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02-21-2012, 06:38 AM   #1
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Pentax DA* 300mm AF Problem?

So I got the DA* 300mm F4 a few days ago, and I STILL have not had time to shoot with it. Weather here in Chicago has been grey and boring, and I had to much stuff going on this weekend.

Anyway - last night I did a quick check for front/back focus, and there was no immediate problem with the lens. Also the image looked incredibly sharp at F4.

HOWEVER something weird happend: I was focusing on my lego figures about 8 ft away (doing the AF test), and the focus missed and then it just kind of got stuck and would not focus?! I adjusted manually and then it snapped into focus. I did not think more about it at the time, but then afterwards I realized the same thing happend a few days ago, when I first mounted the lens. In other words, the focus missed, and then it stayed unfocused - not just extra hunting. This happend, btw, using my K-5.

Surely this can't be normal? Now, it was in the basement, and the light is fairly dim, but I don't recall this happening with other lenses, even my slower lenses I used to have.

I will have to do some testing now, but wanted your thoughts, please...

02-21-2012, 07:31 AM   #2
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I have noticed this on a couple of my lenses also, and all the research I did led to "low lighting" situations. I have retested these lenses and found that was my problem.
02-21-2012, 07:49 AM   #3

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I have not run into this problem with this lens, but then I don't use it indoors. After all, it is a lens best suited for outdoor use. And outdoors, it focuses with a fair degree of accuracy with the K-5. Also keep in mind that if you miss the focus by even a very little bit, it will be noticed due to the narrow DOF of the lens. And if the lens doesn't find something with sufficient contrast to grab hold on, you can't expect it to focus accurately (and that would be true of an AF lens).
02-21-2012, 08:09 AM   #4
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I don't plan on using it indoors, and I am not expecting instant "snap" focus - certainly some hunting is to be expected.

What bothers me is that it stops and stays unfocused. My 70-200mm at F2.8 (almost as narrow DOF at 200mm) has had no problems, other than the occasional miss and hunt. But I don't recall that lens ever just stopping and staying unfocused.

02-21-2012, 08:43 AM   #5
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I don't see it as special issue. I see it not uncommonly on the AF system to hunt for a while and then if it doesn't find anything--- simply give up.

I have a couple of Macros that do this most commonly. they make about 2 passes and if they haven't locked on by then, the AF shuts down rather than
hunting for ever. This is almost always in low light conditons. If it can't lock, best it shut down and let you do it manually and sit there and hunt forever....

I don't think it's much to do with the lens...other than like the macros, it has a very short depth of field, and can snap in and out of focus so quickly that the focus
drive will shot right past the focal point without getting stopped occasionally under difficult lighting.
02-21-2012, 10:55 AM   #6
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I have the same lens on a Kx. I tap the shutter release button instead of holding it down constantly. I too noticed it will hunt and then give up the chase. It works better with intermittent tapping, bright light and higher contrast. In the end, I had to go to a +60 Ám adjustment for autofocus! Not sure if it is the lens or the body but as long as it works (and it does quite well). When frustrated, I switch to manual but with the minimal DOF at f4, that is still a challenge so I "bracket" focus if possible.
02-21-2012, 03:21 PM - 1 Like   #7
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Yes I've had similar instances with my DA*300 which I've owned since it first came out. I've found the AF of the lens to rather sensitive to remaining battery charge (certainly compared the my 16-50). Especially if I've been running the metering for an extended period.

The lens flicked just off the infinity and back for 3 AF button pushes just last Sunday, not bothering to come into the 20' range where the target was... sure enough 10 exposures later the battery indicator on the K-5 dropped its first segment.
02-21-2012, 03:46 PM   #8
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I echo Col's findings. Large lenses dictate more available power to turn the focus motor.


02-22-2012, 06:26 AM   #9
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Very interesting. Thnaks very much for you comments. I doubt I would ever have thought of the battery power being related to the focus. My K-5 was actually low on battery. Whether low light or low battery or both, it sounds like this is not a specific problem to my new lens, which is a relief

Now I just need to get some time and try the lens out! I still can't believe how sharp and contrasty it looked. Costa Rica - here I come (leaving in three weeks)!!!!

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