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12-03-2011, 05:32 PM   #1
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Pentax DA 50-300mm wont focus to infinity???

I've been doing a little bit of astrophotography lately but I'm having focusing issues with my DA 55-300mm on my K-5. It seems that the lens is incapable for focusing to infinity. I've attached a sample of image of what I'm getting when the focus ring on the lens is rotated all the way to the stop. This is the clearest image I can get so I know that I'm not focusing past infinity. Could I possibly have a defective lens? Anyone know if there's a way to fix it? Kinda ticked off that I spent $350 on a lens that can't even focus properly.

Image of the orion nebula taken at 300mm on an equatorial mount:


12-03-2011, 05:50 PM   #2
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Wide-open? Long-exposure? (Stabilized how?) Zoomed all the way in? Possibly not the best combination for sharpness. I would test on a well-lit subject stopped down a bit with a normal shutter speed before concluding it can't focus.
12-03-2011, 06:58 PM   #3
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Yes wide open, 20 second exposure, SR was turned off but camera is mounted on a equatorial mount.

I haven't really had a chance to try the lens in the daylight, not at infinite focus anyway. But I have tried it on the moon before with the same results. The focus point just doesn't seem to move far enough to focus on long distance objects. I know its not the mount as I've tried it with other cameras and my telescope and have gotten much better results.

Here's a (not very good) shot of the Pleiades which you can see is relatively sharp at infinity. Same mount, different camera:
12-03-2011, 07:09 PM   #4
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SR would be automatically disabled at that shutter speed anyway. Don't know much about that lens, but any zoom zoomed all the way in with aperture wide-open will often produce its softest results. And a long shutter will pick up any vibrations due to wind or whatever if your tripod etc isn't rock-solid. But if you just point it at the moon can you get it to look sharp through the viewfinder manually focusing?

12-03-2011, 08:35 PM   #5
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The problem is most likely that you are focused *past* infinity. Virtually all AF lenses allow this to give the AF system room to work. Also, 20 seconda is too long to avoid motion blur at that focal length unless the mount is on a clock drive (and even then, chances are it's not calibrated perfectly).
12-04-2011, 07:02 AM   #6
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QuoteOriginally posted by Marc Sabatella Quote
The problem is most likely that you are focused *past* infinity. Virtually all AF lenses allow this to give the AF system room to work. Also, 20 seconda is too long to avoid motion blur at that focal length unless the mount is on a clock drive (and even then, chances are it's not calibrated perfectly).
I can just copy this. Infinity is typically not the end of the screw, not even necessarily where the infinity symbol is plotted. Check the infinity setting by focussing on something far away (1000x focal length) during the day and check what your camera thinks is infinity. Manufactering tolerances are another reason for this offset. Expansioon of lense elements and AF drive are other reasons. The same holds for EXIF data, focal length or distance readings are not neccessarily correct.
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