results after fixing new sonnar 50 1.5 C focus shift?

[jano]

Hi,

Plenty of examples of the "focus shift" phenomenon with the new Zeiss sonnar 1.5 C.

For those of you that had the lens adjusted, do you have examples of the results from f1.5 through f8 or smaller? Or at least describe what happens?

I have noticed several of my low-lights at wide open seemed oof (and it wasn't a pleasant kind), so despite my hatred for lens tests, I tested mine, and it's about a foot out. Ouch. But I'd like to see/hear/understand what the effect is like once fixed.

Thanks,
Jano

 

[ernesto]

One element, a fraction of milimeter out of place, could mean one foot or more of focus shift depending on the focusing distance. Perhaps a screw that was positioned more or less tight can produce that shift.

I take pictures during dissasembling process, and find references for all the screws. In case there are no visible references I mark them, in order to position every single ring in the right place.

Ernesto

 

[jano]

Thanks for the response, but I'm looking for differences in the image results, not mechanical/physical to the lens, sorry I was not clear 🙂

 

[Marc-A.]

OK, I've read a lot of threads on the famous focus shift of the Sonnar ZM, but I don't get it. Someone can explain me the issue in a understandable manner (for me).
Do the focus shift appears only at f 1.5? Is there focus shift at f2 and smaller aperture?
If you get it fixed by Zeiss, does that means your lens can focus at any aperture? Or does the issue remain, but at other apertures?
I find the Sonnar very desirable, but this focus shift issue "scares" me.

 

[Ororaro]

I think this "optimized for f2.8" thing is ridiculous. I don't believe in focus shift.
Believe me, no company will design and produce a lens with such out of whack characteristics. If the focus shift is bigger then the depth of field, then the optical formula is crap.

The wide-open focus shift is nothing more then the lens going out of the factory very badly assembled.

 

[Marc-A.]

I think this "optimized for f2.8" thing is ridiculous. I don't believe in focus shift.
Believe me, no company will design and produce a lens with such out of whack characteristics. If the focus shift is bigger then the depth of field, then the optical formula is crap.

The wide-open focus shift is nothing more then the lens going out of the factory very badly assembled.

So Ned, in your opinion, it's not a Sonnar problem but just a problem of some Sonnars which weren't tight controlled, right? I'm also dubious about the "optimized for f2.8" argument. I mean, lenses are optimized for particular aperture in respect to sharpness, contrast etc. but not focussing.
Well, I'm lost.

 

[Ororaro]

So Ned, in your opinion, it's not a Sonnar problem but just a problem of some Sonnars which weren't tight controlled, right? I'm also dubious about the "optimized for f2.8" argument. I mean, lenses are optimized for particular aperture in respect to sharpness, contrast etc. but not focussing.
Well, I'm lost.

Of course it's control issue. I'm not an insider but honestly, can it be anything else?

Like the 75 Lux. Apparently they all we're backfocusing out of the factory. (those who say it's not the case, then it might be a batch problem). But it certainly wasn't a "focus shift issue". It was a bad calibration issue.

 

[Roger Hicks]

I think this "optimized for f2.8" thing is ridiculous. I don't believe in focus shift.
Believe me, no company will design and produce a lens with such out of whack characteristics. .

No, I don't believe you -- and nor do several of the top people at Zeiss.

Whether you believe in it or not, focus shift happens. It is an inherent characteristic of some designs that the point of sharp focus shifts as the lens is stopped down, and this is nothing whatsoever to do with the quality of assembly of the lens. This is well known to anyone who knows anything at all about the history of lens design.

The optical formula is crap? Again, sorry, you are merely betraying a certain lack of knowledge of how lenses are designed, made and sold. All optical designs are a trade-off: what you consider 'acceptable' or indeed 'crap' is a matter of personal opinion.

The Sonnar design is a modified Cooke triplet. This allowed a minimum number of glass/air surfaces (high contrast) at the cost of reduced resolution and focus shift. The Leitz Xenon (slightly later, a symmetrical derivative) has higher resolution and less focus shift at the cost of reduced contrast. Yer pays yer money and yer takes yer choice. Original Sonnars were all 3-group; the current 1,5/50 is 4-group.

As it happens, I yesterday tested my f/1.5-optimized Sonnar for d-o-f at full aperture and closest focus. What happens is that because of focus shift, d-o-f increases much faster behind the focused point than in front of it. What I had previously reported as Dr. Nasse's rule of thumb -- think of all the d-o-f being behind the focused point -- is a useful simplification, but d-o-f does grow in fromt of the focused point beyond about f/2.8 as well.

My f/1.5-optimized Sonnar was collected from Oberkochen in May, when I spent several hours speaking to Zeiss designers. It's one of my favourite 5cm lenses, though (for example) the Summicron is sharper and more evenly illuminated.

Cheers,

Roger

 

[jano]

Guys, quit crapping on my thread! You've hashed this through and through (I've read the threads and searched). And if you'd rather talk about it over and over, take it to another thread like an leica M8 or something, or start a new one. 😡

Again, I'm just looking for a perspective of someone who has had the focus shift adjusted to 1.5.

Thank you.
Jano

 

[Roger Hicks]

Dear Jano,

Mine is on 1,5.

The results are as reported above.

What else do you want to know?

If your results are a foot out, I'd query (a) the adjustment and (b) the accuracy of your rangefinder. An inch or two is nearer the mark with my test.

Cheers,

R.

 

[Marc-A.]

Guys, quit crapping on my thread! You've hashed this through and through (I've read the threads and searched). And if you'd rather talk about it over and over, take it to another thread like an leica M8 or something, or start a new one. 😡

Again, I'm just looking for a perspective of someone who has had the focus shift adjusted to 1.5.

Thank you.
Jano

I was also seeking information on fixed Sonnar, and asked for more information on the issue because I don't understand it well. That's too much for you.
Thanks for your courtesy. Good to know. Bubye.

 

[jano]

Roger, your description of what happens was buried inside a long response to issues unrelated to my original question - that's why I just found it after you mentioned now, thanks for pointing it out. It does make some sense, but tough to understand without some sort of illustration. However, there are two issues:
1. have you had this lens as optimized at 2.8?
2. what happens to the focus point on a 1.5 optimized lens as it approach f4 versus the 2.8?

This is what I'm after. I'm not sure why that isn't clear -- quoting myself in the first post, "for those of you that have had the lens adjusted" -- sheesh.

 

[Gabriel M.A.]

Thread, we hardly knew ye...

 

[Roger Hicks]

Roger, your description of what happens was buried inside a long response to issues unrelated to my original question - that's why I just found it after you mentioned now, thanks for pointing it out. It does make some sense, but tough to understand without some sort of illustration. However, there are two issues:
1. have you had this lens as optimized at 2.8?
2. what happens to the focus point on a 1.5 optimized lens as it approach f4 versus the 2.8?

This is what I'm after. I'm not sure why that isn't clear -- quoting myself in the first post, "for those of you that have had the lens adjusted" -- sheesh.

This lens is factory optimized at f/1.5 (I picked it up at Oberkochen). I do not know if it is a conversion or original, but according to Zeiss, there is no difference between a factory original and a conversion. If I understand correctly -- and I think I do -- it is merely a question of the angular position of the focusing cam: a new cam is not needed. This is why I said that if you're a foot out, I'd query either the adjustment or the camera rangefinder.

The test was focusing on a receding line of books. At f/1.5 the tiny amount of d-o-f (an inch/25mm or so, at the closest focus distance) is greater in front of the focused point than behind. At f/2.8 the d-o-f in front is about the same as at f/1.5, but the d-o-f behind is much greater. After that, d-o-f grows in both directions, but faster behind the focused point than in front.

In other words, the focused point recedes as you stop down, but the d-o-f grows (as usual) and by f/4 or f/5.6 it is enough that you don't need to worry, even at the closest focused point. I did not test at f/4 but at f/1.5, 2.8, 5.6 and 11.

With an f/2.8 optimization, the focused point is in front of the rangefinder focus; about an inch or two, I believe, though frankly, I didn't pay much attention bevause this is not a lens that most people will be using at full aperture at the closest focusing distance. At f/2.8 it is spot on. At f/4 to f/5.6, as before, it's not really an issue.

Does this answer your question?

Cheers,

R.

 

[jano]

Makes sense, thank you. One last clarification - "in front" and "behind" the focus -- which is which? 🙂

me -------------- focus point +++++++++++

Is "in front" on the side of the dashes or the pluses? Depending on which which you figure, it could be either. Also, when the focus point "recedes", which direction is it going?

 

[Roger Hicks]

Makes sense, thank you. One last clarification - "in front" and "behind" the focus -- which is which? 🙂

me -------------- focus point +++++++++++

Is "in front" on the side of the dashes or the pluses? Depending on which which you figure, it could be either. Also, when the focus point "recedes", which direction is it going?

Dear Jano,

'In front of' -- towards me/you/the camera.

'Behind' -- on the far side of the subject, away from me/you/the camera.

Recedes -- gets further away from me/you/the camera.

It would astonish me that there was any reason to query this, were it not that my wife, Frances Schultz, often thinks exactly backwards from me on 'front' and 'back' and my late mother always maintained that first should be the highest gear in a car, not the lowest. She was a schoolteacher...

Cheers,

R.

 

[jano]

A point doesn't have a direction, hence the confusion.

Consider your terminology from the POV of the photographer (e.g. vector from you to the point), and you'll see how the words suddenly mean the opposite of your definition.

As far as the cars, I disagree with your mother.

Thanks for the clarification.

 

[Roger Hicks]

Consider your terminology from the POV of the photographer (e.g. vector from you to the point), and you'll see how the words suddenly mean the opposite of your definition.

As far as the cars, I disagree with your mother.

Thanks for the clarification.

For the former, my wife's logic also. Maybe I'm more egocentric: 'forwards' means 'towards me'.

As for the second, I have met one or two others who saw her point. After all, as she said, 'if you're top of the class, you're first in the class, so first gear should be tp gear'. I never agreed with her either, at least, not on that one.

Cheers,

R.