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Old Sep 23, 2010, 2:53 AM   #11
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Originally Posted by peripatetic View Post
From wikipedia


Until recent years focusing of a camera lens to achieve a sharp image on the film plane was achieved by means of a very shallow helical thread in the lens mount through which the lens could be rotated moving it closer or further from the film plane. This arrangement whilst simple to design and construct has some limitations not least the rotation of the greater part of the lens assembly including the front element. This could be problematical if devices such as polarising filters were in use that require to maintain an accurate vertical orientation irrespective of focus distance.
Later developments adopted designs in which internal elements were moved to achieve focus without affecting the outer barrel of the lens or the orientation of the front element.
Many modern cameras now use automatic focusing mechanisms which use ultrasonic motors to move internal elements in the lens to achieve optimum focus.
Again what I needed to know. Thank you very much!

Edit: To make sure I'm understanding correctly, focus is controlled by the distance of the front lens element (the element closest to the body) to the body?

Last edited by jWest; Sep 23, 2010 at 2:56 AM.
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Old Sep 23, 2010, 2:59 AM   #12
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The focus distance is the distance from the subject to the image sensor.

Different lenses adjust the focus distance in different ways. Some lenses move optical elements at one end of the lens or the other, and some even more optical elements within the barrel of the lens. If the lens moves elements at the end of the lens, then the lens will get longer or shorter as the focus distance changes, and the filter mounting ring will rotate as well. If the lens moves elements within the barrel, then the lens won't change in length and the front element won't rotate.
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Last edited by TCav; Sep 23, 2010 at 3:03 AM.
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Old Sep 23, 2010, 3:31 AM   #13
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Basically the elements in the lens move in and out a bit.

Think of trying to burn a piece of paer (or an ant) with a magnifying glass. You need to move it back and forward a bit to get the rays focussed just where you need them.

Same thing with the elements in a lens, to get them precisely lined up so that they focus the image on the sensor you need to move them back and forth a bit so that the image is sharp, depending on how far way is the thing you are trying to focus on.
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Old Sep 23, 2010, 3:52 AM   #14
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OK, I understand now. Thank you both for taking the time to explain.
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