adjust focus of Sears fd zoom 28-70m f/3.5-4.5 lens

fstop · Joined: 11 Apr 2010 Posts: 29

As I described in my earlier post today, I am cleaning and trying to fix a Sears 28-70mm fd lens, f:3.5-4.5.

The haze is mostly gone (but I still cannot wipe it away 100%). The lens has two elements, one at the front which can be removed by unscrewing the lock screw under the rubber grip and then turning the front part of the lens counter clockwise. The rear element group can be unscrewed after removing the mount of the lens (4 cross head screws on the mount side).

I have put it all back, but the focus is out of whack. At 70mm zoom, I adjusted the focus by adjusting the position of the front element (by starting its thread at the right place on the helicoid).

However, the infinity focus at the wider end , 28mm, is not correct. It is as if I should turn the lens focus ring more, beyond its current limit, to get the infinity into focus.

Can somebody give hints on how to fix this problem?

Thanks.

SkedAddled · Expire: 2021-08-12

Post some clear pictures of the lens in question, so that we can all see
what you're working with, and explain what you've already tried.

I've successfully adjusted infinity on two Tokina-made Soligor lenses
I own, but there's no way for us to know what you have without some
clear pictures of what you have in your possession.

fstop · Joined: 11 Apr 2010 Posts: 29

fstop · Joined: 11 Apr 2010 Posts: 29

I have given this another shot. This time, I took out the front element's helicoid and changed position of the focus ring three times, each time rotating by 45 degrees and putting back the helicoid to get infinity in to focus at max zoom (70mm). No luck.

Furthermore, it appears that the focus ring can move go onto the body in only one orientation while allowing the front helicoid to move through its full range in the slot in the focus ring.

Eventually I am now at a position where the front helicoid moves all the way as I change the zoom. At the maximum zoom, infinity is in focus. At minimum (28mm), it is not. To get infinity into focus at the wider end, I would need to turn the focus ring beyond its current limit.

So, does this mean I need to so do something with the rear element?

Note that in this camera there are only two elements. Front one is in the front helicoid. The rear one can be unscrewed off by removing the mount. This one has five pieces of glass in it whose diagram I have posted earlier, but it is here again for convenience:

All these parts to into a threaded tube on the left. So the left most part is the one near the film plane and the right most part is the one toward the front of the lens.

Thanks.

fstop · Joined: 11 Apr 2010 Posts: 29

Can somebody with experience shed on this little question I have in mind? The lens in question has two groups of glass, as I mentioned in my earlier posts.

The one is front is cemented together (I think) and I haven't disassembled it.

The one on the rear is in a black metal barrel. I can unscrew it out as a unit from the rear, and I can disassemble it by unscrewing the little collar on the front side of this unit. Then all lens elements and spacers come out.

Now, this problem of distance being all off in focusing (at longer zoom, if I focus at infinity and set the distance scale accordingly, all other focus distances are off), could it be that I might have disturbed the order of the spacers or elements? Though I did take care not to do so, but can this be ruled out?

As an example, I remember that the elements were not easy to get out of the little barrel and I had shake the last two out. In the process, the second last element came out reversed but I didn't realized that at that time. When I reassembled the lens, the image was all out of focus and clearly something was all wrong. I then sat and looked at the diagram I had made as I took out the elements and pondered on the possibilities and eventually concluded that the either the last or the second last element was reversed. Flipping the second last element solved the problem and the image was then as clear as it was supposed to be. Now, as long as the image is in focus, it looks perfect, only the focus distance is way off.

Now, could I have done something else wrong that would show a clear image, when in focus, but make the focus distance all wrong? Or, another way to put this is, if the focus distance is wrong but the image is correct, does that indicated that only the placement of two lens groups needs adjustment and that the order of glass elements is correct?

Thanks.