[Sandy Hancock]

A few of my (DA) limiteds have a little bit of wobble, and they continue to function perfectly. My FA31 has none that I can elicit though.
I'll check more thoroughly when I get home.

[Driline]

All great shots! .#2 is some serious eye candy. #4 is incredible too.. thanks for sharing..

I think you're mistaken. I believe you meant to say that picture 4 with the cute blonde was the "eye candy" shot

[Apapukas]

Found a potential used copy for sale. I asked him about the wobble and here was his reply: "Never noticed it until you asked, but if you try to wiggle it, you can, just a little. Not sure if that's a defect or just the way they are."

So the question is, how much wobble is too much wobble?

I asked Pentax USA and they said that it is nothing unusual if 31 Ltd has a slight wobble. I don't know what does that mean, but I am sending mine for repair. This wobble (slight) is just annoying.

[todd]

I think you're mistaken. I believe you meant to say that picture 4 with the cute blonde was the "eye candy" shot

Haha! I admit that I thought about saying that.. Skin tones render so well with this lens!

I asked Pentax USA and they said that it is nothing unusual if 31 Ltd has a slight wobble. I don't know what does that mean, but I am sending mine for repair. This wobble (slight) is just annoying.

Yeah, if you're saying a slight wobble is annoying, I would imagine this is the same slight wobble my prospective seller is identifying.... Hrmph.. Thanks!

[twitch]

One of my DA limiteds had slight wobble but my FA limiteds, including the 31, have no wobble at all.

[bossa]

One of my DA limiteds had slight wobble but my FA limiteds, including the 31, have no wobble at all.

Were all of these lenses bought locally and supplied by CR Kennedy?

I had a 31 with some wobble and prior to that a silver one for a week that had too much so I returned it.. in fact the person who bought that lens after me just sold it on eBay for a song. The FA77 I had was rock solid.. no wobble. I'm not sure if it makes a huge difference when the lens is stopped down but it might do at wider apertures.

[Sandy Hancock]

Just checked:

DA15 and DA70 have the most wobble, but they are both stellar performers and focus quick and true.
DA21 and DA40 have a little.
FA31 and FA43 hardly any.
FA77 none at all.

I have no idea if it means anything, but I suspect it doesn't. Relax and enjoy these wonderful lenses.

[todd]

Thanks Sandy.., My DA15 has none though my FA50 seems to have a slight wobble.. It's posts like this and this that cause me some hesitation in buying used.. It would be nice to save some dough but I really don't want annoyances either... OTOH, I suppose my own experience could be an indicator, because I do recall noticing the slight wobble in my FA50 before, but was able to accept and ignore it..

[chesebert]

Found a potential used copy for sale. I asked him about the wobble and here was his reply: "Never noticed it until you asked, but if you try to wiggle it, you can, just a little. Not sure if that's a defect or just the way they are."

So the question is, how much wobble is too much wobble?

For FA31, wobble of any kind may have a negative effect on the picture quality. However the effect is one of degree and needs to be evaluated by looking at actual photos. If you can see the effect only at 100% and with serious pixel peeing, then the lens may be acceptable to you.

[wlachan]

The difference between the FA31 and other Pentax small primes is that the latter have fixed group design, while the FA31 is a floating design.

A fixed group lens moves the whole inner barrel forward or backward to focus. While the while lens group might not be perfectly perpendicular to the sensor due to wobbliness, the optical alignment of the lens itself is maintained.

The FA31 however is different. 3 lens groups are moving independently of each other, and the front or the rear group (or both) may wobble due to less than perfect inner barrel dimensions (a rather loose manufacturing tolerance to reduce cost imo because imprecise barrels were not discarded). Since the dimensions of the inner barrels are fixed, the only way to address the problem is to identify and replace that(those) imprecise barrel(s) (there is nothing to be calibrated by screw etc). For service centers outside Japan, it is quite unlikely they would go this route as it will be time consuming and a hit & miss approach and may take forever because you never know how precise the new barrels will be from Japan. So they might just return your lens claiming it was within spec, or just give you a new one.

As to whether the front or rear lens group affects the optical alignment, I strongly believe it does, and I tested a few of them to confirm, especially at wide open. Whether it matters to anyone depends on your experience, expectation, subject matters, and how bad the lens is. Like the DA16-45/4, I feel Pentax had made a huge mistake by not making them probably because the optical designs are very fine. Then again, I am probably the minority to really care I guess.

[Driline]

If you can see the effect only at 100% and with serious pixel peeing, then the lens may be acceptable to you.

Urine of any kind is never acceptable on my lenses

[bossa]

Just checked:

DA15 and DA70 have the most wobble, but they are both stellar performers and focus quick and true.
DA21 and DA40 have a little.
FA31 and FA43 hardly any.
FA77 none at all.

I have no idea if it means anything, but I suspect it doesn't. Relax and enjoy these wonderful lenses.

Thanks Sandy, could you indicate if these lenses were bought locally through a store new or used via eBay and/or if they were parallel imports? I've never bought a grey/parallel import and am starting to seriously consider doing so. I was quoted $989 for a FA 43LTD from CameraHouse in the Adelaide Central Market whereas I can buy one for $580 from OS. If I were in the US I could even grab one for 380 odd minus the $250 rebate if a K-5ii is bought at the same time. This is a horrifying price differential.

[todd]

As to whether the front or rear lens group affects the optical alignment, I strongly believe it does, and I tested a few of them to confirm, especially at wide open. Whether it matters to anyone depends on your experience, expectation, subject matters, and how bad the lens is. Like the DA16-45/4, I feel Pentax had made a huge mistake by not making them probably because the optical designs are very fine. Then again, I am probably the minority to really care I guess.

Thanks for sharing all this! Were your tests the pixel 'peeing' types of tests that chesebert mentioned?

Would an example of a test be to take a picture in landscape orientation and then try to take that same shot in portrait to compare? Seems like it would be hard to notice a difference unless it was drastic...

[chesebert]

Urine of any kind is never acceptable on my lenses

Thank you for catching the typo. Now, go and use that attention to detail to find yourself a perfect FA31.

[wlachan]

Thanks for sharing all this! Were your tests the pixel 'peeing' types of tests that chesebert mentioned?

Would an example of a test be to take a picture in landscape orientation and then try to take that same shot in portrait to compare? Seems like it would be hard to notice a difference unless it was drastic...

I tested and view the results on my monitor at 100%, I am not sure if that qualified as pixel peeping. What prompted me to check it out was when my FA31 did fine on film, okay on Ds but quite badly on K-m. When I came across a perfect FA31, I stripped them down for comparison and found the flaw.

My tests were done shooting a stucco wall at about 1 meter away with careful alignment in both landscape and portrait orientation on tripod. I repeated the tests many many times before I was convinced of my own conclusion. But as I said, whether a particular lens with less than perfect alignment could pose any practical issue depends on your experience, expectation and subject matter. I often do landscape which can reveal misalignment easily, while portrait is rather forgiving (but wide open off center portrait can be a problem too). With that said, I recommend buying from B&H and use their exchange policy when needed. The performance of a good copy is well worth the trouble imho, especially when there aren't many great lenses to choose from (no offense intended).