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06-28-2012, 06:03 PM   #31
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QuoteOriginally posted by bossa Quote
The wobble in those lenses is the front optical assembly (focus mechanism actually) and doesn't affect the projection onto the sensor by the rear group. It's kinda like a micro shift lens and really shouldn't affect the image quality at all. Mine is pin sharp and has a slight wobble.
Actually it does and the effect is quite significant but I explained this b4 and not going to repeat it again.

07-02-2012, 05:27 AM   #32
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i'm sure there is a bad Leica summicron 35mm f/2 out there somewhere.
07-02-2012, 05:39 AM   #33
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QuoteOriginally posted by wlachan Quote
Actually it does and the effect is quite significant but I explained this b4 and not going to repeat it again.
If you can't be bothered explaining then perhaps you can supply a link?
07-10-2012, 10:07 PM   #34
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I have had a chance to play with few more FA31mm, and between the couple that I owned and the 4-5 that I played with, only one of the FA31mm was free from any wobbliness in the lens hood. With that said, I find no correlation between wobbliness in lens hood and optical alignment (i.e., soft wide open shots).

Wlachan, perhaps you can elaborate on your theory of positive correlation between wide open performance and wobbliness in lens hood.

07-10-2012, 11:15 PM   #35
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It isn't a theory but actual finding after repairing some FA31. Most Pentax primes have a single fixed lens group that moves forward and backward for focusing. All the glasses are screwed together with tight tolerance and optical misalignment is less likely to happen. The FA31 is different because of its floating design which comprises of 3 lens groups that move independently. The design itself is fine but many production samples have wobbly 1st (and 3rd too) lens group that tilts slightly against the 2nd lens group. This results in optical misalignment which gets more obvious toward wide open. The misalignment can also be seen at f8 if severe. Whether this is practically significant depends on one's expectation, shooting habit and knowledge. To make matters more complicated, the front lens group tilts differently as the lens being rotated. The lens being optimized for landscape orientation will suffer in portrait orientation. For this reason, one should look for one with firm hood when buying new. The good news is that since the wobbly hood is due to imprecisely made inner metal barrels, a good one will stay good forever.
07-10-2012, 11:25 PM   #36
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That's good information. Assuming your findings are accurate, how could you explain the disparity between wide open performance of the several lenses when all of them have wobbly lens hood?
07-10-2012, 11:41 PM   #37
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The degree of misalignment of this lens is not only affected by the severity of the actual wobbliness from the 1st and 3rd lens group, but how well the 1st lens group was adjusted. The 1st group is held by 3 tiny screws and that lens group can be shifted along that axis slightly. Though it may not seem much, that tiny bit of adjustment has huge impact on the optical alignment, much more than the wobbliness itself. This means once the 1st lens group was removed, the original optical alignment would be gone forever (screw-in lens groups don't have this issue). What this means is that a FA31 with firm lens hood could also suffer from optical misalignment because 1) the 1st lens group might not be adjusted precisely in the factory, or 2) the 3rd lens group wobble, or 3) both. Since customers have no control on 1), all you can check is the wobbliness of the 1st & 3rd lens groups by shaking the lens.
07-11-2012, 12:01 AM   #38
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Many thanks. On the lens shaking test, I have shaken probably 6-7 FA31mm and I can feel some movements (some more some less) in all of them if you shake the lens hard enough.

On the optical alignment, if the lens was perfectly aligned at the factory, does the alignment change as the lens age? what about external shocks (e.g., normal bumps and whatnot)

07-11-2012, 12:41 AM   #39
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The alignment won't deteriorate due to age or normal wear & tear. Impact damage can however, damage any lens. If you are concerned, always land this lens on the rear instead of the hood.
07-11-2012, 12:43 AM   #40
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QuoteOriginally posted by wlachan Quote
The alignment won't deteriorate due to age or normal wear & tear. Impact damage can however, damage any lens. If you are concerned, always land this lens on the rear instead of the hood.
Any thoughts on the utility of your lens shaking test when all of the 6-7 FA31mm I have shaken produced some form of internal movement.
07-11-2012, 12:56 AM   #41
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The internal movement came from the rear (3rd) lens group. The impact won't be as critical as the 1st group and there are many AF lenses with wobbly inner groups (not just Pentax). The one I have is firm but I also came across some really loose ones. Ideally the whole lens should be firm but if I was forced to compromise, I would rather have a wobbly rear group instead of the front. You can also tell how wobbly the rear group is by pushing it with your finger or lenspen with it pointing down. The shop owner might think you are nut though. However, so long as it doesn't sound loose when shaking slightly, I would be happy (but I reckon this is rather subjective and takes experience). I would not shake the lens violently myself for this test.
07-11-2012, 03:49 PM   #42
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I have a four year old MIJ FA31 that has been a splendid performer until very recently, when it decided not to lock focus at infinity. This is especially true with autofocus, but also with manual focus. After four years, I won't hold the Japanese factory workers responsible. I expect that a small adjustment at the repair center will set things right again.

In response to the OP's question as to whether there are bad copies of the FA31, I would suggest looking at the user reviews on this site to find the answer. They are overwhelmingly positive with many bordering on ecstatic.

Rob
07-11-2012, 05:46 PM   #43
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QuoteOriginally posted by robgo2 Quote
I have a four year old MIJ FA31 that has been a splendid performer until very recently, when it decided not to lock focus at infinity. This is especially true with autofocus, but also with manual focus. After four years, I won't hold the Japanese factory workers responsible. I expect that a small adjustment at the repair center will set things right again.
This indicates the 3 screws holding the focus ring to the focusing mechanism are loose. This is due to insufficiently threadlocked in the factory and should be fixed asap because the contact brush underneath the focus ring would be destroyed when the focus ring was detached. This should never happen in the first place.

QuoteOriginally posted by robgo2 Quote
In response to the OP's question as to whether there are bad copies of the FA31, I would suggest looking at the user reviews on this site to find the answer. They are overwhelmingly positive with many bordering on ecstatic.
Which lens isn't? The truth is those are unhappy with their lenses would dump them on used market quietly.
07-11-2012, 07:55 PM   #44
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QuoteOriginally posted by robgo2 Quote
have a four year old MIJ FA31 that has been a splendid performer until very recently, when it decided not to lock focus at infinity. This is especially true with autofocus, but also with manual focus. After four years, I won't hold the Japanese factory workers responsible. I expect that a small adjustment at the repair center will set things right again.
QuoteOriginally posted by wlachan Quote
This indicates the 3 screws holding the focus ring to the focusing mechanism are loose. This is due to insufficiently threadlocked in the factory and should be fixed asap because the contact brush underneath the focus ring would be destroyed when the focus ring was detached. This should never happen in the first place.
I seem to be suffering from the same problem - I'll get mine to the repairman ASAP.
07-11-2012, 10:07 PM   #45
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QuoteOriginally posted by wlachan Quote
The internal movement came from the rear (3rd) lens group. The impact won't be as critical as the 1st group and there are many AF lenses with wobbly inner groups (not just Pentax). The one I have is firm but I also came across some really loose ones. Ideally the whole lens should be firm but if I was forced to compromise, I would rather have a wobbly rear group instead of the front. You can also tell how wobbly the rear group is by pushing it with your finger or lenspen with it pointing down. The shop owner might think you are nut though. However, so long as it doesn't sound loose when shaking slightly, I would be happy (but I reckon this is rather subjective and takes experience). I would not shake the lens violently myself for this test.
So the take-away is, from manufacturing variations/defects with the most impact on IQ to the least impact, factory misalignment >>> wobbliness in lens hood (1st group) > wobbliness in internal lens barrel (3rd group)

Let me know if that's correct.
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