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Old Jul 1, 2005, 5:15 PM   #1
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As many, I started only recently with digital photo. I try to learn quite fast. There is a question, I couldn't find the answer until now. I'm got confused with the actions taking place in the lense, allthough I understand theinfluence of focal lengthon wide vs telephoto angle:

1. When you zoom in/out the lens moves changing the focal length (from wide to tele)- what particularly is moving (happening) in the lens?

2. When you are focusing (in fact autofocusing by halfpresing the button) is it again the focal length which is adjusted, why is it not changing the zoom ( how does this relates to zoom), and what particularly is being moved in the lens?

If I get answered, then a weight would be liftedoff my mind!!!
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Old Jul 2, 2005, 12:28 AM   #2
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A camera zoom lens is quite a complicated optic. In order to have a variable focal length, the objective lens must move in relation to intermediate elements, which must maintain symmetry between the objective and the focal elements. So the simple answers are only approximations of what is really going on.

1. The objective lens moves forward or backward in relation to other lenses.

2. The focus elements don't change the focal length of the lens relative to the subject, but adjust what part of the field is in focus, relative to the film or sensor plane.

Camerazoom lenses cam be considered to be actually two sets of lenses. One to adjust the focal length between lens and subject, and the othet to adjust focal length between lens and sensor. Somewhere in the middle, there is an intermediate focal plane, which is the basis for the two parts.

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Old Jul 2, 2005, 5:42 AM   #3
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Thanks! As You might expect, some aditional questions arose for me:

1. Is it a common practise to say that by changing the focal length we zoom in/out (move from wide to telephoto), allthough the same meaning could be applied to focusing?

2. Is there 1 single lens in the objective from which movement depends zoom. As I understand the other lenses serve different tasks like geometrical correction, focusing, e.t.c.

As You see, what made me confused, is that they (the authors) used the same term, focal length,when describing both- the zooming and the focusing.

May be You do know some site, where I could learn more about these processes?

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Old Jul 2, 2005, 6:46 AM   #4
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Zooming and focusing are two seperate functions.

When you zoom you change the focal length of the lensbetween wideand telephoto.

When you focus you are changing the point of focus just as you would with binoculars. If your subject is 1ft away the position of the lens in relation to the sensor needs to be different than when the subject is 3ft away. What focusing does is move the lens to bring your subject into focus on the sensor without changing the focal length of the lens.




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Old Jul 2, 2005, 10:24 AM   #5
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Nagasaki wrote:
Quote:
What focusing does is move the lens to bring your subject into focus on the sensor without changing the focal length of the lens.



That's exactly about what I'm wondering- how can the lens be moved, when focusing, without affecting the focal length?
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Old Jul 2, 2005, 10:27 AM   #6
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Marinjo...

I'd suggest you hunt down a copy of a book by Canon called "EF Lens Work III". It's a hard cover advertisement for Canon lenses, but has a full listing of all Canon lenses in production at the time the book was published. In addition to some fabulous photos, it gets into the design and manufacture of lenses. I received a free copy of the book when I bought my 20D. The technology of modern lenses is remarkable. They include some pretty exotic materials, amazing design techniques and close tolerance manufacturing, all of which is described in great detail in the book. Your local library might have a copy. I've seen it advertised on the Web for about $20, if your curiosity is that strong.
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Old Jul 3, 2005, 4:56 AM   #7
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Thanks to all sugestions! My curiosity is partialy satisfied. Of coarse, it would be fine to get a book with images of objective's construction- the lenses and there tasks. How do the lenses move when you are zooming, and how they move when you are focusing. It may seem funny why this is of any importance for me. But this madness started for me, as I allready told, when I found the term "focal length" being applied both for zooming and for focusing.:?

Maris
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Old Jul 3, 2005, 11:10 AM   #8
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Focal length determines the field of view, i.e., zoom. In terms of focus, the focal length enters because it (in part) determines the depth of field.
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Old Jul 3, 2005, 12:09 PM   #9
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The focal length of a zoom lens is controlled by changing the positional relationship between the individual elements or groups in the lens. Focus is controlled by changing the relationship of the entire lens with the sensing element, film or CCD.
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Old Jul 3, 2005, 3:43 PM   #10
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Thanks! Great answer- short and clear. I hope no more questions for my curiosity aboutzooming versus focusing.

Maris
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