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02-20-2012, 05:45 AM   #1
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Focus shift with fast lenses

In another thread lytrytyr pointed out the possible problem of focus shift when focusing fast lenses like my A*85mm f1.4. As I never had heard of this problem before I was puzzled. Doing my homework I found that the problem has been described, like here and here, but I was still as puzzled. If this was a common problem with fast lenses, why had no one even mentioned it in the lengthy praisals of lenses such as the A*/FA* 85mm f1.4 or the A*135mm f1.8? Not to mention the M/A/F/FA 50mm f1.4 lenses.

Only one thing to do. Print a test chart, pull out the tripod and start testing. I have two (well, three, but the F and FA fifties are basically the same) f1.4 lenses, the A* 85mm and the F 50mm.

Tripod 2 sec mirror lock up. Focused manually at f1.4. No refocusing when stopped down. The words "Focus here" in center of original picture. Pictures converted to jpg and pp-ed with auto contrast in Photoshop, no sharpening. Resized and cropped for forum posting. Here's the result, starting with the 50mm.


F 50mm f1.4@f1.4

Name:  50mm f1,4.jpg
Views: 825
Size:  318.1 KB


F 50mm f1.4@f2

Name:  50mm f2,0.jpg
Views: 679
Size:  329.0 KB


F 50mm f1.4@f2.8

Name:  50mm f2,8.jpg
Views: 1086
Size:  362.1 KB


As you can see, there is a bit of focus shift, but the intended focus still sharp due to increased DOF. The tiny focus shift also looks a bit stronger due to a slight back focus to start with. We're talking of fractions of mm's here. I'd say that with this lens, the focus shift problem is measurable but hardly a real life issue.

A*85mm in next post.

02-20-2012, 06:02 AM - 1 Like   #2
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The 85mm proved to be easier to focus. Not a surprise perhaps, because of the enlargement due to longer FL, but also due to the longer throw and better damping of the focus ring.


A* 85mm f1.4@1.4

Name:  85 mm f1,4.jpg
Views: 543
Size:  295.4 KB


A* 85mm f1.4@2.0

Name:  85mm f2,0.jpg
Views: 520
Size:  302.3 KB


A* 85mm f1.4@2.8

Name:  85mm f2,8.jpg
Views: 583
Size:  343.2 KB


Pentax lens engineering at it's peak. I can't see any focus shift at all, can you? To my knowledge, there is no aspherical lens element in this lens. As I have understood it, AL is mainly used in WA lenses. Can it be that focus shift is a problem that decreases with increased focal length?

Apparently, the problem I had with getting the focus right wasn't due to focus shift, but to the Pentax matte focus screen. That's why my problem was solved when I exhanged it. Read about that here.

Looking forwards to the comments from other, and more knowledgeable, users of fast lenses.

Kjell
02-20-2012, 09:21 AM   #3
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I am not sure the test of the 50mm shows focus shift. To me it looks more like you were back focused on the 1.4 shot and stopping down just improved the DOF but kept the focus point the same which to me looks like about half way between the "focus here" and back 2mm mark.
02-20-2012, 12:10 PM   #4
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I can't comment on specific lens but some fast lenses do shift focus when stopped down (not just Pentax). This has to do with optical designs, or limitations. I know this shift might be compensated by AF for some lenses. For instance, the DA14 AF will choose a closer focus from f5.6 than wider aperture.

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