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11-30-2011, 10:36 AM   #1
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Autofocus: SDM vs Screwdrive question

I just got my first lens with built in autofocus and OIS and am using it on my K5. I was wondering if an AF lens determines "focus" differently than an older lens relying solely on the K5's mechanisms. I use just the single center focus point and it has mostly worked well for me shooting a variety of stationary and moving targets. The new lens seems to go off target more frequently than my older lenses, getting something just behind or just in front of the intended focus point instead. I realize the actual center target area is somewhat larger than the red center focus dot in the view finder, but even so....compared to my older lenses. I DO turn off the K5's SR when using the lens's OIS, and under controlled conditions checked and confirmed my micro focus adjustments. When the lens does "hit" I am very happy with the results, more than I expected.

11-30-2011, 10:49 AM   #2
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So if I understand the question correctly you are wondering if your new lens focuses differently than an older auto-focus lens? AFAIK all AF lenses whether older screwdrive or newer ones with motors in the lens all use the camera AF sensor to determine what is in focus. The only difference is in how the lens components are moved to bring the subject into focus. If I'm wrong maybe someone else has a better explanation. Have you tried it using the live view? This uses a different AF mechanism so might show different results if you are trying to track something down. You might also see what happens if you try to use manual focus.

What lens are we talking about here? Maybe someone with experience with that lens has a better idea.
11-30-2011, 10:57 AM   #3
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Have you tried some controlled tests to determine whether the lens is consistently back or front-focusing in the same way? If so, you might be able to solve some of your problems through the K5's focus microadjustment. Also, do you tend to see the problems under certain kinds of lighting conditions? The k5 still has some problems in tungsten light, which may be more pronounced depending on what lens you use.
11-30-2011, 11:15 AM   #4
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What is a lens with builtin AF? The ones I know are old lenses (from the 80s I guess) and definitely don't have IS builtin

Please enlighten me with make and model so I know what you're talking about.

11-30-2011, 12:13 PM   #5
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QuoteOriginally posted by sterretje Quote
What is a lens with builtin AF? The ones I know are old lenses (from the 80s I guess) and definitely don't have IS builtin

Please enlighten me with make and model so I know what you're talking about.

I mean the electrical motors and movable elements needed to focus the lens (and in this case provide optical stabilization as well) are built into the lens itself and does not rely on the screw drive motor in the K5 to move the lens elements into focus. Battery power still comes from the camera. In some cases the lens are made so the K5's screw drive motor can not even be used as a backup in case the lens's AF mechanism fails. Many if not most of the newer lenses are made this way and if you check you'll find Canon and Nikon both make camera models without an autofocus motor in the body. It reduces the body weight but means every lens those cameras use need to have the motors and electronics built into every lens in order to autofocus. (older lenses can usually still be used with manual focus).

I assume the camera and lens "talk" back and forth with each other but I'm wondering if there is a chip or firmware in the lens itself that may affect the parameters that determine proper "focus" . Lenses with the built-in AF seem to vary quite a bit as to how fast they acquire focus, I would guess that could be in part price driven. My K5 usually has focus before I can fire, it may be that the acceptable focus range to "fire" is a bit wider than exactly on target. Most of my shooting involves targets of opportunity but when I have time to really frame a shot the results are quite good.
11-30-2011, 12:41 PM   #6
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It's the same sensor, just a different motor driving the lens. Pentax's SDM is actually slower than the screwdrive system; Sigmas HSM and Pentax's DC are both faster (and all 3 are quieter).

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11-30-2011, 06:39 PM   #7
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I will say the first time I mounted an SDM lens on my camera I didn't know if it was focusing unless I heard the beep from the body. I wasn't wearing my glasses since I was just messing around, so I couldn't tell focus; and the thing was so silent I honestly thought it was broken. Speedwise, I think it's about the same as the screw drive lenses I have. Maybe a little faster than the cheaper DA L lenses, but not as fast as F and FA lenses I've used. My F 50 1.7 is super fast, if a bit jerky.
11-30-2011, 06:47 PM   #8
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QuoteOriginally posted by Don From The Radio Quote
I will say the first time I mounted an SDM lens on my camera I didn't know if it was focusing unless I heard the beep from the body. I wasn't wearing my glasses since I was just messing around, so I couldn't tell focus; and the thing was so silent I honestly thought it was broken. Speedwise, I think it's about the same as the screw drive lenses I have.
I had the same "problem" adjusting for HSM. I was used to "listening" for the focus.

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