# How does a lens focus? Scientifically speaking?



## prodigy2k7 (Jul 29, 2008)

I thought or understoof that your eye's lens physically changes shape (streches?).

How does focsing work? for something that is solid like glass? I am guessing it moves forward and backward but I am not sure.

Also, what the difference in elements/focusing whatnot in macro lenses verses regular lenses.


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## Moglex (Jul 29, 2008)

prodigy2k7 said:


> I thought or understoof that your eye's lens physically changes shape (streches?).


Yes, the curvature of the cornea changes to tune the eye's focus.



> How does focsing work? for something that is solid like glass? I am guessing it moves forward and backward but I am not sure.


Usually only some of the elements move.

If you look at the front of your zoom lens as you change focus, it generally stays put.

If you take the lens off the camera and adjust the focus you can usually see the rear element group move.



> Also, what the difference in elements/focusing whatnot in macro lenses verses regular lenses.


In a macro lens the design of the lens is optimised for close focusing.

It will still usually take perfectly reasonable normal shots in the same way as normal lens with extension tubes will take perfectly reasonable macro shots.

It's just a matter of optimising the design for the purpose that (it is assumed) the lens will be most used for.


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## Helen B (Jul 29, 2008)

For lenses of fixed focal length (often called prime lenses) the usual focusing method is to move the whole lens. There are exceptions, particularly with high-end and low-end lenses. For cheap lenses only one element, often the front element, may move. For some high-end lenses, and almost all zoom lenses, the focusing may be achieved by moving internal elements. 

There are some high-end lenses that combine both effects - ie they move the whole lens, and they move internal elements (floating elements). In those cases the internal element is usually moved to improve the image quality, not to focus. Nikon CRC (Close Range Correction) lenses are like that. There are some lenses, like the old Zeiss 40 mm FLE Distagon (FLE = Floating Lens Element), where the internal movement and focusing mechanism are separate - the user has to set the floating element, then focus.

Macro lenses are similar in that they may be focused by moving the whole lens without moving any internal elements, or they may focus by moving internal elements, or they may have a combination of the two. As well as improving the optical design for different distances, the floating elements may also reduce the focal length as the lens is focused closer. This is common among recent Nikon macro (Nikon call them 'Micro') lenses. This enables the lens to focus very closely without too much lens extension - the mechanism that moves the lens further from the image plane need not have such a large travel as it would have to have if the focal length stayed the same.

The performance of macro lenses at 'normal' distances varies a lot from design to design. The new 60 mm AF-S Micro-Nikkor is outstanding in this respect - it is good at all distances. 

Best,
Helen


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## Helen B (Jul 29, 2008)

Moglex said:


> If you take the lens off the camera and adjust the focus you can usually see the rear element group move.



Early zoom lens designs moved the rear group (the prime group, because early zooms were like a prime lens with a variable afocal converter in front of them) but modern designs rarely move the rear group - they usually move internal elements, and this can be seen by looking in the front of the lens. It may be a very subtle movement.

In my previous post I forgot to mention one other category of lens that has floating elements for focusing - lenses that do not change angle of view when focused. They are used in cinematography, so that when you do a focus pull the angle of view does not change.

Best,
Helen


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## Bifurcator (Jul 29, 2008)

And you might be asking yourself what all these "groups" are and if they're really groups, what's in these groups - groups of what specifically.


I think these pages are a good place to start: 

http://en.wikipedia.org/wiki/Lens_(optics)

http://en.wikipedia.org/wiki/List_of_lens_designs

http://www.digitalartform.com/lenses.htm

http://science.howstuffworks.com/lens3.htm (13 pages)

http://www.opticalres.com/optics_for_kids/kidoptx_p2.html 







http://www.opticalres.com/gentle_introduction.html

http://ezinearticles.com/?How-Does-A-Digital-Camera-Combined-Auto-Focus-Work&id=416048

http://answers.yahoo.com/question/index?qid=20070626140842AAS2Bmx

etc.​


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## Moglex (Jul 29, 2008)

Bifurcator said:


> http://en.wikipedia.org/wiki/Lens_(optics)



Clicking the above yields: _"Wikipedia does not have an article with this exact name. Please search for Lens (optics in Wikipedia to check for alternative titles or spellings."_


Try Wikipedia here, instead.


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## prodigy2k7 (Jul 29, 2008)

Change the link to (optics) its missing the ) at the end


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## Bifurcator (Jul 29, 2008)

Mine works. It goes right there.

 ???


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