CV Nokton 35mm 1.4 Classic SC - Focus Shift Questions

[aufeuer]

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Hi All!

First time posting in a while after recently getting my first digital body, an M 240, and recommissioning my Nokton 1.4.

It was hard to pay attention to any focus shift shooting film before, but it's easier now with the M 240. And I notice now that wide open, at minimum focusing distance, the rangefinder and the focusing of the lens seems tightly coupled and spot on, but as soon as I move to intermediate distances (also wide open), I start noticing some front focusing that I need to compensate for.

I know focus shift is a typical problem of this lens, but I would appreciate if anyone can clarify the following:

1) I heard that focus shift is typically a problem at closer distances rather than farther ones, and mine is the opposite. Also, I know this is typically related to aperture changes, but I'm primarily observing the shift (accuracy of the rangefinder) at the same aperture but different distances. Does this behavior still fall under "normal", as far as focus shift issues are concerned?

2) Would you say that this behavior shows more a problem with the lens, rather than the rangefinder? Or is it still possible that rangefinder adjustment is needed?

Thanks!

[adan]

1) No - that is not "focus shift." Bad focus at different distances is a lens or camera calibration problem.

1a) - the focus shift with the CV Nokton 35 (which I have) occurs with either MC or SC versions (same glass, different coatings).

True focus shift occurs if a fast lens has spherical aberration - at wide apertures, the outer edges of the glass focuses at a different distance than the center of the lens (the best part). Wide open, you get "best average focus (BAF)" at one point, and then as you stop down the lens and remove the light rays from the edges, you get a different "best average focus" point, since you are using only the center of the lens. By 3-4 stops down the issue vanishes, for the most part, hidden by using fewer and fewer edge rays, and gaining more DoF.

Because DoF is smallest at close distances, that is usually where focus shift is most obviously blurry, but it is there at other distances, if it is present.

The lens focus cam can be adjusted to match the BAF at any particular aperture (by the factory, NOT by a user) - my MC Nokton 35mm happens to be "set" for f/2.0 (presumably at the factory) - at f/1.4 it front-focuses, at f/2 it focuses very nicely, and at f/2.8 and f/4 it progessively back-focuses, but by f/5.6 the shifting stops and DoF makes everything look "sharp enough" anyway.

2) It is situational. If only one lens shows the focus problem, then it is likely it is that lens that needs service. The camera RF only needs adjustment if it is having trouble focusing several different lenses in the same way. Unless one has several lenses that are "wrong" by exactly the same amount - statistically very unlikely.

[aufeuer]

2 hours ago, adan said:

1) No - that is not "focus shift." Bad focus at different distances is a lens or camera calibration problem.

1a) - the focus shift with the CV Nokton 35 (which I have) occurs with either MC or SC versions (same glass, different coatings).

True focus shift occurs if a fast lens has spherical aberration - at wide apertures, the outer edges of the glass focuses at a different distance than the center of the lens (the best part). Wide open, you get "best average focus (BAF)" at one point, and then as you stop down the lens and remove the light rays from the edges, you get a different "best average focus" point, since you are using only the center of the lens. By 3-4 stops down the issue vanishes, for the most part, hidden by using fewer and fewer edge rays, and gaining more DoF.

Because DoF is smallest at close distances, that is usually where focus shift is most obviously blurry, but it is there at other distances, if it is present.

The lens focus cam can be adjusted to match the BAF at any particular aperture (by the factory, NOT by a user) - my MC Nokton 35mm happens to be "set" for f/2.0 (presumably at the factory) - at f/1.4 it front-focuses, at f/2 it focuses very nicely, and at f/2.8 and f/4 it progessively back-focuses, but by f/5.6 the shifting stops and DoF makes everything look "sharp enough" anyway.

2) It is situational. If only one lens shows the focus problem, then it is likely it is that lens that needs service. The camera RF only needs adjustment if it is having trouble focusing several different lenses in the same way. Unless one has several lenses that are "wrong" by exactly the same amount - statistically very unlikely.

Awesome, thanks for the detailed response adan! The only reason I ask question #2 is because that Nokton happens to be my only M-mount lens at the moment, so I have no point of reference. I guess I will find out where the problem is as I eventually add more to my kit. Thanks!

[adan]

A quick and rough check of focus calibration with just one lens available is - to set the lens to its hard stop for infinity (as far as it can be turned to the infinity mark on the focus scale). Then look through the viewfinder/rangefinder at something very far away - at least 1 km/1 mile, with a crisp vertical edge. Say, a distant building.

If the two rangefinder images align exactly in that condition, then you know that at least the system calibration (lens + camera) is probably correct for infinity.

A double-check is then to take a picture of that situation at a widish aperture (to minimize DoF). If the building looks sharp in that picture, with the lens focus-scale and camera RF both agreeing that that is the focus point, then the calibration is correct - for infinity. If the picture is not as sharp as possible, then either the lens or the camera has some kind of calibration problem. Statistically, it is more likely that the camera is the problem (it has more moving parts to get out of adjustment) - but it can be the lens.

 

[AceVentura1986]

On 8/13/2019 at 12:54 AM, adan said:

True focus shift occurs if a fast lens has spherical aberration - at wide apertures, the outer edges of the glass focuses at a different distance than the center of the lens (the best part). Wide open, you get "best average focus (BAF)" at one point, and then as you stop down the lens and remove the light rays from the edges, you get a different "best average focus" point, since you are using only the center of the lens. By 3-4 stops down the issue vanishes, for the most part, hidden by using fewer and fewer edge rays, and gaining more DoF.

Because DoF is smallest at close distances, that is usually where focus shift is most obviously blurry, but it is there at other distances, if it is present.

Yes, this matches my experience exactly. Within 2 meters it’s completely usable at f/2 but less so at f/2.8 and unusable at f/4. However, from f/5.6 and smaller or beyond 2 meters at any aperture, it’s in focus. Given that 35mm is more usually reserved for situational, scenic, or street photography the 2 meter distance isn’t a practical issue but more of a theoretical one, at least for me. If I were shooting within 2 meters, I’d probably use a 50mm anyway.