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01-06-2009, 06:48 PM   #1
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Lens choice for faster K20D's AF

After two years of using Nikon D50 kit I've bought K20D body, DA 18-250 (smc PENTAX-DA 18-250mm F3.5-6.3ED AL [IF]) lens (as "all-in-one travel zoom") and was disappointed by AF speed (comparing to Nikon D50 18-55mm kit), especially with indoor shots. I've read thread "Would SDM lens speed up K10D's AF?" also and think that SDM lenses may not help me.
Now I think that better question will be "Which lens can speed-up AF process in my K20D?"
Because indoor shots do not require long zooms I will buy some shorter zoom lens, i.e. *16-50, but will be this lens faster than current? In thread about SDM I've seen good opinions (fast and accurate) about Sigma 70-200 HSM, but 70-200 range isn't good for indoor shots.
What lenses may be acceptable for me in this case?

01-06-2009, 09:58 PM   #2
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QuoteOriginally posted by HEKPOMAH Quote
After two years of using Nikon D50 kit I've bought K20D body, DA 18-250 (smc PENTAX-DA 18-250mm F3.5-6.3ED AL [IF]) lens (as "all-in-one travel zoom") and was disappointed by AF speed (comparing to Nikon D50 18-55mm kit), especially with indoor shots. I've read thread "Would SDM lens speed up K10D's AF?" also and think that SDM lenses may not help me.
Now I think that better question will be "Which lens can speed-up AF process in my K20D?"
Because indoor shots do not require long zooms I will buy some shorter zoom lens, i.e. *16-50, but will be this lens faster than current? In thread about SDM I've seen good opinions (fast and accurate) about Sigma 70-200 HSM, but 70-200 range isn't good for indoor shots.
What lenses may be acceptable for me in this case?
The DA 40 lmt. should be fast.
The same for some of the Sigma's, like 17-70mm f/2.8-4.5 DC; AFAIR.
( And likely Sigma 18-50mm F2.8 EX DC MACRO)


Norm also posted some info, on the 28-70 f/4 :
http://forums.dpreview.com/forums/read.asp?forum=1036&message=29511295

Last edited by Jonson PL; 01-06-2009 at 10:58 PM.
01-07-2009, 02:33 AM   #3
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QuoteOriginally posted by HEKPOMAH Quote
After two years of using Nikon D50 kit I've bought K20D body, DA 18-250 (smc PENTAX-DA 18-250mm F3.5-6.3ED AL [IF]) lens (as "all-in-one travel zoom") and was disappointed by AF speed (comparing to Nikon D50 18-55mm kit), especially with indoor shots. I've read thread "Would SDM lens speed up K10D's AF?" also and think that SDM lenses may not help me.
Now I think that better question will be "Which lens can speed-up AF process in my K20D?"
Because indoor shots do not require long zooms I will buy some shorter zoom lens, i.e. *16-50, but will be this lens faster than current? In thread about SDM I've seen good opinions (fast and accurate) about Sigma 70-200 HSM, but 70-200 range isn't good for indoor shots.
What lenses may be acceptable for me in this case?
A super-zoom indoors is bound to be slow. The aperture is sub-optimal and the light levels indoors usually are lower anyway. So a faster lens will speed-up AF. The 16-50 might be a good choice, but primes like the 35/2 etc. will be at least as fast.

Ben
01-07-2009, 05:10 AM   #4
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I use the Sigma 17-70mm f2.8-4.5 on a k20d, it a good lens but the AF in dim light
can have a little bit of trouble. Its still reasonably good but if you want something that
is pretty quick I would go for a prime or something like is mentioned above.
The Sigma is a great lens and I'm glad I got it, out doors in good light it is reasonably fast
but indoors you need a good bit of light. You would think that a smaller zoom or prime with
large costant aperature and SDM would be a better option.

01-07-2009, 10:38 AM   #5
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I think that my choice will be one of the following lenses (with rough prices in my country):
- ($420) Sigma 18-50mm F2.8 EX DC MACRO (HSM models are available to Nikon mount only);
- ($845) SMC Pentax DA* 16-50mm f/2.8 ED AL [IF] SDM;
- ($775) SMC Pentax DA 12-24mm f/4 ED AL [IF];
- ($400) SMC Pentax DA 16-45mm f/4 ED AL;
- ($600) SMC Pentax DA 17-70mm f/4 AL [IF] SDM.
If I understood correctly, SDM lenses give no advantage to AF speed except silent operation? On other side, I don't require superzooms at this time because I have 18-250 lens already. I think about indoor zoom lens. Therefore, one of the first 3-4 lenses will be optimal choice?
I think that Sigma 18-50 and Pentax 16-45 prices are attractive, especially the first one.
01-07-2009, 11:26 AM   #6
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From my experience, the 21 Limited, 40 Limited and Sigma 30/1.4 all focus pretty snappy.
01-07-2009, 11:39 AM   #7
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I love my Sigma 30mm f1/1.4 EX DG
01-07-2009, 02:02 PM   #8
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Hekpomah, from experience I can say, that the Sigma 18-50/2.8 focuses pretty quickly, even indoors. It is a very good allround lens. Nevertheless, if you really are into indoor shooting without flash, you should consider a faster prime lens, too. Any decent prime lens will give you at least one f-stop more (FA 35/2 for example). Indoors often also means a tighter space and if you want group shots, you need a real wide-angle lens. In that case the 35mm would be too long a focal length. But for single persons, details at the table or whatever, I find it very nice. And due to its compact size, it is not as intimidating as some bigger zooms.

Ben

01-07-2009, 02:47 PM   #9
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Thanks all for replies. I think that zoom lens (i.e. Sigma 18-50/2.8) will be more flexible and less expensive to my budget than fixes (i.e. 35mm lens). And I agree with Ben Edict about indoor wide angle needs.
Now I think how much will be real AF speed difference between, for example, Sigma 18-50/2.8 and my current Pentax 18-250/3.5-6.3 lenses...
01-07-2009, 06:32 PM   #10
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I think most of the issue has been answered, specifically the slow superzooms are too small an apature (i.e. less light) to profide fast AF. Every AF camera works best in higher light levels, therefore to an extent faster lenses speed up AF. Where you may start going backwards is if maximum apature starts becoming a trade off for sharpness. Some people have commented that they cannot get reliable focus confirmation for example with a 50mm F1.2. The lens it self is very fast, but somewhat soft at maximum apature.

Therefore, make sure that if you go for bigger maximum apatures, the lens remains reasonably sharp wide open.
01-07-2009, 08:06 PM   #11
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QuoteOriginally posted by Lowell Goudge Quote
the slow superzooms are too small an apature (i.e. less light) to profide fast AF
Also, they are physically more massive, meaning the screw-drive motor has to work that much harder to focus them. The 18-55 lets in no more light than the 18-250, but has considerably less mass, so it probably focuses more quickly (I don't have an 18-250 to compare with). The DA40 lets in a decent amount of light (f/2.8) and is one of the world's smallest lenses, and it's pretty sharp wide open too, so it is about the fastest focusing lens Pentax has ever made.
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