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08-24-2008, 12:31 PM   #1
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pentax 18-250 not focusing to infinity

My Pentax 18-250 is not focusing to infinity. I can sent it on a distant object and focus. When i check the distance on the lens it will be before the infinity mark. If i move it to the infinity mark it will be out of focus. i am talking about something that is far away like the moon. could the lens be bad.? I have no problem getting sharp close photos. I tried my other lens and they seam to be fine. I have a K20D. I checked the forum and have not found anything in reference.

08-24-2008, 03:30 PM   #2
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perhaps the object you're focusing at is close to infinity, but not exactly infinity? from the photo of the lens, there is a decent size gap between 100ft (30m) to infinity. as long as the output is sharp when autofocused, i would not worry too much

for moon photos, it takes a lot of practice to get the right focus, i've never tried it but i don't think infinity focus will yield you the sharpest photo

otherwise, it's time to call up support
08-24-2008, 03:46 PM   #3
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Doesn't help you much but I've seen quite a few AF lenses that focus slightly past infinity.

Presumably it is to stop the lens suffering a big jolt against the hard stop when the lens hits infinity, so it may be normal if it is only a small amount.
08-24-2008, 04:39 PM   #4
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QuoteOriginally posted by k100d Quote
perhaps the object you're focusing at is close to infinity, but not exactly infinity? [] for moon photos, it takes a lot of practice to get the right focus, i've never tried it but i don't think infinity focus will yield you the sharpest photo
LOL. moon should be infinite enough

Seriously, focusing onto the moon is an easy exercise as it is bright and its border is as contrasty as it will ever get. Should always be at infinite position afterwards. Probably better test procedure than any focus chart.

QuoteOriginally posted by hitechrr Quote
it will be before the infinity mark.
The 18-250 has a focus stop and an infinity mark which is before the focus stop. If I focus at 200mm onto the moon (using this lens), the focus mark will be 1/2 mm before the middle of the infinite sign. Which is quite precise. Note that it is not at focus stop position.

If your's is more than, say 2mm away from the infinite sign (at 200mm), I would consider it non normal.

There are two reason for focus stop position and infinite position to differ:
- room for sloppy factory calibration
- controlled room for top notch lenses to allow to reach infinite focus at very hot temperatures.

08-24-2008, 05:42 PM   #5
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infinity is very close to the 100 mark. just a little past the 100, but before the infinity mark. if you set it at infinity it will be out of focus. i have tried it with things that are 200 or more feet away. when i tried the focus chart the image looks fine.
08-25-2008, 03:17 AM   #6
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QuoteOriginally posted by hitechrr Quote
infinity is very close to the 100 mark. just a little past the 100, but before the infinity mark. if you set it at infinity it will be out of focus. i have tried it with things that are 200 or more feet away. when i tried the focus chart the image looks fine.
200 ft is not enough. 200 ft is exactly half way between 100 ft and infinity. Mine focusses at 0.5mm away from the infinite mark, which is ~6mm away from the 100 ft mark. To come that close to the infinity mark, you need objects as far away as 1200 ft or more.

If your focus is more on the 100ft than the infinity side of the scale with subjects more than ~1000 ft away, then, and only then, your lens' focus scale is abnormal and may be subject for service or return.
08-25-2008, 03:10 PM   #7
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I will have to go to the bay and try to focus on the light house. That is at least 4 miles away.
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