occurs when a filter is used behind a lens that was not designed to have a filter there. A filter belongs only on the front of an Apo Grandagon to get the quality inherent in the lens all the time.

[JPlomley]

But if I focus with the rear filter in place, would this not obviate the need to compensate for the focus shift?

[Helen Bach]

The focus shift is not uniform - it increases as you get away from the axis:

http://gallery.photo.net/photo/5787026-md.jpg

The scale on the left is the distance from the optical axis divided by the distance to the original plane of focus. This applies for a point on the optical axis, but it can be interpreted for other points.

A quick (and careless) estimation for the focus shift from the axis to the corner for a 55 mm lens with no movements, with a 2 mm thick filter suggests that there would be a difference of about 0.4 mm between the plane of focus at the centre and at the edge of the frame. That might not matter.

Best,
Helen

[Bob Salomon]

But if I focus with the rear filter in place, would this not obviate the need to compensate for the focus shift?

It would compensate for the focus shift as long as you also check the focus at shooting aperture but you would still suffer deteriorating image quality based on the filter being there in the first place.

[Ed Richards]

Hence why folks who do this use gelatin.

[Helen Bach]

Or polyester. Like shirts. And pants.