How does focus shift manifest itself?

[Freakscene]

Hmmm...is this something that occurs with every image or is it infrequent? The reason I ask is not knowing which camera, focus system, etc. I don't know what to think but it seems that the focus occurred right in the center of the image but the top half is more in focus than the lower left portion. Almost like a zone of out-of-focus area when compared to the rest of the image. Weird.

If it happens all the time, what can you do to fix it? If it is infrequent, what causes it? Questions that I would like to hear solved as I have never encountered this problem.:angel:

Focus shift is 100% consistent. It is a property of all optical systems. It happens in every lens. You only see it when the focus shift moves the plane of optical best focus away more than the increase in depth of field that occurs when you stop down. Erwin puts describes it for the f1 Noctilux here:
http://www.imx.nl/photo/leica/lenses/lenses/page56.html

My strategy was to make sure that my Noctilux was properly calbrated (it went to Leica for this) and then to check where the depth of field caught up with the focus shift. I found I get soft focus from f2.5-4.5. So I use it wide open to f2. I could also try to learn how much to lean in, but that would bug me, and I have a Hexanon 50/2 that doesn't display visible focus shift, so in practice I just use the Nocti wide open and the hexanon for everything else.

Marty

 

[jplomley]

Really hoping the M10 has an EVF to overcome all of these issues. In this manner, you are essentially focussing right off the sensor. Goodbye focus shift and goodbye calibration issues, each of which plague both of my M9's to some degree.

 

[ampguy]

interesting

interesting

If I'm understanding Erwin's info. right, by properly focusing the 50/1 noct (as with a NEX or extreme focus bracketing with a legacy focus system), then between f2-f5.6 you can get 10%-50% or better center contrast at 10-40lp/mm - when refocused with say a NEX, vs a RF.

When I had my 50/1 noct, I mostly used it on film, and rarely closed down more than f2, and f2.8, and never noticed the focus shift.

If I ever got a Noctilux again, and planned to use it stopped down, I'd use a NEX for it over an M8/M9/RF system any day.

The lens I have noticed with extreme focus shift outside the DOF have been the following:

Nikkor first version 43-86 zoom
Jupiter 3
Yashica GSN - between 1.7 and f2.0

Ones I've seen others have issues with, so never even bothered with an RF were the original non FLE Leica 35/2 asph (not aspherical), Zeiss C-Sonnar 50/1.5, CV 35/1.4, and CV 28/2. But, with the NEX, I'm not afraid to try these now (well, as far as focus shift goes...)

Lenses that others have had issues with, but I haven't on my M6, and M8 are the Leica pre-asph lux 35, 50, and German 75/1.4 (2 units). But the 35, and 50 pre-asphs were sent by Sherry K to Solms for specific alignments by their previous owners. To my knowledge, I was the 2nd owners of both German 75/1.4 lenses, and the first owners bought directly from US stores.

I also tested and experienced no significant focus shift on a CV 40/1.4. Harsh bokeh at 1.4 (to my eyes) didn't lend itself to being a keeper for me.

Focus shift is 100% consistent. It is a property of all optical systems. It happens in every lens. You only see it when the focus shift moves the plane of optical best focus away more than the increase in depth of field that occurs when you stop down. Erwin puts describes it for the f1 Noctilux here:
http://www.imx.nl/photo/leica/lenses/lenses/page56.html

My strategy was to make sure that my Noctilux was properly calbrated (it went to Leica for this) and then to check where the depth of field caught up with the focus shift. I found I get soft focus from f2.5-4.5. So I use it wide open to f2. I could also try to learn how much to lean in, but that would bug me, and I have a Hexanon 50/2 that doesn't display visible focus shift, so in practice I just use the Nocti wide open and the hexanon for everything else.

Marty

 

[ampguy]

It had better be a good evf ...

It had better be a good evf ...

by that time, we will all be measuring focus shift in nm instead of micron/micrometers ...

Really hoping the M10 has an EVF to overcome all of these issues. In this manner, you are essentially focussing right off the sensor. Goodbye focus shift and goodbye calibration issues, each of which plague both of my M9's to some degree.

 

[Mablo]

Having just bought a lens that I fully expect to have focus shift could someone please explain me a good way to test it. Unfortunately I don't have a digital cam that would accept LTM lenses.

 

[Chriscrawfordphoto]

Having just bought a lens that I fully expect to have focus shift could someone please explain me a good way to test it. Unfortunately I don't have a digital cam that would accept LTM lenses.

Put the camera on a tripod pointing at an object that is three demensional and focus on some point on the object. Shoot the thing wide open then at every f-stop on the lens. If the focus is accurate at all stops, there's no shift.

 

[Freakscene]

Put the camera on a tripod pointing at an object that is three demensional and focus on some point on the object. Shoot the thing wide open then at every f-stop on the lens. If the focus is accurate at all stops, there's no shift.

And look closely - make 8x enlargements or use a good lupe.

Marty

 

[rxmd]

There is some error in there too, I was using it more as an illustration, of the Nocti. I can show you photos of a lens align, but they're 100% uninteresting.

Sorry to be devil's advocate here, but what does it say about a phenomenon that the pictures where it manifests itself are the 100% uninteresting ones?

 

[ampguy]

well

well

The lensalign tests would be very interesting to me, but only if shot using a NEX. If shot with a RF system designed 100 years ago, there will be so much slop in the system, that the lensalign results would have minimal value without initially doing extensive focus bracketing and examining at high magnification.

What is flawed with Chris's suggestion is that upon reviewing the film results, one may think the 3d object is in focus, but it won't show where in the focus window the object lies, in otherwords, you can have focus shift, but having the subject of focus being covered by DOF, but it's still important because things in front of, and behind the subject will go in and out of focus sooner or later than they should or would at different apertures.

Basically, if you're still using a RF to focus, anything that is HCB sharp, is sharp enough. Plus if you're using film, film is never flat, so you have field curvature in your results regardless of your optics.

Did I miss anything?

Sorry to be devil's advocate here, but what does it say about a phenomenon that the pictures where it manifests itself are the 100% uninteresting ones?

 

[rxmd]

The lensalign tests would be very interesting to me, but only if shot using a NEX. If shot with a RF system designed 100 years ago, there will be so much slop in the system, that the lensalign results would have minimal value without initially doing extensive focus bracketing and examining at high magnification.

What is flawed with Chris's suggestion is that upon reviewing the film results, one may think the 3d object is in focus, but it won't show where in the focus window the object lies, in otherwords, you can have focus shift, but having the subject of focus being covered by DOF, but it's still important because things in front of, and behind the subject will go in and out of focus sooner or later than they should or would at different apertures.

Basically, if you're still using a RF to focus, anything that is HCB sharp, is sharp enough. Plus if you're using film, film is never flat, so you have field curvature in your results regardless of your optics.

Did I miss anything?

A number of comments:

Curvature of field is a property of the lens. It has nothing at all to do with film flatness, and the latter is not much of a practical problem in 35mm cameras to begin with; basically tension and a well-designed film path take care of that.

People have been using f/1.5 and faster lenses for a while, with rangefinders for the better part of 80 years, with open-aperture SLRs for fifty-five. If these systems were fraught with slop and intrinsic inaccuracies to the extent that rangefinders are too sloppy for 85/2, 75/1.4 or for that matter 50/1.5 lenses on principle, or that stopped-down viewing was the only way to get acceptably sharp images, I guess people would have complained a lot more than they actually did. :angel:

If now people suggest that the only way to get sharp images is to hunt for the image on a 15x enlarged live view on a miniature glossy LCD, always remembering to stop down first and focus later and to zoom out occasionally for framing, maybe there is something fundamental about taking pictures that I've been missing all the time.

It rather seems that focus shift is just one of the more recent fads introduced with the advent of pixel-peeping, and that the fact that you have to run tests with specially-arranged scenes at close distance ranges to even demonstrate the thing at all supports this view more than anything...

 

[dave lackey]

This is an interesting topic, focus shift. And I appreciate the helpful comments.

Still, and I apologize, but I am not grasping what it is or how it manifests itself yet. The image above still shows soft, but generally acceptable focus except for the area shown below:

So, is this really focus shift where only one quadrant seems adversely affected? Does it occur this way or is it usually more uniform?

Again, sorry to continue with questions.

 

[jplomley]

by that time, we will all be measuring focus shift in nm instead of micron/micrometers ...

Inconsequential. I've been using some of my Leica glass on the EP2 with EVF, glass that does exhibit focus shift when stopped down on my M9's. These shifts do not plague the EP2 as I am focussing the image right on the sensor with the lens already stopped down, so I am compensating for any shifts. With the magnification option of the EVF, my focussing accuracy is spot-on. Too bad I can't make this claim for a camera 10x the cost of an EP2.

 

[ampguy]

right

right

just curious, what are the 1 button magnifications of the u4/3 cameras? On the nex it is 7x, another press it is 14x mag.

by that time, we will all be measuring focus shift in nm instead of micron/micrometers ...

Inconsequential. I've been using some of my Leica glass on the EP2 with EVF, glass that does exhibit focus shift when stopped down on my M9's. These shifts do not plague the EP2 as I am focussing the image right on the sensor with the lens already stopped down, so I am compensating for any shifts. With the magnification option of the EVF, my focussing accuracy is spot-on. Too bad I can't make this claim for a camera 10x the cost of an EP2.

 

[ampguy]

well you're a bit confused with the nex

well you're a bit confused with the nex

With the nex live view, and focus shifting lens, you don't have to stop down or select your aperture first. And, even if it would be dark on an slr, the nex gains up so you can focus accurately at fast. The nex has variable, and a 1 button 7x mag. I use 7x or sometimes no mag most of the time, the 14x mainly for testing.

On SLRs, advanced and pro ones have always gone the extra mile for focusing accuracy, which includes a variety of screens, often more than 20 or so, and stop down of lens capability, if not the default. I have never owned an SLR out of many that didn't either a) require you to stop down to meter (and optionally focus, or focus check) - like Nikomats and even up to the F3 with non AI lens, nor have I had an SLR that didn't have a stop down DOF preview if the default setup was to view wide open (auto).

So the SLRs, or the usage you're implying, is probably for the casual tourist user, who even then probably selected the SLR for better focusing than a RF.

RF's were cool because they were smaller than SLR's for a while. Now we have the NEX and it is smaller than a RF.

A number of comments:
...

If now people suggest that the only way to get sharp images is to hunt for the image on a 15x enlarged live view on a miniature glossy LCD, always remembering to stop down first and focus later and to zoom out occasionally for framing, maybe there is something fundamental about taking pictures that I've been missing all the time.

...

 

[Freakscene]

This is an interesting topic, focus shift. And I appreciate the helpful comments.

Still, and I apologize, but I am not grasping what it is or how it manifests itself yet. The image above still shows soft, but generally acceptable focus except for the area shown below:

So, is this really focus shift where only one quadrant seems adversely affected? Does it occur this way or is it usually more uniform?

Again, sorry to continue with questions.

The photo is a bad illustration.

Think of depth of field as a theoretical band of acceptable sharpness in space that varies in width and with distance from the film or sensor plane as you stop down. It is not a flat field; lenses have a curved plane of optimal sharpness. You should get sharp photos wide open, although designers typically make compromises about this kind of thing too, as you can find out reading about the Zeiss 50/1.5 Sonnar. At close range and wide open the focus point of fast RF lenses is often ever-so-slightly in front of the point at which you focus - if the lens is poorly adjusted and this is too great, this can result in softness, but this is 'out of specification collimation', not 'focus shift'. As you stop down the focus point shifts away or towards the film or sensor plane (depending on the lens' design, but it is almost always away from you). You run into problems when this shift is greater than the increase in depth-of-field. The f1 Noctilux, for example, focuses correctly at f1, but depth-of-field doesn't catch up with focus shift after f2 until f5.6. This is especially apparent on the M8 and M9.

There are some good web tutorials on focus shift; Sean Reid and Lloyd Chambers in particular have great ones, but their sites are pay only access.

Marty

 

[rxmd]

With the nex live view, and focus shifting lens, you don't have to stop down or select your aperture first. And, even if it would be dark on an slr, the nex gains up so you can focus accurately at fast.

You have to stop down first. On a rangefinder lens stopping down and selecting the aperture are the same thing obviously. If you want to shoot at f/5.6, you'll have to focus at f/5.6, otherwise your focus will shift - assuming, of course, that focus shift was an actual problem to begin with.

On SLRs, advanced and pro ones have always gone the extra mile for focusing accuracy, which includes a variety of screens [...] nor have I had an SLR that didn't have a stop down DOF preview if the default setup was to view wide open (auto).

Then again, one might ask why the lever in question is called DOF preview. It would seem that using it for focus shift avoidance is so irrelevant in practice that it didn't even make it into most SLR manuals, photography textbooks etc. for the last 50 years or so.

RF's were cool because they were smaller than SLR's for a while. Now we have the NEX and it is smaller than a RF.

Yeah, it's small and everything, but I'm sorry to say operating it with manual lenses makes me feel rather dorky. I guess asking for a viewfinder is asking too much. I'm simply not a friend of chimping, and chimp focusing is worse.

Maybe it's just me. I don't mind working on the groundglass of a 4x5. It's darker and forces me to use a magnifier with wideangles, but at least it's larger than an oversized postage stamp and doesn't force me to zoom in and out to get the composition right, chasing the zoom window around the scene all the time. Also the final output is way better, so that the clumsiness of the operation is offset by the quality of the result somewhat.

It's nice that you like how with your EVIL camera you can shoot your lenses without being afraid of focus shift. But given how focus shift is in practice such a non-issue to begin with, this part of it does seem like a solution in search of a problem.

 

[Sparrow]

For me focus shift first manifested itself on the internet, up to that point it was just one of those theoretical things I knew about but never came across in the real world

... then along came this interweb paranoia where everything, however unlikely in practice, must be tested to destruction, maybe it's just one of those displacement activities the net is so good at

 

[Freakscene]

Focus shift has always been apparent with very fast lenses and lenses that push their design parameters e.g. fast normal Sonnars. The biggest reason that "focus shift avoidance is so irrelevant in practice that it didn't even make it into most SLR manuals, photography textbooks etc. for the last 50 years or so" is because we were using film until ~2000 or so. Film has depth so it buffers the point of focus and 35 mm film was mostly printed relatively small, where shift is less obvious. Then everyone got digital, where you have no depth in the sensor to buffer shift, you can look at everything on the screen at 500% and a 17" wide printer is commonplace. Suddenly everyone notices it a lot more. Photography has changed, not optics.

Marty

 

[Chriscrawfordphoto]

For me focus shift first manifested itself on the internet, up to that point it was just one of those theoretical things I knew about but never came across in the real world

... then along came this interweb paranoia where everything, however unlikely in practice, must be tested to destruction, maybe it's just one of those displacement activities the net is so good at

A lot of people online seem to think that denying reality makes them sound sophisticated. It doesn't, it makes you look foolish or blind. Given how many photos I see on RFF and elsewhere where someone photographed a person and the ears are in focus and the eyes are not, I think blindness is likely in many cases. Whether caused by focus shift, an out of adjustment camera, or simply a poor job of focusing, a lot of people seem unable to see that out of focus images suck and so they post them online for all to see. Some lenses do have focus shift, its easily verified by simply taking pictures. I don't get why that is so hard for some people to understand.

 

[ampguy]

I first experienced focus shift sometime around 2005 or so, and didn't even know there was a word for it then.

I had picked up some Nikon non AI lenses, and since I didn't know if I was going to keep them or not, I didn't have them upgraded, although the factory kits, and services to upgrade them were widely available (back then).

I only had an FT2, and F3, so with both of these I had to stop down to meter. I had a few different screens for the F3 and tried them all, but I was getting different focusing results when I would change the aperture and have to refocus slightly, which was very noticeable with the 43-86 zoom, but I also saw it on some primes (50/2 H, 28/2.8 non CRC, etc.) where just a touch up of focus rotation when viewing stopped down after changing from another aperture.

I printed and scanned these to verify that the cameras were ok, that what I was seeing in focus was in sharp focus, and what wasn't in sharp focus but a bit soft.

Later I read about it, subscribed to Sean Reids stuff, read the Puts stuff on it, talked about it with repair folks and how some lens like the Leica 35 asph (first one), and C-Sonnars just had it, but could be sort of setup or optimized around preferred apertures.

All of the above was on film.

If I had all wide open metering SLR setups (AI/AIS+) and minimal focus shifting lens, I would have never noticed it, but I would possibly wrongly conclude that some lens just aren't as sharp as others, when in fact they just needed a degree of focus adjustment one way or the other.

Another test I did with my particular Yashica GSN showed that with tripod and multiple roll setup, was that my 1.7 and f2 had shift at minimum distance. Not sure if all GSN's do, but my particular one does, visible in the standard scan, without loupe or magnifying.

I think that if your focusing is good, and what you focus on comes out focused, you shouldn't worry about it. But if your shots that you think you've focused properly come out softer than you think they should be, then I would do tests.