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08-13-2009, 05:33 PM   #1
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Lens focuses fine at close distance, not sure about infinity?


I recently acquired an FA 100-300 f4,5-f5,8. A very fine lens, with good performances.

I tested it at short focus distances and get great results, handheld. However, focusing at long distances (a few hundred meters) seems to give me average results, really hit-or-miss. I took Moon shots with a tripod and got some nice sharp images, but the best pictures were with an aperture between f16 and f18. If I manually focused the lens to infinity, focus was off, the camera set the focus point just a tad before infinity.

When taking pictures of trees at a few hundred meters (near infinity, right?) I sometimes get a sharp picture, with lots of details, but sometimes it seems impossible to get a clear shot. The lens is usually used around f9 (in daylight), handheld with SR on, and a shutter speed that's high enough considering the focal length.

I have the K20D so focus adjustments are possible. However, since focus doesn't seem to be off in every situation, I'm not sure about this. I haven't yet run a test with a focus chart, but I plan to.

I know the lens can deliver extremely sharp results with distant subjects at 300 mm, so it's not a matter of the lens performing poorly optically.

Any thoughts on the best tests/adjustments to make?

08-13-2009, 07:51 PM   #2
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I don't know if it is the case, but haze and heat waves can really mess things up with a long lens trying to get a faraway shot. Just a thought.
08-14-2009, 04:25 PM   #3
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I have the same experience, too. I suggest that you shoot some distant objects on a tripod and then do the AF adjust based on that.

In my case, the AF for near objects also seems to be OK after the adjustment, some 80 or 90% of the time. Good luck.
08-14-2009, 09:27 PM   #4
Damn Brit

It's possible that when you were shooting the moon, AF might have been thrown off by the slight halo effect caused by any haze. f/16 would be about right for moon shots
I think.

08-16-2009, 12:43 PM   #5
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Thanks for the feedback. I will try AF adjustments with distant subjects and see how that leaves me at close distances

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