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Thread started 03 Apr 2009 (Friday) 00:33
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50mm prime lens tests

 
wimg
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Apr 03, 2009 23:59 |  #16

joe mama wrote in post #7662881 (external link)
Interesting. I was under the impression that the floating RE of the 85 / 1.2L offset the shift.

The RE of the 85L is not a floating element; all but the rear end are floating, AFAIK. The RE is static, and AFAIK it serves to eliminate vignetting, not focus shift. At least, AFAIK, that is what these fixed elements at the back of a lens are used for normally. Note that the literature mentions a floating construction, not a floating element, and the 1 element I am sure about that it is not floating is the rear one. It is conceivable that elements 5 & 6 (which is a group), or elements 5, 6 and 7 move independently of the rest, thus constituting the floating construction. I haven't been able to verify that, however. To me it looks like the front 7 elements move when focusing.

Anyway, 1/4 of DoF at close distances only at F/2 and less at the rest of the apertures from F/1.4-F/1.6 to F/2.5 is well within tolerances for a fast lens, so nothing to worry about. I was amazed by this, however, because my 50L was calibrated to 1/6 of DoF at F/2.

The 85L is a modified Double Gaussian slightly telecentric design if I read the schema correctly, and the 50L is a modified Double Gaussian design as well. These designs are known to have strong SA especially at very large apertures. Used with deliberate care, this is what introduces great bokeh, at the expense of a focus shift potential at large apertures.

I've long been a pundit of correcting for the shift via software. That is, since the 50L reports focus distance back to the camera, and the camera also knows the f-ratio the camera is set to, the AF algorithms can *easily* be programmed to adjust for the shift.

In a way, this is what happens, just not in-camera. I understand from the Canon technician who calibrated both my 85L and 50L that these are the hardest lenses to calibrate, which is done with software on an optical test bench. Hardest of the two to calibrate, actually of all Canon lenses, is the 85L apparently.

Anyway, one interesting take I've heard on the issue is that the 50L "should be" calibrated for accurate AF wide open, and thus the shift would be visible when framing closely stopped down using center AF.

It is not only calibrated for wide open performance, at least, it shouldn't be. If that was the case it would be a 5-minute job rather than the hour and a half it took, by a trained technician.

However, a miscalibrated 50L might be dead-on at f/2, show slight front focus wider, and slight back focus narrower,

It should always be back focus, not front focus, when caused purely by SA. You'll find that the copies that have front focus generally have the worst problems: they tend to get focus wrong at almost any aperture and any distance. At least that is the conclusion I have drawn from all the problems I have read about here, on other forums and have spoken about with fellow shooters.

and the focus shift might go unnoticed, with people mistaking the shift for wide open softness. Don't know if it's true, but it sure sounds good. : )

That would be rather extreme softness, in my view :D. I couldn't live with that :D.

After the second round of calibration, when I personally delivered the 50L and spoke with the technician to explain my problem, and what I thought could cause it, it is fine, and well within spec as mentioned above. I did find and do find that focus shift was always worst at F/2 with all 5 copies I tested, both on 40D and 5D, although others had different results. These were all copies from different batches with different date codes.

In any event, by far the most important aspect to sharp pics at wide apertures is critically accurate AF, and at 50mm f/1.2, that's not the easiest thing to do, especially using off-center AF points, and even more so in low light.

Yes, agreed.

That's why I was so disappointed with the 5DII.

I would expect it to work better than the classic 5D's AF. It seems the 5D II system takes different wavelengths into account. I expect there to be less problems with AF in fluorescent lighting, just like the 50D, as compared to the 40D :D.

While it's a great camera, to be sure, I was really hoping for a killer AF system. I'd love just a few more AF points more spread out,

This is hard with a FF body, due to the angle at which the light comes in. I hope they'll manage to do something about this sooner rather than later, however.

with all having the same performance as the center AF.

Yes please, although I generally am fine with center point AF - used to this from the old analog days after all :D.

While I'm not saying that fast 50mm primes are incredible wide open, I think most people's disappointment with them stems more from the difficulty in achieven accurate AF than it does with the performance of the lens.

I would agree. I am also aware that the AF sytem employed by cameras is better and faster than anything I could achieve in 10X the time. I can only do as good, with luck, or aided by Live View or an Angle Finder in extremely slow mode. :D

For example, this gallery shows the potential of the Canon 50 / 1.4 wide open on the 5D:

http://www.pbase.com/j​oemama/on_a_good_day (external link)

Perfect? Of course not. But not too shabby, actually. The 50 / 1.2L has less difficulty nailing it wide open than the 50 / 1.4 (don't know why), but I prefer how it renders the image regardless (most likely contrast/bokeh). Still, if Canon made a 50 / 1.4 II that nailed the focus more consistently below f/2, I think a lot of people would call it a killer lens, even if the optics remained unchanged.

My main problem with the batch of 50 F/1.4s I tested was focusing close-up, combined with the (lack of) bite below F/2.8 at other distances, except for one which was ok at F/2. By that time I had already seen how the 50L rendered beyond the "problem zone", so I knew it was just a matter of finding the right one. When I found one that just backfocused by 2 cm at all distances and apertures, i.e., focus shift that was negibly small and masked by "normal" backfocus, I got that and had it calibrated, twice, to get it to work as it should :D.

Kind regards, Wim


EOS R & EOS 5 (analog) with a gaggle of primes & 3 zooms, OM-D E-M1 Mk II & Pen-F with 10 primes, 6 zooms, 3 Metabones adapters/speedboosters​, and an accessory plague

  
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joe ­ mama
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Apr 04, 2009 00:25 |  #17
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wimg wrote:
I would expect it [5DII AF] to work better than the classic 5D's AF. It seems the 5D II system takes different wavelengths into account. I expect there to be less problems with AF in fluorescent lighting, just like the 50D, as compared to the 40D.


Extremely interesting! I have most certainly noticed that the 5D AF does struggle more in non-multichromatic light, such as fluorescents. Man, I'm so freakin' poor right now, but since I shoot in that kind of light a lot, the 5DII might be a worth it. Say, do you have any spare money lying around? : )


--joe

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Apr 04, 2009 00:33 |  #18

joe mama wrote in post #7663975 (external link)
Extremely interesting! I have most certainly noticed that the 5D AF does struggle more in non-multichromatic light, such as fluorescents.

As did 350D, 400D and 40D ...

Man, I'm so freakin' poor right now, but since I shoot in that kind of light a lot, the 5DII might be a worth it. Say, do you have any spare money lying around? : )

About as much as you do, it seems :D ;).

I did test a 50D a while back, and I really hope the 5D II performs similarly when it comes to AF with different light sources (I don't expect the same speed, but that's fine :D). It did mention something vague like that in the specs, just like it did with the 50D. After selling off a bunch of lenses I can finally afford a 5D II, so I ordered one and I am keeping my fingers crossed :D.

Kind regards, Wim


EOS R & EOS 5 (analog) with a gaggle of primes & 3 zooms, OM-D E-M1 Mk II & Pen-F with 10 primes, 6 zooms, 3 Metabones adapters/speedboosters​, and an accessory plague

  
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Apr 04, 2009 00:50 |  #19
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wimg wrote in post #7664020 (external link)
As did 350D, 400D and 40D ...
About as much as you do, it seems :D ;).

I did test a 50D a while back, and I really hope the 5D II performs similarly when it comes to AF with different light sources (I don't expect the same speed, but that's fine :D). It did mention something vague like that in the specs, just like it did with the 50D. After selling off a bunch of lenses I can finally afford a 5D II, so I ordered one and I am keeping my fingers crossed :D.

Kind regards, Wim

I hear the 5D is a lot better than the 5DII. So, when you're disappointed with the 5DII, let me know and we can swap. I'll throw in an extra battery to make it fair. : )


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Shuko
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Apr 04, 2009 02:51 |  #20

wimg wrote in post #7663824 (external link)
The RE of the 85L is not a floating element; all but the rear end are floating, AFAIK. The RE is static, and AFAIK it serves to eliminate vignetting, not focus shift. At least, AFAIK, that is what these fixed elements at the back of a lens are used for normally. Note that the literature mentions a floating construction, not a floating element, and the 1 element I am sure about that it is not floating is the rear one. It is conceivable that elements 5 & 6 (which is a group), or elements 5, 6 and 7 move independently of the rest, thus constituting the floating construction. I haven't been able to verify that, however. To me it looks like the front 7 elements move when focusing.

There are at least 3 kinds of floating constructions. One where a group in the middle moves (24L etc), another where rear half moves (14L etc) and then there's the fixed rear element, which is used on 85 and 50mm.

From Canon Lens Work:

"The floating system in these lenses differs from that of wide-angle lenses in that it leaves the rear lens group fixed and extends the remainder of the lens system during focusing."

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Best for the last! Check what else it says in the book. ;)

"Another application of the floating effect is to prevent spherical aberration, which tends to become significantly large at close focusing distances with large aperture lenses. This is the main reason why a floating system is employed in lenses such as the EF 50mm f/1.2L USM, EF 85mm f/1.2L II USM, and EF-S 60mm f/2.8 Macro USM."

Should I sent in my 50L so they can add the missing lens? ;)

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Apr 04, 2009 18:12 |  #21

joe mama wrote in post #7664089 (external link)
I hear the 5D is a lot better than the 5DII. So, when you're disappointed with the 5DII, let me know and we can swap. I'll throw in an extra battery to make it fair. : )

Nah, already got a 5D, and 6 spare batteries if I am not mistaken :D, so I'll pass on that generous offer :lol:.

Kind regards, Wim ;)


EOS R & EOS 5 (analog) with a gaggle of primes & 3 zooms, OM-D E-M1 Mk II & Pen-F with 10 primes, 6 zooms, 3 Metabones adapters/speedboosters​, and an accessory plague

  
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Apr 04, 2009 18:56 |  #22

Hi Shuko,

Thanks for this information! I always forget how much info there is in Lens Work.

Shuko wrote in post #7664374 (external link)
There are at least 3 kinds of floating constructions. One where a group in the middle moves (24L etc), another where rear half moves (14L etc) and then there's the fixed rear element, which is used on 85 and 50mm.From Canon Lens Work:

"The floating system in these lenses differs from that of wide-angle lenses in that it leaves the rear lens group fixed and extends the remainder of the lens system during focusing."

IMAGE NOT FOUND
HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO


Best for the last! Check what else it says in the book. ;)

"Another application of the floating effect is to prevent spherical aberration, which tends to become significantly large at close focusing distances with large aperture lenses. This is the main reason why a floating system is employed in lenses such as the EF 50mm f/1.2L USM, EF 85mm f/1.2L II USM, and EF-S 60mm f/2.8 Macro USM."

Should I sent in my 50L so they can add the missing lens? ;)

Ok, I thought so, with the 85L that is. This means that the floating construction is the front 7 elements, after all the last element is the fixed one :).

Anyway, I'll check out my 50L and 85L again, as soon as I am fit enough to go up and down the stairs: I had knee surgery on Wednesday, and I am not very mobile yet.

What is interesting is that the aspherical element in the 50L is the last element in the lens, where with the 85L it is the third. It would mean that the asphrical element is the fixed one in the 50L :D. Alternatively, all elements are floating, but then, so are they with most lenses :D. And no, I don't think there is an element missing from your 50L :D. At the Canon Camera Museum it mentions that the last element in the 50L is there to minimize aberrations when focusing, so maybe it is fixed. I don't want to know how, in that case, how images would have looked like without that last element, or how focusing would have worked :D.

The EF-S 60 employs IF, but maybe the design has both IF and a floating construction with a fixed last element, which is possible I suppose. It has two diaphragms as well: the second one is there to prevent flare.

Kind regards, Wim


EOS R & EOS 5 (analog) with a gaggle of primes & 3 zooms, OM-D E-M1 Mk II & Pen-F with 10 primes, 6 zooms, 3 Metabones adapters/speedboosters​, and an accessory plague

  
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Apr 04, 2009 23:35 |  #23
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Shuko wrote:
"Another application of the floating effect is to prevent spherical aberration, which tends to become significantly large at close focusing distances with large aperture lenses. This is the main reason why a floating system is employed in lenses such as the EF 50mm f/1.2L USM, EF 85mm f/1.2L II USM, and EF-S 60mm f/2.8 Macro USM."


Interesting!

wimg wrote:
Nah, already got a 5D, and 6 spare batteries if I am not mistaken , so I'll pass on that generous offer.


Man, I try to be nice to some people, but what do I get in return? : )

I had knee surgery on Wednesday, and I am not very mobile yet.


Suck it up and get to testing. Did Ronnie Lott let a little thing like a finger get in the way of what needed to be done? : )


--joe

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Apr 05, 2009 06:05 |  #24

joe mama wrote in post #7669206 (external link)
Interesting!

Man, I try to be nice to some people, but what do I get in return? : )

:p

Let's see, let me be equally nice: you'll get me a good used 1Ds III, low shuttercount, and I'll trade you a 2 classical 5Ds with a bunch of spare batteries. Together, at 24 MP, that is more than the MP-count of the 1Ds III!

:lol:

Suck it up and get to testing. Did Ronnie Lott let a little thing like a finger get in the way of what needed to be done? : )

Well, I don't let my finger get in the way either ... :D

Kind regards, Wim ;)


EOS R & EOS 5 (analog) with a gaggle of primes & 3 zooms, OM-D E-M1 Mk II & Pen-F with 10 primes, 6 zooms, 3 Metabones adapters/speedboosters​, and an accessory plague

  
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Apr 05, 2009 11:28 as a reply to  @ wimg's post |  #25

joe mama wrote in post #7662881 (external link)
That is, since the 50L reports focus distance back to the camera, and the camera also knows the f-ratio the camera is set to, the AF algorithms can *easily* be programmed to adjust for the shift.

But how does the camera know what f-stop you will use? This makes the assumption of M/Av mode with AF confirmation immediately preceeding shutter action (and without enabling safety shift). Forget using the AF-On button, custom function #4, DOF bracketing, and welcome to necessitated AF between parameter changes - I haven't shot that tied down since my 300D. :confused:


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Apr 05, 2009 15:30 |  #26
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Chez Wimpy wrote in post #7671288 (external link)
But how does the camera know what f-stop you will use?

Since the camera tells the lens what f-stop to use, the camera knows the f-stop being used before the lens does.

This makes the assumption of M/Av mode with AF confirmation immediately preceeding shutter action (and without enabling safety shift). Forget using the AF-On button, custom function #4, DOF bracketing, and welcome to necessitated AF between parameter changes - I haven't shot that tied down since my 300D. :confused:

I don't see how. Any time the f-stop on the lens changes, the camera knows before the lens. If the f-stop changes, then there may have to be an adjustment to the focus. However, as the entire idea behind a software correction to the focus shift is that it is predictable, then the camera need not refocus for a changing aperture, but rather adjust the focus by a predetermined amount instead.

And, since the focus shift applies only for close focusing stopped down shooting using center AF, I don't imagine that any lag between the exposure lock and the focusing corrections would be an issue for that type of photography.


--joe

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50mm prime lens tests
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