In the footsteps of William Fox Talbot

[mickeyobe]

Dave,

You are correct about moveable rear elements. Now, it seems there is all kinds of element travel back and forth within zoom lenses.

I think that the test that I mentioned earlier, that is focus an image of the sun on a piece of paper will show the point on the lens from which to measure for stated focal length.

Focusing that image on a ground glass at the film plane of the camera might allow one to mark the lens at infinity.

Mickey

[daveh]

Mickey, you're right of course - unless the lens won't focus on infinity. Some cameras used to have the film plane marked: have there ever been any lenses that carried a "centre" mark?

[colray]

I have just done some more tests and it looks like there could be a problem with one elements because on my latest using full 5x7 coverage, one side is pin sharp and it fall off over the rest of the image. I will post a picture soon

[nikonbob]

That's a bit of progress. If the lens was parallel to the focal plane then the lens could have been improperly assembled at some point, factory or otherwise. You may have to disassemble and reassemble the lens making sure that the individual elements are centered, sitting properly in the place.

Bob