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02-25-2016, 12:43 PM   #1
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Auto focusing in dim light

I use a K50 with a Pentax 18-135 mm lens. Under dim light I find that the auto focus mechanism can take forever to work. I found that by twisting the zoom ring back and forth a little, focusing is usually much faster. Does anyone know why this happens? What's the connection between the auto focusing mechanism and the zoom mechanism?

02-25-2016, 12:47 PM   #2
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shorter zoom FL is relatively Faster Brighter (f3.5 vs f5.6 at extremes )
02-25-2016, 01:20 PM   #3
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The 18-135mm is an all-around zoom lens. Like all all-around zooms, the lens' weakness is the low light conditions. Full stop.

In low light you want to use a fast lens: i.e., large aperture (f.14 or f1.8).

In very low light, AF will not work. You need to use a fast lens and MF. For that purpose, MF fast lenses tend to be the best, because their focus ring is usually very accurate and precise.

Like you I have the DA18-135mm (and DA18-270mm). When I take it for travel, I always add a fast lens, to cover low light conditions when I cannot use flash: e.g., sunset, early evening, indoor when flash is prohibited.

Hope that the comment my help..
02-25-2016, 01:43 PM   #4
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QuoteOriginally posted by mattt Quote
shorter zoom FL is relatively Faster Brighter (f3.5 vs f5.6 at extremes )
the other thing, of course, is that at higher magnifications (135 end) there is also less for the AF to latch on to - it's more likely everything is very out of focus and the AfF can't find a phase difference which it needs.

02-25-2016, 03:39 PM   #5
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Yes, the lens is an issue. The newer bodies are a little better at AF at the lowest light levels. Be sure to have the AF assist light set to "on."
02-26-2016, 07:00 AM - 1 Like   #6
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Thanks for all your advice. It's very helpful. The problem is especially frustrating when I want to take a flash photo of friends who are in dim light. They all sit there, big-eyed, waiting for the photo to be taken and the camera won't focus. My wife's cheaper point-and-shoot camera has no such problem.

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