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02-11-2009, 01:35 PM   #1
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K20D hunts in low light

I've had the K20D for just over a month and hadn't really tested it in low light until a couple of days ago when I received my brand new 16-45mm f4.

I pop the lens on, turn the camera on in AF.S, point it at the wall which is lit by a single fluorescent lamp, and the hunt begins when I press the shutter button. It sounds like a chainsaw and as if something will break as the lens turns back and forth repeatedly, and it just won't lock the focus. It happens in P, Sv, Tv, Av and TAv modes.

I tried the same with my 18-55mm AL II and the 55-300mm ED with the same results.

With bright light the focus locks right on, that happens with all of my lenses.

Is this normal? Anyone ever heard of this hunt with K20D? Being new to the camera, I am almost certain that I am not doing something right but I can't figure it out.

Any help is appreciated.


Last edited by EsBee; 02-11-2009 at 01:37 PM. Reason: updated
02-11-2009, 02:16 PM   #2
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This is absolutely normal. Don't tell me, the wall is white or otherwise uniformely painted, on top. AF needs two ingredients to work: a minmal light level and contrast - otherwise it can't work. That's true for all cameras working with a passive AF system.

What you can do to assist the AF function is, to add a good external flash with AF illuminator, like the Pentax 360 or 540 or the Metz 48/54/58 modells. The AF illuminator projects red stripes on the object and the camera can AF on those. With that kind of help you can even get a white wall focused.

Ben
02-11-2009, 02:22 PM   #3
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Try pointing it at a picture or calendar on the wall.
It focuses by detecting edges (contrasting colors). A white wall has no edges. All SLRs have this issue...
02-11-2009, 03:11 PM   #4
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What Ben and kenyee have said. No camera can achieve focus lock when contrast is not present.

You will find that your K20D tends to hunt more in low light situations. However, I can say from personal experience that a Pentax will achieve focus lock in situations where other cameras would simply give up.

02-11-2009, 04:28 PM   #5
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Thank you all, it makes sense now.

It is indeed contrast, or lack there of, but I kept thinking light. Until my 16-45 showed up, I was playing with an M 50mm f1.4 indoors just fine with its big aperture and manual focus. And of course, the outdoors were never an issue with my kit AF lenses.

Thanks again.
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