focus shift - please explain it

Thanks for posting that. Much of it went way over my head (particularly as I was skimming), but on page 32 he finally explained the "bullseye" effect I saw so often in photos from the original CV Nokton 35 / 1.2 (version 1).

JHutchins said:
An interesting discussion of the relationship between focus shift, spherical undercorrection and bokeh here from Zeiss's director of lens design:
http://www.zeiss.com/c12567a8003b8b6f/embedtitelintern/cln_35_bokeh_en/$file/cln35_bokeh_en.pdf
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135format said:
That's cleared it up. So basically it's due to a poorly corrected/designed lens. Or a lens designed for one purpose which doesn't suit another. i.e. it wasn't designed to be used that wide open or that closed down but they stuck an aperture in there which could be used like that.
And basically I guess all lenses will suffer to an extent unless they perfect correction at all apertures and all distances which is unlikely.
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All lenses are balancing a range of considerations, including cost. In the Hexar AF, under-corrected spherical aberration and focus shift are accepted as part of the tradeoff, and you have a highly respected lens, quite sharp wide open, and the sophisticated IR rangefinder automatically adjusts for the focus shift.

As I read the interesting Zeiss article, I suspect most all fast primes have focus shift. I wonder if any other AF systems take into account focus shift?

"Focus on the nose if stopped down" is an interesting approach.
 
ampguy said:
Anyone who has used a stop down (metering, DOF preview, etc.) SLR, and observed critical focus, will have seen it.

So those who haven't seen it, have either a) bad eyesight, b) wide open SLR or RF use only, or c) have seen it, but haven't been bothered by slightly out of focus subjects.
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I just tried that, and I don't agree .. one focuses at full aperture, then when stopped down the DOF covers any error ... oh, and I don't have bad eyesight, a gear fixation or any prints I'm ashamed of ...
 
ok

ok

Not questioning if DOF covers it for your photographic needs, but are you able to see the shift of exact focus from wide open to several stops down, with your particular setup?

Not everyone wants to take photos like yours, some want to know their exact single point of focus, and base their front and rear DOF based on that exact point, which changes with aperture, and which can be seen with say a Nikon F3 with center line image focusing screen, and bright enough light to stop down a fast lens and still focus.



Sparrow said:
I just tried that, and I don't agree .. one focuses at full aperture, then when stopped down the DOF covers any error ... oh, and I don't have bad eyesight, a gear fixation or any prints I'm ashamed of ...
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Two lenses I use frequently, the 75 Summilux and the 50/1.5 Jena Sonnar are known for focus shift and, in fact, display it. I notice it principally when at the near end of the focus range. There, the solution is easy -- just focus, as you would normally, then lean in a tiny bit before shooting.
 
ampguy said:
Not questioning if DOF covers it for your photographic needs, but are you able to see the shift of exact focus from wide open to several stops down, with your particular setup?

Not everyone wants to take photos like yours, some want to know their exact single point of focus, and base their front and rear DOF based on that exact point, which changes with aperture, and which can be seen with say a Nikon F3 with center line image focusing screen, and bright enough light to stop down a fast lens and still focus.
Click to expand...

As soon as you stop down "several stops", the exact point of focus becomes pretty much irrelevant and, in fact, hard to see in a print, let alone on a focusing screen. Moreover since whatever divergence is there is eaten by DOF at that point.

It's when you stop down less than "several stops" (one or two at most) that you may be able to discern it, or, in fact, probably not. In the end it doesn't really matter in practice with SLRs.
 
ampguy said:
Not questioning if DOF covers it for your photographic needs, but are you able to see the shift of exact focus from wide open to several stops down, with your particular setup?

Not everyone wants to take photos like yours, some want to know their exact single point of focus, and base their front and rear DOF based on that exact point, which changes with aperture, and which can be seen with say a Nikon F3 with center line image focusing screen, and bright enough light to stop down a fast lens and still focus.
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... well I tried a 50 105 and 135 on a F3 with the H type screen but failed to replicate your observation, the small variation is covered by the DOF and at full aperture where DOF is at its minimum there is obviously no change at all with a SLR


 
Focus shift you said?

A few weeks ago I sent my M9-P to Leica because all my pictures taken at full aperture with 50/2, 90/2 APO, 75/1.4... all Leica lenses were out of focus. This was at close distance as well as medium or long distance.

Leica told me that I was having a focus shift issue????

What the ... are we talking about? Same lenses with film Leica bodies no issue then a $8000 camera supposedly top of the art and we have a focus shift.

I was told by Leica that this was absolutely normal. You see, the digital sensor is absolutely flat while the film in a M7 or any M film body is not. Well I am learning something.

Wait, I was advised to stop down to get a sharp picture, to play with the camera in order to find the right point of focus (may be they should remove the rangefinder and cut the cost). Yeah right. Get an M9-P and a Noctilux for something near $20.000,00 and you have to stop down to have a focused picture. I never though of that. So stupid I was using a Noctilux at full aperture. Who am I to think I can get a sharp picture. I just have to stop down, right?

Then why should I buy a Noctilux and not a Summicron or even a Voigtlander 50mm. You get it I love to spend money ...

I really apologize if my words may hurt some of you but this is driving me crazy and please correct me if I am wrong. I may have missed something.

Right now I am thinking of going back to film, sell the M9-P or why not buy a Nikon D 800, shame on me!
 
Bananasplit that does sound like rubbish from Leica. Which Leica division was it? I use my C Sonnar 50 without trouble. Can you post some test shots? I've regularly used my 135 at max aperture admittedly only f4 and here is a version 5 Summicron at f2 min distance focussed on the Peroni tab on the glass.


L1000812.jpg by Richard GM2, on Flickr
 
Try this

Try this

Setup a photo where you're photographing the top of a fence from min distance out to 50 yards +.

Put a subject at about 15 yards next to the fence and focus on the eyes.

Take 2 photos at the same exposure values. First focused wide open, Second focused stopped down.

Compare the front parts of the prints (fence area near lens) and see if you can spot a difference.


rxmd said:
As soon as you stop down "several stops", the exact point of focus becomes pretty much irrelevant and, in fact, hard to see in a print, let alone on a focusing screen. Moreover since whatever divergence is there is eaten by DOF at that point.

It's when you stop down less than "several stops" (one or two at most) that you may be able to discern it, or, in fact, probably not. In the end it doesn't really matter in practice with SLRs.
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bananasplit said:
.............Get an M9-P and a Noctilux for something near $20.000,00 and you have to stop down to have a focused picture. I never though of that. So stupid I was using a Noctilux at full aperture. Who am I to think I can get a sharp picture. I just have to stop down, right?............
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The f1 Noctilux I had focused amazingly perfectly at full aperture (Techpan fine film, tripod etc) and your Leica lenses should focus perfectly at full aperture also. In the case of the f1 Noctilux, even with film, focus shift occurred with smaller apertures, to a degree that increasing depth of field did not compensate. Since I bought it for full aperture use this was acceptable.

The worst focus shift I experienced was with the Zeiss 50mm/f1.5 which was not optimized for full aperture because the focus shift in this lens is huge. Optimizing it for full aperture would have made sense to me, but I disowned it.

As far as I know, Leica lenses are supposed to work optimally at full aperture. As you said, why else would you bother with them?
 
bananasplit said:
A few weeks ago I sent my M9-P to Leica because all my pictures taken at full aperture with 50/2, 90/2 APO, 75/1.4... all Leica lenses were out of focus. This was at close distance as well as medium or long distance.

Leica told me that I was having a focus shift issue????



I was told by Leica that this was absolutely normal. You see, the digital sensor is absolutely flat while the film in a M7 or any M film body is not. Well I am learning something.

Wait, I was advised to stop down to get a sharp picture, to play with the camera in order to find the right point of focus (may be they should remove the rangefinder and cut the cost).
Click to expand...

May I inquire where this "Leica" is located? With your wording such an absurd commentary could be expected from a plumber but definitely not from any Leica service.

A rangefinder mechanism needing a readjustment is one thing, to blame such lenses like the 50/2, 90/2 APO and 75/1.4 for focus shift or being out of adjustment is totally another thing! And this "absolutely normal"??? :eek: "the digital sensor is absolutely flat while the film in a M7 or any M film body is not." :eek: "I was advised to stop down to get a sharp picture, to play with the camera in order to find the right point of focus" :eek:

The camera in concern is an >$7K one.. the lenses in concern are all top $$$$ ones.. and these were absolutely normal?? And they did not "correct" them but told you to "stop down". Holy Grail!!

I'd be reporting these immediately to Solms.. This is the most ridiculous commentary to be heard from an authorized service, be it Leica or any other brand.... or it was a bad joke...
 
Here's a test sample that I found interesting. Watch how the plane of best focus shifts away from the camera as you go from f/1.4 to f/2.8 on a Zeiss 50 f/1.4 ZF. (Mouse over the f-numbers at "Bokeh Fringing" near the bottom of this Photozone.de page to see the effect.)

Also note the different false coloration of the OOF foreground vs. the OOF background.

In this example and all others I can find, the increased DOF makes up for the shift, so the original focus point is still sharp. But... 1) The optimal focus plane does shift back, and 2) I can imagine a lens for which the increase in DOF does not cover the focus shift. That would be a bad design.
 
ColSebastianMoran said:
Here's a test sample that I found interesting. Watch how the plane of best focus shifts away from the camera as you go from f/1.4 to f/2.8 on a Zeiss 50 f/1.4 ZF. (Mouse over the f-numbers at "Bokeh Fringing" near the bottom of this Photozone.de page to see the effect.)

Also note the different false coloration of the OOF foreground vs. the OOF background.

In this example and all others I can find, the increased DOF makes up for the shift, so the original focus point is still sharp. But... 1) The optimal focus plane does shift back, and 2) I can imagine a lens for which the increase in DOF does not cover the focus shift. That would be a bad design.
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Bad design? It'd be a design. The focus shift would be a disadvantage (one that could be mitigated by using a camera with a live viewfinder) but it would bring with it its own advantages. Every lens design is a tradeoff and looking at any lens over one dimension (like susceptibility to focus shift/correction for spherical aberration) is a mistake becuase poor performance on any given dimension is usually a result of a tradeoff.

The Planar is actually very well corrected for spherical aberration and thus not particularly prone to focus shift. In the Makro-Planar it essentially doesn't exist (unsurprising since it's designed to be used at the near end of its focus range). Sonnar designs, by contrast, are highly prone to focus shift, but contrastier & more flare resistant (even without coatings) and smaller and, besides, have a really lovely look.
 
PKR said:
When using an 8X10 for table top work, compositions were always made with the lens wide open. Once a basic placement of the subject was finished - the lens (210mm f 5.6 WA Schneider) was stopped down (22-45) and a "hot light" was used to refocus the lens, at a small aperture, for accurate FP and DoF (the camera Standards were readjusted for best DoF -Scheimpflug).

In most instances the "hot light" was a 500W Quartz. If the subject matter was sensitive to heat from the "hot light" (food photography) a series of very expensive 8x10 Polaroids would be used to confirm critical focus and DoF.

Focus shift is a real problem.

https://www.schneideroptics.com/ecommerce/CatalogSubCategoryDisplay.aspx?CID=169
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The point of that exercise was to verify DOF and whether you had enough of it, which is critically important. Focus shift comes a distinct second in relevance, being pretty much unmeasurable at f/45, table top or not.

The Schneider page and brochure you linked to don't even mention focus shift.
 
ampguy said:
Setup a photo where you're photographing the top of a fence from min distance out to 50 yards +.

Put a subject at about 15 yards next to the fence and focus on the eyes.

Take 2 photos at the same exposure values. First focused wide open, Second focused stopped down.

Compare the front parts of the prints (fence area near lens) and see if you can spot a difference.
Click to expand...

What lens do you propose using?
At which apertures?
What behaviour do you expect except "there is more in focus"?
 
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