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I am trying to get a greater understanding of how lenses work. Basically I bought this lens, the Sigma 10-20 f4-5.6:

Sigma 10-20mm 4-5.6

The construction has really fascinated me. 14 Elements in 10 Groups?! What exactly does that mean? Are the elements the blades?

What does this do the optics?

If I think about a pair of glasses there is effectively just one element.

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So, a conceptual problem complicates answering this. The "blades" are not the individual lens elements as pictured in your diagram, or the simple lenses in your glasses. The blades are the things that comprise the mechanical aperture diaphram. –  mattdm Apr 13 '12 at 18:42
The different lens elements and groups in a compound lens are discussed here: What does the number of elements and groups in a lens mean?, but maybe at a level which presumes a more-advanced starting point. –  mattdm Apr 13 '12 at 18:45
Ah thanks @mattdm I did a search - but the lack of my understanding didn't find the correct search terms –  Rob Apr 13 '12 at 18:48
Since @coneslayer provided a nice answer, I'm going to edit the title to include both. –  mattdm Apr 13 '12 at 18:57
It's like razor blades: The more the better! –  Unapiedra Apr 13 '12 at 19:12
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1 Answer

up vote 7 down vote accepted

The "elements" and the "blades" are two completely different things.

An "element" is a single piece of glass in the lens. Most of what's shown on the diagram are the lens elements. Some elements are colored pink, to indicate that their shape is aspherical. Other elements are colored blue, to indicate that they are made of a special type of glass. Others are uncolored, because they're just ordinary "spherical" elements made of typical kinds of optical glass. Your eyeglasses have a single element for each eye. Generally, using more elements, and using exotic types of glass or aspherical elements, allows the designer to better correct aberrations, or optical faults, in a lens.

When two or more elements have their surfaces in contact with each other (instead of having a gap between them), they form a group.

The number of blades refers to the number of diaphragm blades, that form the aperture stop. The diaphragm is an iris that you can make bigger or smaller, to control the amount of light that can pass through the lens. The location of the diaphragm is noted in the drawing by the vertical lines near the middle of the lens, above and below the central axis.

The number of blades is not shown in the diagram. The diaphragm looks like this:

Lens diaphragm from Wikipedia

This diaphragm has 6 blades; you can see that the aperture (opening) is a hexagon (6 sides). The number of blades is one of the factors that affects the bokeh of the lens. Your eyeglasses do not have any blades, because there's no adjustable diaphragm in eyeglasses.

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By the way, don't feel like you should get some deep understanding of the lens from its diagram. I think very few people could tell you the strengths or weaknesses from looking at it (I couldn't). I think lens makers mostly publish the diagrams to say "Look at all the parts in this lens! Some of them are fancy and hard to make! That's why we charge so much!" –  coneslayer Apr 13 '12 at 19:07
Lenses is one of those things where you pretty much do have to spend more money to get better. –  rfusca Apr 13 '12 at 19:14
Yeah, it's good to spend money wisely, but I think lens reviews and user reports tell you more than the lens diagram will. (FWIW, everything I've seen indicates that this lens is a very good value, and I plan to rent one for vacation this summer, with an eye toward buying it later.) –  coneslayer Apr 13 '12 at 19:15
@Rob Being a retrofocus lens, there are two parts that are relatively easy to understand separately. There is a long-focus lens at the rear, where the low-dispersion glass is concentrated. Long lenses are more subject to chromatic aberrations. The front elements are essentially a wide-angle converter module, and they really need to bend the light. Being relatively huge in diameter in relation to their focal (or, rather, virtual focal) length, they are more prone to spherical aberration and coma, so aspherical elements are very appropriate (and, yes, harder to grind than spherical elements). –  user2719 Apr 13 '12 at 19:23
Now, you'd need to know a little bit more than the average layman to make sense of it all, but if you do know that little bit you can make at least a "go/no go" appraisal of the design, at least in relation to its price—how much is "real design" and how much is $1500 direction-sensitive AC power cord. If it seems to make sense, it probably does. –  user2719 Apr 13 '12 at 19:28
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