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08-21-2007, 10:15 AM   #1
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Manually focusing @ infinity

How do people set "infinity" accurately when focusing manually or is it just the case that it doesn't need to be exact, as in perfectly in-line with the infinity mark on the lens ?



08-21-2007, 10:55 AM   #2
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You have to have distance scale on your focusing ring, or then judge distance. You can do it with AF as well. It's quite easy. There is something called hyperfocal distance, which is different for every focal lenght and f stop. If you calculate this and focus accordingly then everything from about 1/3 of that distance to infinity will be in focus. To help you find out hyperfocal distance check page like DOF Master, or similar.
It's useful and it works
Good luck,
08-21-2007, 10:56 AM   #3
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Last week I was using my Pentax ZX-5n with a SMC F 70-210 lens. I was shooting in Manual focus and found that the lens I was using required me to "back off" just slightly from the full "infinity" direction. The scene I was focusing on was about 500 yards away so I am assuming that "backed off" position was my Infinity. Using the viewfinder and my OLD eyegball, some of the photos were not as sharp as others.
08-21-2007, 11:41 AM   #4
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I've taken a lot of astrophotos with manual lenses so Infinity is something I use often. I think my experience has something in common with normal daylight photos. Most lenses will manually adjust, "just beyond infinity." For a manufacturer to make the lens stop exactly on the mark and be exactly at infinity would be difficult so they add a little extra range so the operator can find true infinity.

Setting focus by depending on the Infinity mark is not good enough for the night sky, even looking at stars through the viewfinder is tough. So I usually take some test shots (or use preview) and zoom in on the LCD display to see how well focused the image is. It is an iterative process, sometime it takes me 10 minutes to get well focused stars.


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