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November 1, 2006 2:26 PM   Subscribe

Why does a good camera use more than one lens? and what is the physics phenomena associated with it?
posted by joshuak to Science & Nature (14 answers total)
 
You're oversimplifying a bit. You could build the best damn camera the world has ever seen, with a single fixed lens. In fact, I'm pretty sure that's what you'll find on spy satellites. For regular hand-held cameras, though, the designer asks himself "Do I give it a permanently attached lens or a removable one? If it is permanently attached, is it a fixed focal length, or a zoom?" And if you buy a camera with a removable lens, you ask yourself "Do I want to mount a zoom on this and call it a day, or carry around several regular lenses?"

The issue comes down to the particulars of a lens' optics. You can build a regular lens—that is, one with a single focal length—and optimize the optics to that focal length. Or you can build a zoom lens that covers a range of focal lengths.

One of the key advantages of the single focal-length lens is the aperture: the physics involved make it impossible to build a zoom lens that "stops down" as much as a regular lens, so it can't admit as much light, so it's "slower." A really good regular lens may stop down to f1.2; the best zoom lenses stop down to about f2. That's the difference between shooting at about 1/50th sec and 1/15th sec.

There are other issues, like pincushioning, that I think are easier to control for with regular lenses than zooms.

Serious photographers, for whom photographic quality ranks higher than convenience, want to be able to use the best lens for the current situation. Regular folks may be willing to put up with the optical compromises of a permanently attached lens.
Zooms have gotten a lot better than they used to be, and there have been some pretty serious cameras released in the past few years with permanently attached lenses.

As always, wikipedia is your friend
posted by adamrice at 2:46 PM on November 1, 2006


See this discussion of zoom versus fixed focal length lenses and the tradeoffs involved in lens design.
posted by caddis at 2:49 PM on November 1, 2006


Are you asking about compound optics which are assembled from multiple pieces of glass (or tissue, as in the eye), or interchangeable "lenses," (which are also typically compound optics)?
posted by Good Brain at 2:59 PM on November 1, 2006


Thank you for that, but I still don't get it completely. Are the multiple lenses for zoom purposes or to correct aberrations? And what is the "appropriate physics phenomena"?
This is for my physics class and no one has found a good answer to this question.
posted by joshuak at 3:01 PM on November 1, 2006


And I'm not sure about compound optics or interchangeable lenses, the only information I have is what I wrote in the original question.
posted by joshuak at 3:02 PM on November 1, 2006


Some of the lenses in the overall lens are for zooming and some are for correction. One of the phenomena which may answer your physics question is the variation in diffraction at various wave lengths - think prism.
posted by caddis at 3:04 PM on November 1, 2006


When you say more than one lens, do you mean interchangable lenses or do you mean more than one piece of glass within a given lens?
posted by caddis at 3:09 PM on November 1, 2006


The short answer is, issues like correcting for many optical abberations on one end of the zoom lens also affects the other end of the range, and the problem is dramatically exacerbated with longer zoom lenses. Thus, on an interchangable-lens system you'll get the best optical performance by switching between lenses with the specific focal length(s) and apertures you need, rather than trying to use one permanent mega-zoom that attempts to do everything comparatively poorly. For more details, Erwin Puts writes extensively about the problems involved with correcting optical aberations while designing lenses (but he's an unabashed Leicaphile, so take his words with a grain of salt).

And lots of "good" cameras use a single lens (usually fixed at a single focal length). For instance, the lenses on the Ricoh GR series are nothing short of legendary, and they're not interchangable-lens cameras at all. However, most non-interchangable lens consumer cameras are designed to meet specific price-points and are overloaded with bloated featuresets, and they wind up using cheaply-made zoom lenses that are optically junk.

On preview:

Are the multiple lenses for zoom purposes or to correct aberrations?

Both. You get better-corrected lenses by limiting your focal lengths (i.e. the "zoom") on your lens, so you'll end up needing multiple lenses to cover a large set of given focal lengths.
posted by DaShiv at 3:12 PM on November 1, 2006


"I'm not sure about compound optics or interchangeable lenses"

Then it's impossible for us to answer this question for you.

"This is for my physics class"

Then it is inappropriate for us to answer this question for you.
posted by adamrice at 3:27 PM on November 1, 2006 [1 favorite]


Different lenses have different focal lengths and give different depth of fields (depths of field?). I googled photo focal length depth of field example to get some sites:

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
http://www.luminous-landscape.com/tutorials/dof2.shtml
http://www.mir.com.my/rb/photography/fototech/htmls/depth.html
http://www.dpreview.com/learn/?/key=depth_of_field
Go to a decent library or bookstore for more examples.
posted by theora55 at 3:28 PM on November 1, 2006


If you are trying to ask why do camera optics have more than one lens as part of its optical system, then the reason is that the designers are trying to build an optical system that is compact as possible to give the best image quality for the price. The multiple lenses are there to control for chromatic and geometric aberrations that result from the fact that all optical materials interact with electromagnetic waves in a frequency (wavelength or color) dependent manner.

The boundary effects that result from light crossing an interface between two different materials is called refraction. This is a function of the wavelength of the incident light, and of the materials that are present on either side of the boundary. This effect is equavalent to impedance in electrical circuits.

(N.B.: Diffraction is the result of the wavefronts bending and interferring when they pass by a sharp edge.)

Not to be a snark, on preview, it's much better if you had asked a more detailed question after doing some legwork, than to post, verbatim, a homework exercise. But you knew that...
posted by scalespace at 3:33 PM on November 1, 2006


The problem was that I did try to find the answer myself but spent a lot of time and ended up with pretty much nothing.
posted by joshuak at 3:39 PM on November 1, 2006


And I'm not sure about compound optics or interchangeable lenses, the only information I have is what I wrote in the original question.

Then go back to your teacher and tell her you don't understand the question.
posted by timeistight at 3:52 PM on November 1, 2006


I think what you are asking about is what is called a compound lens. (Try googling it.) A compound lens is made by putting two or more simple lenses in series. There are several reasons for doing this.

The first is spherical aberration. Most glass lenses are ground as spheres because that is the easiest shape to make from glass. But a sphere is not the ideal lens that you talk about in physics. Rays of light that pass through the lens away from the center line will focus at a slightly closer distance than rays that pass through the center of the lens.

The physical property of a lens is described by the law of refraction or Snell's Law (sometimes called Decarte's Law). This states that the angle of light passing through a lens will bend proportional to the angle by which it enters and the index of refraction. A light ray that enters the center of the lens passes straight through without bending. The farther away from the center of the lens, the ray is bent more because it strikes the lens surface at a greater angle because of the spherical shape. This is the desired effect because you want rays on the edges to bend more to focus at the focal point. While a sphere is a good shape for a lens, it is not the perfect shape.

The ideal lens must be made aspheric, but aspheric lenses are much more difficult to make. It is cheaper to use a second spherical lens in series with the main lens to correct for the spherical aberration of the first lens. Aspheric lens are sometimes used for plastic eyeglass lenses which can be made by molding, unlike glass lenses which have to be ground.

The second effect is chromatic aberration. The index of refraction of glass is dependent on the wavelength of light. This is the effect that creates a rainbow from a prism. This means that light of shorter wavelength, blue, bends more than longer wavelengths, red. The result is that the blue light focuses closer than the red light so the image is fuzzy with spread out colors.

Again this can be explained by Snell's Law. The amount of bending is dependent on the index of refraction. Since different colors have different indices of refraction, Snell's Law says they will bend differently and spread out. A second lens can compensate for the spreading and bring it back into focus.

Chromatic aberration is the concept behind those "Blue Blocker" sunglasses you see advertised on late night TV. These orange lenses are supposed to filter out the blue light so that the remaining colors focus more sharply on your retina, because the lens of your eye exhibits chromatic aberration.

Note that reflecting telescope mirrors are not subject to chromatic aberration because they do not rely on refraction.

A high quality camera lens may consist of up to five simple lenses in series in order to best average out all of the aberration effects.

So the answer is that compound lenses are used to compensate for the non-ideal properties of lenses. The point is that the real world is more complex that the simple explanations in introductory physics.
posted by JackFlash at 6:06 PM on November 1, 2006 [1 favorite]


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