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Old Apr 22, 2004, 10:04 AM   #1
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Default Infinity focus is how far?

Do lenese have an infinity focus? like maybe landscape are all objects say 20 feet away in focus? Or trying to take pictures of fireworks can you just turn the focus ring to a point or mark?
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Old Apr 22, 2004, 11:23 PM   #2
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I read somewhere that some cameras focus beyond infinity (not that there is such a place in this realm). That is to say the mechanics of the lens actually travels beyond the point where distant objects would be in focus.

The obvious result of this is that if you turn the ring to the stop, there is no focus point.

Unfortunately, I don't remember the rational for this design aspect. One explanation may have been to compensate for how temperature may affect the lens's focus point

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Old Apr 23, 2004, 12:55 AM   #3
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Correct that's what I also read (temp).

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Old Apr 23, 2004, 5:19 AM   #4
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IMO the beyond infinity focus is for design tolerance in the lens or temperature expansion/contraction like Frank explained, but the most logical explanation is in case the photographer add accessories like teleconverter lenses to the camera which might not bring parallel rays of light to the same focal plane... (a large DOF will take of this problem).

BTW the other need is for IR use which have a different focusing point on the lens!
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Old Apr 30, 2004, 3:50 PM   #5
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"Beyond infinity" is there - not sure if it's a design "feature" or a natural consequence of the way it works or what. But I was taking some shots of "distant" items and to speed things up I put teh camera on "MF" and just swung the ring full-trave... and found that it wasn't in fucus. Had to rotate it back a few mil. AF finds the "infinity" point fine, but you can still push it "too far" if you then turn the ring in the same direction.

This is with a 35-135 USM - a very nice lens so far. So I guess it's a feature and I'll just have to remember it and not "assume" that full-rotation is "infinity".
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Old Apr 30, 2004, 7:41 PM   #6
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If the object is 15, 20 or 50 feet away is the camera just going to focus to infinity? Maybe it depends on the lense?
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Old May 2, 2004, 5:05 PM   #7
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Yes, it does depend on the lens. Multi element lens have a variation in focus which is dependent on temperature for one thing.The change of focus due to temperature will only beapparent when a large temperature change is experienced. Also aperature size or "f" stop will allow for varying "depth of focus" depending on the focal length of the lens. It is not possible to state an exact distance that represents infinity but you can assume that it is probably around 100-200 feet or more for the usual range of lens and typical "f" stops thatthe typical photographer uses. In addition, attached filters can cause a slight shift of focus.

You can test a lens by manually focusing on a distant object with the aperature set at the smallest "f" number(largest aperature)and then without changing the focus photograph a picket fence line or similiar prespective and noting the nearest picket or object that is in focus.Try different lenses and different "f" stops and you will see the depth of focus for each combination. Then notice that you can change the focus to a closer objectwhile still have the most distant object in focus. The closest focus which will still have infinity in focus is commonly referred to the parafocal point for a given lens and aperature.

The focus range or "depth of focus" is the region within which acceptable focus is achieved and is a function of "f" stop andfocal length of lens. The focus control permits the photographer to select such things as an out of focus background by using a small "f" number(large aperature) and a longer focal length lens( or zoom in with a zoom lens).


PS If you are interested in the auto focus methodology which works by examining contrast of the image check out the following: http://www.navitar.com/download/how_af_works.pdf

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