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01-18-2023, 06:29 PM   #1
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How does autofocus on the 28-105 compare to other modern Pentax lenses?

Tried this on Reddit, got a lot of views but no responses. I hope that doesn't mean there just aren't many Pentaxians out there that own these lenses. I hate to even have to ask questions like this, but due to a lack of reviews, especially video reviews that would show auto focus performance, there's nothing for it but to try to ask in forums.


After testing my older lenses on the k1-ii, including the smc fa 50mm 1.4, 77mm limited etc. I have found the older lenses to struggle with autofocus more than the newer lenses (obviously), while also being quite loud.

As I look to expand my lineup with better glass, I was hoping I could get a comparison on autofocus to the 28-105. The auto focus on this lens is quiet, reasonably quick, and relatively accurate. It feels very usable.

So my question is, how does the autofocus on the other full frame modern lenses compare? Especially on the 70-200 2.8 and the 85mm 1.4, 24-70 2.8 etc.

Is the autofocus on these modern full frames about the same as the 28-105? Is it better? Worse?

Pentax gets plenty of flak for it's autofocus before the k3 III, and I find I agree that with older lenses on the k1 it is pretty hit or miss, but I find the 28-105 usable even for sports if I'm working at it. If the other modern lenses are as good or better, that would be comforting to know before I lay out a lot of money on them.

Thanks!

01-18-2023, 06:41 PM - 3 Likes   #2
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The DFA 28-105 has a modern DC autofocus motor. With the exception of the two blazingly fast PLM-driven zooms (the new DA* 16-50 and the latest version of the DA 55-300), any difference from the other modern Pentax lenses is splitting hairs.

Much more important should be the design features you value most. Is ultimate image quality, or a faster maximum aperture, more important than portability?

Last edited by Sandy Hancock; 01-18-2023 at 07:07 PM.
01-18-2023, 06:50 PM - 1 Like   #3
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It's fine.. as has been every lens since the 18-135. It's the older SDM lenses DA* 60-250, DA*200 etc. with the rep for being slow. And the DA 18-135 was years ago.
01-18-2023, 07:13 PM - 1 Like   #4
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Love my 28-105 - perfectly fine on autofocus with K-1.

Was out taking pictures of snow today (we got 3 feet in the last few days!) and it worked just fine.

01-18-2023, 08:23 PM - 1 Like   #5
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I can only speak for the DFA*50 and yes I can say that it’s DC autofocus is more reliable and consistent than any of the screw mount 50’s I have (FA 1.4, FA 1.7, F 1.7, FA Macro) and is comparable to what you are experiencing with the DFA 28-105 which I also have.
01-18-2023, 09:29 PM - 2 Likes   #6
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QuoteOriginally posted by vector Quote
I can only speak for the DFA*50 and yes I can say that itís DC autofocus is more reliable and consistent than any of the screw mount 50ís I have (FA 1.4, FA 1.7, F 1.7, FA Macro) and is comparable to what you are experiencing with the DFA 28-105 which I also have.
Just for the sake of accuracy, the DFA*50 has a large ring motor, which is designated as SDM, not DC.
01-18-2023, 09:43 PM - 1 Like   #7
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The DFA 28-105 focuses pretty nicely. The only modern lens I have to compare (that doesn't have screw drive that is), is the HD DFA 21 f2.4. The DFA21 makes a similar noise but is noticeably quicker than the DFA 28-105 and at least as accurate (and probably a bit better).

01-18-2023, 09:48 PM - 1 Like   #8
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Hi PBlogic,

I'm sure you should go through this website's official reviews of any lens you're considering ... it's often Bernard Dery doing them, and he'll nearly always address autofocus.
01-19-2023, 12:13 AM   #9
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You can tell by the sound.
Old lenses sound like coffee grinder: "GRRRR...RRR..GRRR bip", but without the smell of coffee.
Old SDM lenses sound like wings in the air: "Shhhhh..shhhhhhh bip" or "Sssshhhssquicksquicksquick bip" when the lens SDM motor is about to give up.
DC lenses sound like trouser zipper: "bZzz..zzz..zzzzt..bip"
But the DC motor on my DFA 21 limited sounds more like " hesitate...anyone pressed the shutter button? Oh yeah hurry up bshhhhhshhhh..(sorry I can't hear you)... bip".
I don't know why but my DFA 21 always has a small lag to start, then it's decently fast and silent. DFA28-105 motor starts immediately, no lag.
01-19-2023, 05:45 AM   #10
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I'am a bit baffled about the statement concerning older lenses on the K-1. I have been using the F 28-80, the F 70-210 and the Tamron 28-105 (A79) on my K-1 without any AF issues beside the fact they are all slot driven. Else they are reasonably fast and focus is on the spot.
01-19-2023, 07:02 AM   #11
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QuoteOriginally posted by Papa_Joe Quote
I'am a bit baffled about the statement concerning older lenses on the K-1. I have been using the F 28-80, the F 70-210 and the Tamron 28-105 (A79) on my K-1 without any AF issues beside the fact they are all slot driven. Else they are reasonably fast and focus is on the spot.
I also read things (about the 100 macro, screw drive AF) such as "unfortunately screw drive AF is slow". I compared my 100 macro AF with my DFA 70-210 f/4 and my DFA 70-200 2.8 (silent motor), and the 100 macro AF is significantly faster at the same subject distances, to the point that I consider using the 100 macro for shooting sports instead of the using the 70-210. I also have the FA 35 2, also screw drive and I things it the fastest focusing of all 10 lenses I have.
01-19-2023, 08:52 AM - 2 Likes   #12
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QuoteOriginally posted by biz-engineer Quote
I also read things (about the 100 macro, screw drive AF) such as "unfortunately screw drive AF is slow". I compared my 100 macro AF with my DFA 70-210 f/4 and my DFA 70-200 2.8 (silent motor), and the 100 macro AF is significantly faster at the same subject distances, to the point that I consider using the 100 macro for shooting sports instead of the using the 70-210. I also have the FA 35 2, also screw drive and I things it the fastest focusing of all 10 lenses I have.
My problem with the 100 macro is not that it's slow focussing, but the it sometimes misses focus and goes into some kind of non-productive search cycle. that often doesn't go back to the actual focus point, going back and forth between different points of out of focus distances.
01-19-2023, 09:58 AM   #13
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QuoteOriginally posted by normhead Quote
My problem with the 100 macro is not that it's slow focussing, but the it sometimes misses focus and goes into some kind of non-productive search cycle. that often doesn't go back to the actual focus point, going back and forth between different points of out of focus distances.
Never had this problem, except in live view but that's with any lens when the live view image is completely out of focus the contrast detect based auto-focus needs to perform a loop to determine if the subject in front or behind the plane of focus.
01-19-2023, 10:03 AM   #14
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QuoteOriginally posted by biz-engineer Quote
Never had this problem, except in live view but that's with any lens when the live view image is completely out of focus the contrast detect based auto-focus needs to perform a loop to determine if the subject in front or behind the plane of focus.
It doesn't happen a lot, but enough for me to remember it happens.
01-19-2023, 03:11 PM   #15
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QuoteOriginally posted by biz-engineer Quote
You can tell by the sound.
Old lenses sound like coffee grinder: "GRRRR...RRR..GRRR bip", but without the smell of coffee.
Old SDM lenses sound like wings in the air: "Shhhhh..shhhhhhh bip" or "Sssshhhssquicksquicksquick bip" when the lens SDM motor is about to give up.
DC lenses sound like trouser zipper: "bZzz..zzz..zzzzt..bip"
But the DC motor on my DFA 21 limited sounds more like " hesitate...anyone pressed the shutter button? Oh yeah hurry up bshhhhhshhhh..(sorry I can't hear you)... bip".
I don't know why but my DFA 21 always has a small lag to start, then it's decently fast and silent. DFA28-105 motor starts immediately, no lag.
Or without bip. That sound I switched off...itís already noisy enough.
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