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focus shift - please explain it
Old 04-29-2012   #1
135format
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focus shift - please explain it

I repeatedly see talk of focus shift in some lenses but I am curious why adjusting aperture actually changes the focus point. Can someone explain the theory of why it happens and does it happen in all lenses to a lesser degree. And if not why does it happen in some and not in others.
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Old 04-29-2012   #2
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You've probably seen the simple diagrams showing a lens focusing an object at a distance, with the lines of light meeting at a point where the film is. The film has to be where the lines meet at a neat point.

The reality is that the lines don't neatly meet at a point. They don't meet exactly. They approach each other in a bundle called a caustic (probably an archaic term). The designer chooses a point for focus that gives the best compromise. The lines from the wider part of the lens (i.e. widest aperture) meet at a different spot, so that as the aperture is closed the optimum spot is different. The degree of this is different for different lens designs, and is probably most exaggerated at very close focusing distances which is why tests at minimum focus distance might not be the best indication of general focus correction that's needed. The effect is much more pronounced with very wide aperture lenses.

There was a diagram on Erwin Puts's site but I can't find it now. I'll have a look for a diagram.

Edit: http://toothwalker.org/optics/spherical.html

Last edited by john_s : 04-29-2012 at 14:55. Reason: Addition.
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Old 04-29-2012   #3
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http://mansurovs.com/what-is-focus-shift
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Old 04-29-2012   #4
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Focus shift is caused by the internet.

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Old 04-29-2012   #5
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Quote:
Originally Posted by BobYIL View Post
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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Old 04-29-2012   #6
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Quote:
Originally Posted by f16sunshine View Post
Focus shift is caused by the internet.

Amen. I doubt I would have known of it without the internet, and the C Sonnar is my favourite lens.
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Old 04-29-2012   #7
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Quote:
Originally Posted by f16sunshine View Post
Focus shift is caused by the internet.

My Sonnar is unsharp at f1.5. I think it's not too bad to know the scientific reason for that.
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Old 04-29-2012   #8
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Quote:
Originally Posted by tom.w.bn View Post
My Sonnar is unsharp at f1.5. I think it's not too bad to know the scientific reason for that.
Knowing the scientific reasons for something is fine, I guess it's the exaggerated bogeyman attention that focus shift has been getting that is the problem.
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Old 04-29-2012   #9
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It's also Leica's fault.
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Old 04-30-2012   #10
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It's also Leica's fault.
No .... Nikon's fault surely!
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Old 04-30-2012   #11
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No .... Nikon's fault surely!
I thought it was Obama's fault.

Or San Andreas' fault.
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Old 04-30-2012   #12
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Quote:
Originally Posted by 135format View Post
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.
"poorly corrected/designed lens" to a certain extent would rather be unfair; as what we call as focus shift is actually caused by residual spherical aberrations; i.e. corrected for most of the aperture range except the first one or two. Such lenses usually exhibit quick drop in resolution toward edges at wide apertures. The C-Sonnar, 35 Summilux pre, Summicron 35 V.1 (8-elements) just to name few.

However focus shift can be observed also with some well corrected lenses exhibiting excellent characteristics wide open: The former version of the Summilux 35 asph. for example. Especially the use of hi-res sensors began to accentuate such faults which were overlooked in the film era as many lenses do suffer of focus shift -except the apochromatics- however in negligible amounts.

BTW, some of the lenses having residual spherical aberrations could be the ones having the most "character"; especially for B&W. Especially the ones I mentioned above have always been and still are among the sought-after lenses for many.
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Old 04-30-2012   #13
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Roger and Frances site describes focus shift in Sonnar review.

been also cautious (for years) about this "horrid" problem, but reading the article realized how trivial it actually is, also on digital. Sonnars (except very early ones) are optimized for f1.5, where the DOF is narrowest. once you stop down, DOF increases and hides practical shift effect. I doubt meaningful focus issue like "nose in focus, eyes unfocused" will not happen in f2.8 and beyond.
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Old 04-30-2012   #14
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Quote:
Originally Posted by jarski View Post
Sonnars (except very early ones) are optimized for f1.5, where the DOF is narrowest.
What is a very early one? Mine is not even 2 years old and it's optimized for 2.8
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Old 04-30-2012   #15
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Quote:
Originally Posted by tom.w.bn View Post
What is a very early one? Mine is not even 2 years old and it's optimized for 2.8
hmm strange, things have changed again it seems. R&F comment: "... Later lenses -- all after early 2007 -- were therefore calibrated for sharp focus at f/1.5"
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Old 04-30-2012   #16
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Digital made focus shift easier to recognize because few people enlarged their 35mm film images to the same sort of poster-size equivalents that 1:1 viewing on a monitor allows with nothing more than Ctrl+Alt+0.

Of course once you become aware of it and start shooting test scenes and such to see how bad (or otherwise) it is, you're trapped on that merry-go-round (I'm not saying it's not important - for some lenses focus shift across certain aperture ranges can be a show-stopper).

One lens I have (the MD 45/2 Rokkor) that is quite mediocre on the ol' "topo maps taped to a wall" sharpness/centering test has given me some of my absolute favourite landscape images in actual use (in this case not because of focus shift, but field curvature at close range)

IMO the important thing is to know how a particular lens behaves ahead of time and not be caught out by surprise (either on digital or film). If it's characteristics don't suit the way the you shoot, use something else!

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Old 04-30-2012   #17
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Quote:
Originally Posted by jarski View Post
been also cautious (for years) about this "horrid" problem, but reading the article realized how trivial it actually is, also on digital. Sonnars (except very early ones) are optimized for f1.5, where the DOF is narrowest. once you stop down, DOF increases and hides practical shift effect. I doubt meaningful focus issue like "nose in focus, eyes unfocused" will not happen in f2.8 and beyond.
The difficulty with focus shift in a lens like the 50/1.5 Sonnar is that if the lens is adjusted for accurate focus at f1.5, as you stop down the depth-of-field increases, but it shifts backwards sufficiently that the point you focus on is in front of the plane of acceptable sharpness - and therefore unsharp. If you focus on an eye you end up with a sharp ear. The increase in depth-of-field in the 50/1.5 Sonnar does not overcome the focus shift until ~f4.5 as this test shows quite clearly: http://photo.net/leica-rangefinders-forum/00aKSW

Focus shift is not imaginary, or an invention, but it's also not fatal. Understanding it can facilitate you being able to better use of lenses that display it, and make photos that look like you want them to, which is the point after all.

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Old 04-30-2012   #18
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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/c12567a8003b8b6...5_bokeh_en.pdf
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Old 04-30-2012   #19
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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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Old 05-01-2012   #20
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Focus Shift is problematic for those taking pictures of cats.
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Old 05-01-2012   #21
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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).

Quote:
Originally Posted by JHutchins View Post
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/c12567a8003b8b6...5_bokeh_en.pdf
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Old 05-01-2012   #22
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Quote:
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Focus Shift is problematic for those taking pictures of cats.
Focus on the nose when stopped down, unless it's a persian
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Old 05-01-2012   #23
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Quote:
Originally Posted by 135format View Post
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.
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.
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Old 05-01-2012   #24
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Quote:
Originally Posted by ColSebastianMoran View Post
... "Focus on the nose if stopped down" is an interesting approach.
So what can they do in Iran where they are all Persians, or is it Iraq?

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Old 05-01-2012   #25
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Quote:
Originally Posted by ampguy View Post
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.
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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ok
Old 05-01-2012   #26
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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.



Quote:
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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 ...
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Old 05-01-2012   #27
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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.
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Old 05-01-2012   #28
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Quote:
Originally Posted by ampguy
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.
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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Old 05-01-2012   #29
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Quote:
Originally Posted by ampguy View Post
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.
... 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


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Old 05-01-2012   #30
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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!
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Old 05-01-2012   #31
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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.


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Try this
Old 05-01-2012   #32
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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.


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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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Old 05-01-2012   #33
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.............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?............
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?
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Old 05-01-2012   #34
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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).
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"??? "the digital sensor is absolutely flat while the film in a M7 or any M film body is not." "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"

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...
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Old 05-01-2012   #35
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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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Old 05-01-2012   #36
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Quote:
Originally Posted by ColSebastianMoran View Post
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.
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.
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Old 05-01-2012   #37
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Originally Posted by f16sunshine View Post
Focus shift is caused by the internet.
Yep. With that said, time to move on now. Nothing else to see here.
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Old 05-01-2012   #38
rxmd
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Quote:
Originally Posted by PKR

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/ecom...y.aspx?CID=169
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.
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Old 05-01-2012   #39
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Quote:
Originally Posted by ampguy
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.
What lens do you propose using?
At which apertures?
What behaviour do you expect except "there is more in focus"?
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Old 05-01-2012   #40
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Quote:
Originally Posted by Freakscene View Post
The difficulty with focus shift in a lens like the 50/1.5 Sonnar is that if the lens is adjusted for accurate focus at f1.5, as you stop down the depth-of-field increases, but it shifts backwards sufficiently that the point you focus on is in front of the plane of acceptable sharpness - and therefore unsharp. If you focus on an eye you end up with a sharp ear. The increase in depth-of-field in the 50/1.5 Sonnar does not overcome the focus shift until ~f4.5 as this test shows quite clearly: http://photo.net/leica-rangefinders-forum/00aKSW

Focus shift is not imaginary, or an invention, but it's also not fatal. Understanding it can facilitate you being able to better use of lenses that display it, and make photos that look like you want them to, which is the point after all.

Marty
Ok, but if I focus on something at 1.5, take a shot, then step down to say 2.8, does refocussing correct it - or is this lens always out of focus - even if it appears in focus in the viewfinder - from F2~4?
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