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02-18-2014, 02:20 PM   #1
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Help! Why won't f/2.8 200mm autofocus?

Just bought a used Pentax f/2.8 200mm from KEH, in LN- condition, to use with my K-30.

Problem -- Outdoors, in daylight, the autofocus doesn't seem to work. It hunts back and forth 2 or three times, often not getting a sharp focus at all. Tried my usual spot focus and select focus (at 5 and 11 active points) . Also tried it on my K-01, same problem. Tried it at different apertures, on M, TAv, Av, and P... on AF.S and on AF.C... makes no difference.

The manual focus works perfectly, nice and smooth. And, on autofocus, the lens motor is very quiet.

What's going on? Am I doing something wrong -- or do I have a bad lens? My other telephoto, a 55-300mm, works correctly on both cameras. Any ideas?

02-18-2014, 02:22 PM   #2
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Send it back.
02-18-2014, 02:44 PM   #3
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Sounds like it has the dreaded SDM-itis.

Send it back, it needs an expensive repair.
02-18-2014, 03:06 PM   #4
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QuoteOriginally posted by boriscleto Quote
Send it back.
QuoteOriginally posted by Jonathan Mac Quote
Sounds like it has the dreaded SDM-itis.

Send it back, it needs an expensive repair.
Me three! KEH has a very good return policy. Sorry your lens was a disappointment.


Steve

02-18-2014, 03:14 PM   #5
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Tell me about "SDM-itis" -- are all SDM-focus lenses bad... or is this a rare occurence? If KEH sends me one that works, will it go bad in future ... rarely, likely, or always?
02-18-2014, 03:24 PM   #6
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QuoteOriginally posted by jon404 Quote
are all SDM-focus lenses bad
No
QuoteOriginally posted by jon404 Quote
is this a rare occurence
Also no but many if not most don't have issues, the owners with the failures are always first to tell you all about it. Some lenses are known troublemakers however.
QuoteOriginally posted by jon404 Quote
If KEH sends me one that works, will it go bad in future
Anything with a focus motor in it from any brand will eventually fail, the question is how much the lens has been used already and what the reasonable life expectancy for them are, and that isn't well known yet and would be different for every design. Some are more failure prone than others, and recent updates with heavier parts in a few models at least, mean the current newly produced SDM stuff in general is much more durable.

If you want to be sure you are getting the most life out of an SDM lens shop at B&H or Adorama for new ones, KEH is a very good place for used though.

EDIT: Personally I only buy A or FA lenses used since you can easily judge mechanical wear in all the parts or AF clockwork as soon as you get it, and with the screw drive focus the working parts are all in the camera body so a lens will outlast several body upgrades usually. I've seen some FA lenses with a ton of use or cheaper models get a bit sloppy from wear but never seen one actually wear out.
02-18-2014, 04:17 PM   #7
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Thanks PPPPPP42 -- am after the 2.8/200mm for sports, as by all accounts it is supposed to be better for that than my 55-300mm. One thing I noticed today, though -- the f/2.8/200mm is a LOT heavier! But it also seems to be better built -- smoother, quieter. Perfect for manual focus -- but I truly need autofocus to keep up with fast-moving unpredictable soccer action, at f/2.8 or f/4.
02-18-2014, 04:31 PM   #8
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QuoteOriginally posted by jon404 Quote
Thanks PPPPPP42 -- am after the 2.8/200mm for sports, as by all accounts it is supposed to be better for that than my 55-300mm. One thing I noticed today, though -- the f/2.8/200mm is a LOT heavier!
Any lens with low f minimum number will be heavy.

Short explanation: it takes more elements to achieve a low f, and all those elements are made from glass. That's where the weight comes from.

Detailed explanation: f number is the ratio of focal length to aperture diameter. Let's suppose you want f/2.8 out of a 200mm lens. That means the aperture has to have diameter about 72mm. That's going to take some big heavy pieces of glass! Alternatively, what really matters is the effective aperture opening. You'll notice that the 200mm lens has a barrel that is actually narrower than 72mm. What lens designers do is put magnifying elements in the lens that mean the end looks like its 72mm but really, it's a big smaller. Those elements make the lens a bit more compact but obviously add weight too.

02-18-2014, 04:38 PM   #9
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You could have bought the other EX+ DA*200 that they have. Cheaper too.
If you are scared off by SDM there is an FA*200 (sscrewdrive) isted in the 35mm page at KEH. it's listed as BGN so expect some marks on the fragile silver paint.
02-18-2014, 06:12 PM   #10
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Bargains from KEH do tend to be a bargain as they are usually pretty conservative in their quality rating. I would probably call on bargain to find out what the issues were and if its just really crappy looking or if it has glass issues or excessive slop in parts or something. Front elements can have a lot wrong with them and still work fine but rear elements should really be basically perfect. Its always interesting judging savings vs quality on bargain.
02-18-2014, 06:20 PM   #11
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QuoteOriginally posted by jon404 Quote
Thanks PPPPPP42 -- am after the 2.8/200mm for sports, as by all accounts it is supposed to be better for that than my 55-300mm.
If you need it for sports then you absolutely need the FA*200 rather than the DA*200, because for some reason the SDM AF is really slow on this model. Other Pentaxians warned me, but I still decided to try it for myself. If athletes are running toward you, forget it - AF speed there is a joke. If the kids are riding around in slow-moving bumper boats, then it's great (plus it's WR!). I had one for about 2 weeks (max) and sold it to someone who wasn't shooting sports (because it's a fine lens otherwise).


OTOH the DA*300 has a significantly faster SDM AF, and the optical quality is even a little better than the DA*200. The FA*300/4.5 and F*300/4.5 (the one I have) are excellent both optically and AF-wise as well. Since (unlike Canon) Pentax' SDM isn't fully optimized for sports, these screw-drive lenses seem to AF faster too. I also don't like the DA*300 quite as much optically, but it's close. Nevertheless, sports shooter Ron Hendricks said its SDM is still fast enough for soccer, and he often shoots in the rain so he likes the WR. Strangely enough, when I use my 200mm I think it's the right focal length, but when I use my 300mm I think the same thing! So I think you can just choose one and then use it.



It's odd that you had AF issues, because the SDM usually doesn't fail on primes like the DA*200 and DA*300. DA* zooms like the 16-50 and 50-135 I have are much more likely to have issues, though fortunately I haven't had a problem so far.


BTW, quality primes like these should typically be shot stopped down by just 1/3 or 2/3 stops, as long as that gives the DOF you want. For example, I like to shoot my F*300 at f/5 or f/5.6, but on my FA*85 I may stop down to f/2.5 to f/4.5 when the athletes fill the frame (just to get the DOF I want at that FL).

Last edited by DSims; 02-18-2014 at 09:46 PM.
02-18-2014, 06:30 PM   #12
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QuoteOriginally posted by jon404 Quote
Just bought a used Pentax f/2.8 200mm from KEH, in LN- condition, to use with my K-30.

Problem -- Outdoors, in daylight, the autofocus doesn't seem to work. It hunts back and forth 2 or three times, often not getting a sharp focus at all. Tried my usual spot focus and select focus (at 5 and 11 active points) . Also tried it on my K-01, same problem. Tried it at different apertures, on M, TAv, Av, and P... on AF.S and on AF.C... makes no difference.

The manual focus works perfectly, nice and smooth. And, on autofocus, the lens motor is very quiet.

What's going on? Am I doing something wrong -- or do I have a bad lens? My other telephoto, a 55-300mm, works correctly on both cameras. Any ideas?
I have a DA* 200 and do not have any autofocus issues with it. I have only used it outdoors and the only times it did not lock focus spot on is shooting birds in trees where it could hit a branch in the way, but that is not really the fault of the lens. I agree with everyone else, you should return it. A few months ago, I passed up on a LN- DA* 50 -135 at a good price, fearing it may be vulnerable to what you experienced. I guess I feel more comfortable buying a non-SDM lens used. I did recently buy a DA 21 mm limited in LN- condition from KEH and I could not tell it apart from a brand new lens, so their grading is reliable but an issue could always slip through the cracks with a used product.
02-18-2014, 11:50 PM   #13
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Had an immediate response from Keh, allowing a return and exchange for another DA* 200, which I had asked for. Excellent service!

I like the lens enough to try to work with a slow autofocus. We'll see how the replacement one works out. Put the defective one on my K-01 today, which has a Hoodman loupe gaffer-taped on the back, and it works very well in manual focus. Took a train picture at sunset, and f/2.8 really helps... was able to keep 1/640 at a TAv ISO 125. See image and a 100% crop from it, below. Unedited, unsharpened. Wish the K-01 was fast enough at autofocus for soccer! Who knows, I'll try it with the replacement lens later on.
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02-20-2014, 03:59 PM   #14
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QuoteOriginally posted by jon404 Quote
Had an immediate response from Keh, allowing a return and exchange for another DA* 200, which I had asked for. Excellent service!

I like the lens enough to try to work with a slow autofocus. We'll see how the replacement one works out. Put the defective one on my K-01 today, which has a Hoodman loupe gaffer-taped on the back, and it works very well in manual focus. Took a train picture at sunset, and f/2.8 really helps... was able to keep 1/640 at a TAv ISO 125. See image and a 100% crop from it, below. Unedited, unsharpened. Wish the K-01 was fast enough at autofocus for soccer! Who knows, I'll try it with the replacement lens later on.
Best of luck with your replacement. I hope it autofocuses well. Glad to hear KEH came through for you. I bought my DA 21 mm Limited from them. It was graded as LN- and it looks & feels brand new so no thoughts of a return. This makes me feel better knowing that if my next purchase from them has a problem, they will rectify the problem.
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