How to master focus shift?

[Mikec]

Equipment can always be improved. Currently, I have a lens that has focus shift. Rather than trying to look for the perfect lens (which is silly), I want to learn from the RangeFinder masters. I want to know the lens so well that I can compensate for the focus shift sufficiently.

Questions that I have,
- Is focus shift usually aperture dependent?
- Can focus shift change from back focus to front focus (aperture the same) as focus distance change?
- Does focus shift amount change as focus distance change?
- What tips do you have for mastering the lens and its focus shift characteristics?

Thanks!

 

[scott kirkpatrick]

You probably should say just what lens you are trying to work with and how you know you have focus shift rather than a focus error (always off in the same direction). But I'll take a stab at summarizing what seems to be the common pattern and what can be done about it.

- Is focus shift usually aperture dependent?

If it is what frequently happens with early f/1.4 designs, yes. The light that passes through the outermost parts of the lens focuses at a different distance than the light through the inner parts of the lens. Stop it down, and the sharpest parts of the image are at a different distance. Complaints about some copies of the Leica 35/1.4 are commonly heard, and even greater shifts are seen in the recently introduced CV35/1.4. Such designs get sharper and more contrasty when you stop them down, but this sharp image, say from the part of the lens that you use at f/5.6 is drowned out by the 4-stops stronger soft image from the f/1.4 part of the lens.

- Can focus shift change from back focus to front focus (aperture the same) as focus distance change?

I think the direction of the shift is probably always the same but the visibility of the effect changes as the depth of field increases to mask it.

- Does focus shift amount change as focus distance change?

same as above

- What tips do you have for mastering the lens and its focus shift characteristics?

If you can't simply avoid such lenses, or you love the one you have (a CV 35/1.4???), then you have to learn when to trust the rangefinder -- is it correct for f/4 and above, or for f/1.4? Then learn the correction for a headshot. You shouldn't have to worry about focusing 6-10 ' away. And otherwise try to shoot scenes with objects at various depths, so that something will be in focus. Or focus deep in the scene, so that the larger nearer objects are rendered more softly. That looks pretty natural.

Hope this is close to what you wanted to hear about.

scott

 

[Mikec]

I am using the Leica 28mm Summicron and I noticed that the focus plane appears to move further away at f/2.8 and f/4.0. At smaller apertures, I suspect DOF makes up for the focus shift.

Scott, thanks for the great information.

I would just shoot at f/2 but the 1/4000th is not fast enough sometimes. I just need to practice my "focus, then lean back". I wouldn't want to avoid the lens because I really like how it "draws".

 

[harmsr]

Mikec,

The 28 Summicron is not a lens known for focus shift. I would suggest that you describe here how you are checking it and with which camera. I STRONGLY suspect that you have a lens or body which needs to go back into Leica for calibration.

The common lenses which have focus shift in the Leica line-up are the 35 Lux ASPH, 50 Noct, 75 Lux.

I can't remember anyone ever complaining of focus shift with the 28 Cron.

Best,

Ray

 

[Paratom]

as far as I understand focus shift means that a lens focuses fine at a certain f-stop (at all distances) but when you change the f-stop you will get front/or back focus.
Now you could calibrate a lens so it focuses correct wide open and you get some focus shift when stopping down, or you could correct it for a different f-stop (lets say you could optimize a f1.4 lens for f2.8)
In the Leica M range I found two lenses to show focus-shift: Noctilux and 35/1.4(asph).
I use the 28/2.0asph and havent seen any problems caused by focus shift. I also suggest get your lens checked.
Tom

 

[Mikec]

Hmm... Interesting.

So far, at f/2, focus appears to be spot on. I am a RF newbie so user error is highly likely as well. As I stop the lens down, I might be moving the camera. I am using a M8.2.

I have not done any controlled testing. This is simply observations from reviewing photos and remember the ones that I wanted to shot at f/2 and needed to stop down to f/2.8 or f/4.

I am going to do some controlled testing with LensAlign and see what I find. In the meantime, I am still enjoying the camera and lens tremendously. I just thought focus shift is what I need to learn to compensate for.

 

[Mikec]

I paid more attention to my focusing technique and discovered the following,
- I usually start from infinity and pull focus back. I found that I don't pull focus back enough if I am rushed. This would definitely affect my perceived back focus.
- I need new glasses. My eye sight is just not that good.
- I need to practice focusing more.

What's a good RF focusing protocol? When pulling focus back from infinity, is it a good idea to pull past intended focus point then back?

 

[Paratom]

I paid more attention to my focusing technique and discovered the following,
- I usually start from infinity and pull focus back. I found that I don't pull focus back enough if I am rushed. This would definitely affect my perceived back focus.
- I need new glasses. My eye sight is just not that good.
- I need to practice focusing more.

What's a good RF focusing protocol? When pulling focus back from infinity, is it a good idea to pull past intended focus point then back?

I dont think you need to go over the point and then back (like recommended for SLRs) just focus until lines are alligned and shoot.
Do you use a magnifier? I really like the Leica 1.25 magnifier. It does work with 28mm.

 

[hef]

I would also agree with t strenq.... get a 1.25 magnifier... i'm a +2 on the diopters on most cameras, and this really helped me with my 28 elmarit focusing more quickly.

 

[Mikec]

Sounds like a good plan. I will try adding the 1.25x magnifier to see if I can focus better.