35mm APO Summicron-M - Focus Shift Characteristics Query

[Lee S]

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Hi all,

I’m reaching out to see if everyone has tested for focus shift on the 35mm APO Sunmicron-M. 

At 150cm distance my 35mm C-Biogon (which is widely  considered well corrected (only bettered by the APO Summicron-M/VM Lanthar) exhibits 8cm of focus shift from wide open to F5.6. 

Peak sharpness and DOF increases at  F5.6 but the sharpness of the intended focus point is lower than wide open sharpness which irks me (know I’m being picky, it’s still damn sharp although not optimal).

Expect due to an FLE and better correction focus shift should be less than the C-Biogon.

If anyone has tested or can test the 35mm APO Summicron-M at this distance I would be very thankful. 
 

Thanks 🙏 L.

Edited October 25, 2024 by Lee S

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[lct]

I have no experience with the 35/2 apo but i have never seen the least focus shift out of my Biogon 35/2.8. How did you check it, with your rangefinder? Just curious.

[Lee S]

4 hours ago, lct said:

I have no experience with the 35/2 apo but i have never seen the least focus shift out of my Biogon 35/2.8. How did you check it, with your rangefinder? Just curious.

Using a slanted test target with repeating black lines. Focus using live view or rangefinder then take a photo wide open then stop down and take another photo and compare. I used the rangefinder which I found was very well calibrated for this distance. 

The C-Biogon is fantastic with respect to focus shift, what I’m identifying is very very subtle and you have to pixel peep to 1000%+ to detect it. It’s a none issue really. 

With a lot of other lenses the focus point shifts backwards and a loss of sharpness is apparent from a quick glance . 

I’m curious if the 35mm APO improves on this performance given it’s , FLE and better correction.

[roelandinho]

The Zeiss 35 mm C Biogon f/2.8 is one of a few lenses I have used where I have never noticed any focus shift in real-world use. After reading your post I just did a test myself with this lens and can confirm your findings. Focus using the rangefinder is spot on at f/2.8 and moves backwards from there. I guess I'll start compensating for this from now on (like I do with almost all of my other lenses) when not using the full aperture. Actually, I am sort of pleased with this discovery: this lens, which is in my mind the overall best performing lens I have ever used, apparently has the potential to do even better still... 

I have never used either of the 35 mm APO's (Leica or Voigtlander). They are probably better corrected for spherical aberrations, which should reduce focus shift. But on the other hand they're both f/2 lenses, which makes them much more difficult to correct. It would be interesting to have this tested by someone who owns all of these lenses... My guess is the APO's still have detectable focus shift (if you know how to test for this and are critical enough) but I'm really not sure if it's more, less, or the same as the Zeiss.

Edited October 26, 2024 by roelandinho

[Lee S]

45 minutes ago, roelandinho said:

The Zeiss 35 mm C Biogon f/2.8 is one of a few lenses I have used where I have never noticed any focus shift in real-world use. After reading your post I just did a test myself with this lens and can confirm your findings.

Thanks for confirming and glad it’s helped you get the best out of this fantastic lens.

As you mentioned, nobody has ever noticed it in real world and my extremely critical test indicate it’s only a minor drop in resolution/sharpness. 

Agree with your view on the APO’s due the larger aperture it has more ‘potential’ to focus shift but guess it depends on level of Spherical Aberration and how the floating elements can compensate.

Hopefully someone can test the APO’s too!

Edited October 26, 2024 by Lee S

[Lee S]

54 minutes ago, roelandinho said:

I guess I'll start compensating for this from now on (like I do with almost all of my other lenses) when not using the full aperture.

What technique do you employ to compensate for the shift?

Do you move the focus ring slightly or move back? 

I think I’d struggle to remember or gauge it, particularly by moving the focus ring, it’s very sensitive compared to the level of focus shift at that range.

I guess practice makes perfect though!

L

[roelandinho]

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1 hour ago, Lee S said:

 

What technique do you employ to compensate for the shift?

Do you move the focus ring slightly or move back? 

I think I’d struggle to remember or gauge it, particularly by moving the focus ring, it’s very sensitive compared to the level of focus shift at that range.

I guess practice makes perfect though!

L

I usually use the focus ring to pull the focus plane a little bit closer than what the rangefinder indicates (at apertures where the best focus is further than what the rangefinder indicates). I suggest you practise this a bit first. In my experience there is quite a bit of leeway at 35 mm f/4 or smaller. 
 

Depending on subject matter and composition, you can actually get a “double” benefit from compensating: in the case of a subject against a background, compensating for focus shift will increase the contrast of the subject *and* at the same time decrease background contrast. Even if you compensate a bit too much, the subject contrast is maybe again about equal to what it was with rangefinder-indicated focus, but with a less sharp background, which is still gives better subject separation. You really have to overdo it by quite a lot to make it much worse… 

Edited October 26, 2024 by roelandinho