I was taking photos of the stars tonight and felt that the AF would not work in this case, so I set the lens to Manual focus.
When I tried focusing on either the moon or a distant star, I noticed that the point where the subject was in clear focus was when the focus was set to about 90% of the total movement of the lens. What I mean is that I could turn the manual focus to a point where the star was in focus and could then continue turning the focus further which resulted in the star going out of focus again. So at its maximum position, what would be in focus other than the furthest thing I can think of, a star?
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#1. "RE: Where is inifinity?" | In response to Reply # 0
aolander Nikonian since 15th Sep 2006Fri 10-Oct-08 01:43 PM | edited Fri 10-Oct-08 01:55 PM by aolander
Many lenses focus "beyond infinity" (wherever that is! ), so your findings are not unusual. I believe they are made this way to allow for changes in the actual point of infinity focus from expansion and contraction of the lens due to temperature changes.
#2. "RE: Where is inifinity?" | In response to Reply # 0
MotoMannequin Registered since 11th Jan 2006Fri 10-Oct-08 07:15 PM
I've read that infinity has a technical definition with respect to lens design, something like 1000 x focal length, but honestly I don't think that's the situation. All my primes render distant objects like stars sharpest when the focus ring is turned all the way to the stop, which is marked infinity. Zooms I believe allow some play for the focusing elements to move past the ideal infinity focus point, either to allow the camera's AF system to account for variation in manufacturing tolerances or for vari-focus designs which change focus point depending on the zoom setting.
To solve your problem, you could use AF to lock onto a bright distant object like the moon, or set focus manually (give your eyes some time to adjust for darkness).
Because of these issues, I try to use my primes for astrogroghy.
Larry - a Bay Area Nikonian
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#4. "RE: Where is infinity?" | In response to Reply # 0
Len Shepherd Nikonian since 09th Mar 2003Sat 11-Oct-08 03:19 PM
Technically infinity is as far away as it is possible to be.
I am not familiar with your lens but "infinity" focus is opposite the infinity mark (number 8 on it's side) if it has an infinity mark.
Longer lenses usually focus beyond infinity so that when using manual focus longer distances can be more accurately focused with an out of viewfinder focus effect beyond infinity.
As already mentioned focus can also vary slightly in very hot conditions.
Some "economy independent" wide angle lenses used to have infinity at about 3x minimum marked focus, which is closer than mathematical infinity.
Occasionally a focus ring is not accurately set up and needs a slight adjustment to match the focus scale.
Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.
#5. "RE: Where is inifinity?" | In response to Reply # 0
mikeac Registered since 19th Nov 2007Mon 13-Oct-08 01:45 PM
>So at its maximum position, what would be in
>focus other than the furthest thing I can think of, a star?
Thinking back (twenty-five-odd years!) to school physics lessons, I think a lens focused at infinity would be set to focus PARALLEL rays of light to a point on the plane of the sensor. The rays reflected from any point near your lens will be DIVERGENT - spreading out at an angle from that point. For points further away, the angle of spread will be smaller. So as you consider points ever more distant, the angle of spread will get smaller and smaller, tending to 0 degrees at infinity - meaning the light rays will be parallel. I assume that focusing a lens beyond infinity would cause it to focus CONVERGENT rays. But I don't think convergent rays are something that would normally occur from light reflected off an ordinary object.
For your practical purposes, I suspect autofocusing on the moon, then switching to manual, would probably give you a good enough approximation.
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