Mjd-djm
Established
I picked up one of these lenses a few weeks ago. After shooting a few rolls on my IIF and M6 TTL I can see it has a front focus issue despite the rangefinder being aligned at infinity. I realise these lenses were often optimised for f2.8 but this copy is only hitting focus at f4 and even then only just. At f1.5 sharpest focus is around 15-20cm in front. I have removed the optical unit and cleaned it and I can see a single shim. It has outer diameter of 32mm, inner diameter of 30mm and height of 1mm. Obviously I need a shim height of less than 1mm. If 1mm is too much then how much do I need to reduce by? Is 0.5mm to much? Should I work in 0.1mm increments? Also should I sand /machine down the current shim? Or can I make my own? Any help I greatly appreciated. I know I can get an expert to do it but I’d like to try myself. Cheers!
Ko.Fe.
Lenses 35/21 Gears 46/20
If it is front focus you are adding shims, not removing.
I cut extra shims from paper, using original shim as template.
I cut extra shims from paper, using original shim as template.
Mjd-djm
Established
Ok, that sounds much easier. I read that with Russian lenses which typically back focus unless shimmed require an added shim to focus correctly. If this is true then I assumed to correct front focus you would need to remove shims.
Mackinaw
Think Different
We had a guy on this forum years back who used copper tape to shim a lens.
Jim B.
Jim B.
Mjd-djm
Established
Before I do this I just want to double check... to correct front focus I add shims? This is the opposite of other posts I’ve read. Normally when focusing a lens the optics need to move away from the camera to focus closer. If I add a shim surely the focal point will move further forward and worsen the problem?
xayraa33
rangefinder user and fancier
This lens has some natural focus shift, so be aware of that.
In the past I had good luck shimming lenses with Teflon tape, just use it trial and error style till you find the sweet spot.
In the past I had good luck shimming lenses with Teflon tape, just use it trial and error style till you find the sweet spot.
Ko.Fe.
Lenses 35/21 Gears 46/20
Then you are taking focus test image on one meter of focus checking ruler under 45 degree, if in focus part is in front of the zero mark it is front focus. If lens is in focus behind of zero mark it is back focus.
Adding shim moves focus to longer distance, removing brings focus to closer distance.
Adding shim moves focus to longer distance, removing brings focus to closer distance.
Mjd-djm
Established
Just read this on another forum...suggesting I need to reduce the shim. Any other opinions on this?
“Front or back focusing can usually be adjusted by taking away or adding shims between the lens element collar and the lens barrel. Front focusing is a result of the lens elements being too far from the image plane. Therefore they need to be moved closer to the image plane by either taking away existing shims (if there are any) or machining away a small amount of the lens element collar. Backfocusing is a result of the lens elements being too close to the image plane. Therefore they need to be moved farther from the image plane by adding shims. These are pretty straight forward adjustments for competent technicians.”
“Front or back focusing can usually be adjusted by taking away or adding shims between the lens element collar and the lens barrel. Front focusing is a result of the lens elements being too far from the image plane. Therefore they need to be moved closer to the image plane by either taking away existing shims (if there are any) or machining away a small amount of the lens element collar. Backfocusing is a result of the lens elements being too close to the image plane. Therefore they need to be moved farther from the image plane by adding shims. These are pretty straight forward adjustments for competent technicians.”
TenEleven
Well-known
You need to remove shims. Look at what the lens does when you focus closer, it becomes longer. Thus the optical elements are further away from the film plane the closer you focus. If the focus is too close you're extended too far from the plane.
I'd make sure of a couple of things first, though:
- Make sure my camera has the correct flange distance
- Make sure the RF agrees on infinity
But as others have said Sonnar lenses have focus shift. It is however so mild on film (I am assuming you're using film) that it won't be noticeable. On digital it's probably the best idea to collimate the lens for good focus at f/2.8 when it has stabilized.
Okay with all that said I usually collimate my sonnar lenses so that they "touch" infinity by f/1.5 and hit by f/2.0. This means (again on film) that the focus lands where the RF says it will for all apertures, except wide open where you will have to nudge it a tiny bit up close. But even if I forget to do so, I don't usually notice it. Film is much more forgiving and these lenses were made for film, something that often gets omitted in the great focus-shift debate.
Also if you can manufacture shims I'd put the existing shim away and just make a new slimmer one. Perhaps use a digital camera to sanity check infinity focus against a known good lens. That way you can always fall-back to your old shim if something goes south.
Also of course you can only adjust infinity the infinity position, all the other focal distances are falling where they will since you can not adjust the "gain" of the lens, so if your lens doesn't have the appropriate focal length due to someone messing with it or sample variations you will likely only get it to agree with the RF at one distance and it will slowly drift apart elsewhere.
I'd make sure of a couple of things first, though:
- Make sure my camera has the correct flange distance
- Make sure the RF agrees on infinity
But as others have said Sonnar lenses have focus shift. It is however so mild on film (I am assuming you're using film) that it won't be noticeable. On digital it's probably the best idea to collimate the lens for good focus at f/2.8 when it has stabilized.
Okay with all that said I usually collimate my sonnar lenses so that they "touch" infinity by f/1.5 and hit by f/2.0. This means (again on film) that the focus lands where the RF says it will for all apertures, except wide open where you will have to nudge it a tiny bit up close. But even if I forget to do so, I don't usually notice it. Film is much more forgiving and these lenses were made for film, something that often gets omitted in the great focus-shift debate.
Also if you can manufacture shims I'd put the existing shim away and just make a new slimmer one. Perhaps use a digital camera to sanity check infinity focus against a known good lens. That way you can always fall-back to your old shim if something goes south.
Also of course you can only adjust infinity the infinity position, all the other focal distances are falling where they will since you can not adjust the "gain" of the lens, so if your lens doesn't have the appropriate focal length due to someone messing with it or sample variations you will likely only get it to agree with the RF at one distance and it will slowly drift apart elsewhere.