Focus shift

[ramosa]

All,

I am hoping to achieve a conclusive understanding of focus shift. I have never experienced the problem firsthand, but see that the topic comes up fairly common in posts (e.g., regarding the Lux 35mm). I always thought it mean that, when focusing a lens at a certain f-stop (e.g., 4), the focus would be off, being either front or back focused. But, when recently reading an explanation on digilloyd, I think I was wrong. It seems that focus shift "is*a displacement of the sharp plane of focus when the lens is focused wide open, but the image is made with the lens stopped down." Such a problem is significantly better than what I had thought, as you can compensate for it simply by focusing/refocusing at the aperature at which you plan to shoot. Is that correct? Thus, if I had a Lux 35mm*with focus shift, I could focus and shoot at**f/1.4, focus and shoot at f/2, focus at f/2.8 to shoot at f/2.8, etc.

Thanks,

R

 

[Nettar]

Hmm... Concerning your comment that "if I had a Lux 35mm with focus shift, I could focus and shoot at f/1.4, focus and shoot at f/2, focus at f/2.8 to shoot at f/2.8, etc," my response would be the following. On a rangefinder camera you'd struggle to refocus in this way, because you are constrained by the focus settings of the rangefinder mechanism. On a camera that focuses through the lens, via a mirror or an EVF, your focusing accuracy is constrained by the depth of field, and so if you focus stopped down then there may be errors that are of concern. Either way, focus shift is at least a bother, if not a problem. Nettar

 

[Ronald M]

Changing aperture makes no difference as to focus shift on a RF camera. Focus is not achieved by the lens, buy by RF mechanism and lens cam contact to it.

Changing aperture on a slr lens or view camera lens will allow you to compensate for focus shift.

 

[Gary Sandhu]

Changing aperture makes no difference as to focus shift on a RF camera. Focus is not achieved by the lens, buy by RF mechanism and lens cam contact to it.

Changing aperture on a slr lens or view camera lens will allow you to compensate for focus shift.

I think you mis-typed:

Changing aperture CAUSES focus shift.

On a rangefinder, you can't compensate by merely looking at the focus patch/rangefinder image: you can on a (manual) focus SLR or view camera. The digiLoyd comments don't apply to rangefinders.

I had a silver 35 lux that focussed perfectly at 1.4, then back focussed until aperture f8 when the depth of field compensated.

Interestingly, my hexar AF compensates for focus shift.

 

[250swb]

Is that correct?
R

No, as the others have said, the rangefinder mechanism does not see the focus point, it is simply a mechanical system that indicates the focus position of the lens in the viewfinder. f16 or f0.95 it makes no difference what aperture your lens is set at or when you focus it, the rangefinder mechanism has no way to compensate or know what the aperture is.

Steve

 

[Jockos]

On a rangefinder, you can't compensate by merely looking at the focus patch/rangefinder image: you can on a (manual) focus SLR or view camera. The digiLoyd comments don't apply to rangefinders.

Most semi-modern SLR lets you focus with the aperture open, and closes it when you take the picture. Thus you wont be able to see the focus shift unless you do a DOF preview.

 

[Ljós]

No, as the others have said, the rangefinder mechanism does not see the focus point, it is simply a mechanical system that indicates the focus position of the lens in the viewfinder. f16 or f0.95 it makes no difference what aperture your lens is set at or when you focus it, the rangefinder mechanism has no way to compensate or know what the aperture is.

Steve

Yes, a good way to understand it is the word Steve chose here: the rangefinder does not "know" at which stop the aperture is set.
With an SLR, you could stop down the lens (use aperture preview lever), and could see what you are going to get. (However, the viewfinder would go dim, depth of field would increase, and for all practical purposes I do not think a helluva lot SLR users can avoid the unwanted effects of focus shift via focusing stopped down, particularly not when things go fast.)

What's funny about this whole focus shift phenomenon is how little it was known to the majority of photographers (amateurs like me) until some years ago. I remember when Zeiss introduced its lens line for Leica M, and the fact that their lenses were designed for minimized focus shift was featured quite prominently in their campaign. At that time I (and many others, if I remember correctly) thought: wow, so they found the solution for a problem no one really has had a problem with so far in actual shooting.

Of course focus shift had been a tangible problem with some lenses in some situations as long as there is photography. One factor for the greater buzz now is that digital makes it easier to discover that focus is off... plus that the perfectly flat sensors pose harsher criteria for perfect focus than film which has some "depth" (layers) and a bit more wiggle room.

 

[baycrest]

I'm with Gary on this one.

I have a 35 Lux and I experience FF at 1.4, perfect focus at 2.0 and then a BF from 2.8 thru to 5.6. At f/8, the DOF covers the focus shift and I get a sharp picture once again.

So I need to adjust the focus patches to compensate for the shift for f/2, 2.8-5.6. Its a bother and sometimes I don't hit it correctly.

The newest 35 Lux still has focus shift, but apparently its less noticeable. Go figure!

 

[kully]

The focus point shifts on all my lenses (from a 60s Nikkor 85/2 to my CV 35/1.2 and ZM 50/1.5) as I change aperture (GF1 + m-adapter + liveview makes this easy to see). However, the depth of field normally covers it.

Slight differences from absolute spec in the lens + camera can amplify this, or in the case of the ZM 50/1.5 it is inherent in the lens design that the focus shift is not going to be covered by DoF.

Before you match a camera to a lens, I might suggest getting your camera focussing right and then matching the lenses to that.

 

[ramosa]

All,

Thank you. My original understanding was correct. I understand now that the digilloyd explanation would only make sense for a DSLR or view camera--and not a RF.

Pardon my need to clarify this. But the digilloyd information caused some dissonance.

R