Reid Review: 28 2.0 Ultron focus shift

One reason that Leica have moved towards using aspherical elements is to minimise spherical aberration, which also minimises focus shift. The C-V lens shows some spherical aberration and therefore focus shift. For many uses, no problem. You can also learn to get around it. 'Sharpness' is over rated, after all.

Marty[/QUOTE]

The interesting thing here is that I am getting away from the Leica Asphericals, just becouse of the bokeh and higher flare. I had the 21/2,8 Asph, the 24f2.8 Asph and the 35f1.4 Asph and in the end decided that I did not like these lenses with bl/w film! I kept my 35f2 Asph for the times when I need bitingly sharp 35 shots, also kept my 50f1.4 Asph for the same reason - but I cant say that I like them as "regular" lenses. The 75f2 Apo-Asph has a more "blended" quality and that one I have kept because of that. The arrival of the CV and Zeiss lenses has given us a choice of how we like the image to look, beyond mere sharpness.
 
aizan said:
when does the focus shift become an issue on the m8? f2 and minimum focus...?
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When you stop down, the focal plane of the lens does not coincide with the rangefinder any more, but is somewhere behind the subject.
That was something I didn't notice on my two copies of the Ultron. I returned them for other reasons, though.

Cheers,
Uwe
 
Tom A said:
One reason that Leica have moved towards using aspherical elements is to minimise spherical aberration, which also minimises focus shift. The C-V lens shows some spherical aberration and therefore focus shift. For many uses, no problem. You can also learn to get around it. 'Sharpness' is over rated, after all.

Marty
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The interesting thing here is that I am getting away from the Leica Asphericals, just becouse of the bokeh and higher flare. I had the 21/2,8 Asph, the 24f2.8 Asph and the 35f1.4 Asph and in the end decided that I did not like these lenses with bl/w film! I kept my 35f2 Asph for the times when I need bitingly sharp 35 shots, also kept my 50f1.4 Asph for the same reason - but I cant say that I like them as "regular" lenses. The 75f2 Apo-Asph has a more "blended" quality and that one I have kept because of that. The arrival of the CV and Zeiss lenses has given us a choice of how we like the image to look, beyond mere sharpness.[/quote]

Tom - sure, how photos look is an aesthetic consideration - and in aesthetics anything goes.

The Leica 75/2 AA and to a lesser extent the 50/1.4 ASPH are designed differently to the earlier ASPH and aspherical lenses - to some extent the control of spherical aberrations is more moderate - the 35 Summilux ASPH and aspherical and the 35 Summicron ASPH in particular are over-corrected for spherical aberrations, which introduces problems of its own. They are also extremely high contrast designs. Combined, these do seem to make them more flare prone than other designs.

What is most interesting is that the only Leica lenses that display sufficiently little focus shift on the M8 for it not to be visible are the 75/2AA and the 50/1.4 ASPH. To limit focus shift to a point where it is not measureable when determining MTFs, a design needs a floating element _and_ an aspherical element.

Buy what you like, of course.

Marty
 
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aizan said:
when does the focus shift become an issue on the m8? f2 and minimum focus...?
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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. With the CV 28/2 you should get sharp photos at f2, although designers typically make compromises about this kind of thing too. At close range and wide open the focus point of fast RF lenses is often ever-so-slightly in front or behind 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). You run into problems when this shift is greater than the increase in depth-of-field. I haven't tested the C-V 28/2, but the f1 Noctilux, for example, focuses correctly at f1, but depth-of-field doesn't catch up with focus shift until f5.6. This is especially apparent on the M8. The answer for me is simple - use the Nocti on the M8 at only f1 or f5.6 or larger.

Anyone who uses a digital camera and says that focus shift is irrelevant in real-world photography either:
  • doesn't mind where their plane of focus lies
  • habitually uses apertures at which depth-of-field has caught up with the focus shift
  • isn't looking carefully enough.
  • uses lenses whose focus point and depth-of-field always correlate (typically slower lenses)
There is nothing wrong with any of those approaches but it is wrong to state that focus shift is only theoretical or that it is irrelevant.

We could also just petition all lens manufacturers to redesign all the lenses in their ranges to make all lenses include a floating element and one or more aspherical elements.

Marty
 
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I don't subscribe to his site, but I would guess that he mounts the camera to a tripod when conducting the tests, correct?

Depth of field at 3 feet for a 28mm lens at f/2.0 is roughly 4 inches. However, I suppose that if your point of focus is at the near point or rear point of the depth of field, then you wouldn't have any margin for error.
 
All,

Very interesting and useful thread. thanks for this all.

I have the CV 35 1.4 and really like it's compact size and on film use - I've not noticed this focus shift but then again 1) I'm more concerned whether "i got the shot" than absolute sharpness; and 2) I have been using 100ASA films so i often get some blur from slow shutters anyway.

Using a loan m8 of late did not make me want to rush out and get the 35 f2 ASPH. The 50 1.4 ASPH however is a different beast and makes me want to save up for it. Completely different look to my Canon 50/1.4 LTM but wow...impressive but not overly clinical. Interesting to note Tom's point about the newer vs original ASPH designs.
 
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These threads seems to happen every time Sean Reid does a review and every time it must be pointed out that he only does his reviews on the M8, unless you plan on using this lens on an M8 then his reviews are of little benefit to you. Every time these threads start someone says that a bad review from Sean Reid will effect sales, it will not in any way shape or form, I think to much credit is placed on the power of his reviews and the amount of people that actually pay for them to which not many film only users would bother subscribing. Toms reviews always hold far more real world film relevance and they are free. The lens does not have focus shift on any film camera for which this lens was designed. Great lens, great value
 
Palaeoboy said:
These threads seems to happen every time Sean Reid does a review and every time it must be pointed out that he only does his reviews on the M8, unless you plan on using this lens on an M8 then his reviews are of little benefit to you. Every time these threads start someone says that a bad review from Sean Reid will effect sales, it will not in any way shape or form, I think to much credit is placed on the power of his reviews and the amount of people that actually pay for them to which not many film only users would bother subscribing. Toms reviews always hold far more real world film relevance and they are free. The lens does not have focus shift on any film camera for which this lens was designed. Great lens, great value
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To say that his reviews have little value to film-only users is maybe a bit extreme. He is very detailed about how he evaluates these lenses, and more importantly, shows you the actual data so that you can see for yourself and form your own opinion. In addition, his reviews also include many actual photos taken with the lens in question, not just shots of walls, so that you can get a sense of how the lens "draws" in day to day use. Despite the focus shift, the actual photos taken with the lens look very nice, and he compliments the "look" of this lens throughout his review.

Tom's photos are also an excellent resource, and although a bit less "scientific," give a very good sense of how the lens performs on a film camera in a wide variety of situations.

With all this information, one gets a very good sense of how a particular lens performs in a variety of settings. Maybe focus shift is less of an issue with film, but its good to know about it, and then decide how important it will be to your style of shooting and the equipment that you use...
 
The lens does not have focus shift on any film camera for which this lens was designed.

Focus shift occurs independantly of the camera; it is an optical property of lenses. It may or may not be significant for use on film cameras, but without relevant testing we can't tell.

Marty
 
Every time these threads start someone says that a bad review from Sean Reid will effect sales, it will not in any way shape or form, I think to much credit is placed on the power of his reviews and the amount of people that actually pay for them to which not many film only users would bother subscribing. Toms reviews always hold far more real world film relevance and they are free. The lens does not have focus shift on any film camera for which this lens was designed. Great lens, great value
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It will affect sales, IMO. I have an R-D1 and I'm considering an M8, so focus shift on a digital body does matter to me. Just how many pure film shooters do you imagine are out there nowadays, anyway? Outside the confines of the RFF, I would imagine the number is shrinking daily.

And focus shift is focus shift, regardless what camera you mount the lens to. Film is more forgiving because there's more depth to film emulsion than there is a digital sensor, but in my experience with motion picture cameras, some lenses do indeed have noticeable focus shift even when using film.
 
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ZeissFan said:
I don't subscribe to his site, but I would guess that he mounts the camera to a tripod when conducting the tests, correct?

Depth of field at 3 feet for a 28mm lens at f/2.0 is roughly 4 inches. However, I suppose that if your point of focus is at the near point or rear point of the depth of field, then you wouldn't have any margin for error.
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First of all, I must say that this discussion is much more even-keeled than I had feared. I'm pleasantly surprised.

Not only are my res. tests done on a tripod but they're done under controlled conditions and using a very extensive focus bracketing methodology to rule out the possibility that a focus error will confound the results. The overall methodology has evolved over years of testing many, many lenses.

Focus shift is real and it will matter to some photographers for some work. That said, I did much of a (several thousand dollar) shoot with the CV 28/2.0 earlier this year and was very happy with the results overall. I love the way the lens draws.

That said, it is worth knowing that this lens does shift focus and that the change is most noticeable around F/4 and F/5.6. The aspherical 28/1.9 does not show this same behavior. Will it matter to all photographers? - not necessarily.

I try to report everything I can about a lens. Then the photographer can decide how that fits with what he or she needs.

It's possible that a bad sample of this lens could show focus shift but I suspect that this is a result of spherical aberration within the design itself. Stephen is sending me two more copies that I'll test for comparison.

Bad lens samples tend to do things like misfocus wide open or show signs of decentering. I've not yet encountered focus shift as a result of sample variation. But I'll make sure.

I'll quote from my own LUF post in response to a question about this subject:

"Thanks for the article comments. Lens design is certainly not simple and I won't pretend to be a lens designer here. But note the following trend among fast CV (rather than VC) lenses:

Focus shift:

CV 35/1.4
CV 40/1.4
CV 28/2.0


Little or no focus shift:

CV 28/1.9 Aspherical
CV 35/1.2 Aspherical
CV 35/1.7 Aspherical
CV 50/1.5 Aspherical

Interesting trend - yes? Now consider the nature of spherical aberrations. Here's an interesting page that I linked in the review: http://www.vanwalree.com/optics/spherical.html

Keep in mind that at apertures like F/2.8 and F/4.0, the 28/2 is going to show strong resolution *somewhere*. It just won't be at the plane where the lens was focused wide open. That fact sometimes seems to keep some people from recognizing focus shift in certain lenses. They may point to examples made at, say, F/4.0 that show excellent resolution. But of course, the plane where that focus exists isn't the same one they'd see if the lens was wide open."


Cheers,

Sean
 
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Freakscene said:
The lens does not have focus shift on any film camera for which this lens was designed.

Focus shift occurs independantly of the camera; it is an optical property of lenses. It may or may not be significant for use on film cameras, but without relevant testing we can't tell.

Marty
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I was just about to make the same comment.

Cheers,

Sean
 
Palaeoboy said:
T. Every time these threads start someone says that a bad review from Sean Reid will effect sales, it will not in any way shape or form.
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This was not a "bad" review. It looked at many aspects of the 28/2 - including various strengths.

Cheers,
 
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Sean,

Thanks for your detailed reply. While I have not noticed any focus shift in my Ultron 28/2, maybe I do not know what to look for. I have simply been very impressed with the photographs that I have taken with this lens and that was good enough for me.

Couple of questions...

How much does the focus shift... are we talking a few millimetres or a couple of centimetres?

There seems to be a correlation between aspherical designs and lack of focus shift, so am I right to assume the Zeiss ZM series would also display some focus shift since the lens are traditional spherical designs or are the Zeiss designs 'better' than the less expensive CV designs?

Thanks,
 
I don't subscribe to Sean's site, but I do like his reviews on Luminous Landscape, and he's clearly doing good work. But I do fear that an internet culture has developed, of pixel-peeping fussiness among people who are preoccupied with the idea of perfection, at the expense of seeing the big picture. (Sean, I certainly don't think you're one of these people.)

A good example is the new Pentax DA35 f/2.8, which was widely regarded as a miracle lens on the forums, until a couple of sharpness tests came out, saying it was actually a weak performer as a "standard" lens. But I bought it anyway, and it's freaking stunning. Every picture, when you look at the whole, is wonderful, at all apertures, with superb contrast, dynamic range, and color. In other words, it takes great pictures.

This is why I liked Michael's review of the Panasonic LX3 on LL, as well--he took a bunch of pictures and looked at them and declared them good; thus, he declared the camera good. This seems to me a healthy attitude toward the equipment we like to use.

I'm sure there are many photographers for whom absolute sharpness at all apertures is a necessity, and these photographers should consider Sean a fantastic resource. But it bothers me that there also seem to be so many photographers who will turn their noses up at a useful tool because they read somewhere that it's soft at f/4, when such a "problem" would never affect their work in any significant way. Their loss, I suppose, but personally, I think that, if you can't take good pictures with CV gear, then you can't take good pictures, period.
 
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