Dealing with focus shift on a rangefinder

[cptrios]

I've got a family heirloom Contax 50/1.5 Sonnar (along with a 35/2.8 Biogon and a 135/4 Sonnar) that I've always wanted to be able to put into use. The IIa it belongs to has a jammed shutter, and while I've managed some decent shots with it on a Kiev 4a, I really don't enjoy using the camera (which doesn't bode well for an eventually-repaired IIa, I suppose).

I've finally wound up with an M-mount body, and one of the first things I did was order a cheap Fotofox RF-coupled LTM adapter. It's hardly a precision instrument (I don't care about the lens enough to drop $300 on an Amedeo adapter, impressive though they may be), but it's functional. After some preliminary tweaking of the lens itself, thanks to its handy internal mount distance adjustment, I appear to have somewhat accidentally landed a proper calibration with my RF at f/4 and above. It's possibly still a tiny bit front-focused, but good enough for the moment. However, thanks to the legendary focus shift of this particular lens, the focal plane moves progressively closer to the camera as I open up the aperture. Dramatically enough that I've been a bit surprised despite knowing it would happen.

So...how do you all tend to deal with this? I figure the way to go is just to develop a feel for an extra turn toward infinity on wider f-stops, and I don't suppose there's really much else that can be done. But I'm curious to know if anyone's got any tricks up their sleeve! Also...would anyone suggest I calibrate the lens to focus correctly wide open and then hope the extra DoF will take care of things at narrower apertures as long as I'm not shooting too close up?

 

[Patrick Robert James]

Usually it is better to set it for proper focus at 1.5. I rarely use apertures between say 5.6 and 1.5. Outside- 5.6 and up. Inside- 1.5. At 5.6 and above, you'll be fine even if the lens is calibrated for 1.5. It really depends on how you intend on using the lens.

I'd screw around with the adapter before I messed with the lens though. If you get your Contax fixed then you will have to redo the lens if you mess with it.

 

[warden]

I'd adjust it to focus correctly at a wider aperture than the f4 you currently have, but an aperture that you actually plan to use. I have a ZM Sonnar f1.5 that is calibrated to focus correctly at f2.8 and that's fine as I rarely use f1.5 anyway, but if I did I would have the lens adjusted for f1.5.

In my case on the rare occasion that I'm using the lens wide open I'll focus first and then lean in slightly and that does the trick. It'll take practice to learn your lens and how to improve focus within the limitations of the lens.

 

[cptrios]

Hmm maybe I'll try to calibrate it for somewhere between f/2 and f/2.8, since I probably won't use it wide open that often either. And leaning in slightly is a stupidly obvious thing that I never thought of!

The good thing about this lens is that it's very easy to adjust, and I have it marked in case I need to return it to its original setting. The adapter is, unfortunately, not adjustable at all...but at $60 for a rangefinder-coupled CRF adapter, beggars can't be choosers.

 

[brbo]

I had a ZM Sonnar 1,5/50 with a wobble and had it fixed by Zeiss in Germany. At the same time I asked them to calibrate it for correct focus at f1.5.

I now use old Contax mount Sonnar 1,5/50 with Amedeo adapter since it allows me to focus down to 0.7m (vs. 0.9 for ZM). This Sonnar is optimized for f2.8-4.0 and I much prefer it that way. Yes, you need to compensate for focus shift when shooting at wider apertures, but if you optimize the lens for wide open aperture you will also need to compensate when shooting smaller apertures. Dof does not cover the shift. Or maybe it’s just me, I want my pictures reasonably sharp when shooting at smaller apertures and can accept slightly missed focus when pictures are slightly soft to begin with because of a wide aperture.

But since you are able to modify the lens yourself, I’m sure you will quickly find your preferred setting.

 

[Sirius Glass]

Sometimes if the depth of field I want is at, for example f/8 I will expose at f/9 to f/11 to make sure that I capture the focus range I want.

 

[cptrios]

Or maybe it’s just me, I want my pictures reasonably sharp when shooting at smaller apertures and can accept slightly missed focus when pictures are slightly soft to begin with because of a wide aperture.

This is my thought, too. Wide open, this lens certainly has an effect to it that may or may not be desirable depending on the circumstances, but nailing focus doesn't really enter into the equation. The only issue is when something in front of the subject is clearly sharper than the subject itself. A bit of a lean would probably solve that issue, but I think I might give the whatever-you-call-it calibration ring an extra half-turn anyway.

 

[brbo]

How is that going to help with focus shift?

 

[cptrios]

I'd love to have the IIa repaired, but the frequency with which I'd use it is almost definitely not worth the expense. Maybe someday!