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01/12/2005 10:18:40 PM · #1 |
I'm just curious does anyone have any good pics which show what the differences are between the types of glass(ED and whatever else)
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01/13/2005 02:20:36 AM · #2 |
Originally posted by colema19:
I'm just curious does anyone have any good pics which show what the differences are between the types of glass(ED and whatever else) |
Your on the internet where all subtleties are lost.
Message edited by author 2005-01-13 04:14:03.
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01/13/2005 04:05:01 AM · #3 |
Ummmm...
You can't really take pic's to show the different types of glass.
You can compare lenses, but not the individual properties of the glass types.
Even the most simple lens (50mm prime for e.g.) has 7 seperate glass elements. The properties of each is combined to give the end result, and the individual elements are not identifiable in the end result without fairly sophisticated equipment. Certainly taking photos and scaling them down for posting on a website wont show you anything meaningful about a single glass type.
Looking at something like the 70-200 F/2.8L IS USM has 21 elements, 4 of which are UD glass.. That's a lot of variables!
I suppose if you worked for Canon and had the ability to replace a UD element in a lens temporarily with a standard glass one you could get an idea of the difference, but that's not likely to happen.
Cheers, Chris H.
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01/13/2005 06:56:27 AM · #4 |
Sorry I wasn't clear in the first post. What I mean is, what the effect is on the resulting photo between using something with low dispersion versus ED or UD glass elements?
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01/13/2005 06:41:19 PM · #5 |
Originally posted by colema19:
Sorry I wasn't clear in the first post. What I mean is, what the effect is on the resulting photo between using something with low dispersion versus ED or UD glass elements? |
If all other factors are equal and under the right conditions perhaps nothing. Start adding factors like stray light, reflections, glare, flash bounce or in the case of many wide angle lenses, edge distortion and the difference can be a great shot or one you have to throw away.
Message edited by author 2005-01-13 21:18:12.
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01/13/2005 07:43:22 PM · #6 |
There is a bit of misunderstanding about the effects of glass types on a lens̢۪s performance. A lens designer̢۪s job is to try and reduce all the aberrations of the lens, this includes geometric distortion BTW. What the designer has to work with are degrees of freedom. Degrees of freedom are such things as the curvature of a surface the spacing between two surfaces and the index and dispersion of the glass for each lens. If you allow the designer to choose glasses that are more exotic then he (or she) can design a lens with fewer surfaces that will perform as well as one with more surfaces. It should be noted that this is no limit to a lens̢۪s performance if it does not use exotic glass, it just means that it will need more elements to achieve the same quality.
There are old lenses that are great that did not use the low dispersion glass that is available today but which produce very high image quality. In the end the Canon uses low dispersion glass for their top end lenses because they can make them cheaper that way, it can be cheaper to use one relatively expensive lens instead of two less expensive lenses.
You also can not replace one type of glass in a lens design with another, unless you change a whole bunch of other parameters as well. A lens is a very careful balance of elements, each contributing to the over all aberration of the final image, the trick is some elements contribute a negative amount and others a positive amount, a lens that works has the sum of all the aberrations from all the elements add up close to zero. If you change any one element you mess up the whole balance and you have a junk lens.
Now days it is a computer that does this balancing act, with input from a lens designer. There is still a lot of art to a good lens design because you need to tell the computer program where to start from, and this makes all the difference in the world.
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01/13/2005 08:46:49 PM · #7 |
These newer low dispersion glass types permit using fewer lens elements. There are several advantages as a result: 1. less reflected light in the optical path. Each surface reflects some of the incident light causing a reduction in contrast among other things. 2. Fewer elements means lower production cost (but probably higher engineering costs). 3. Low dispersion glass has the potential to reduce fringing (not associated with normal CA) and improve edge definition as a result. 4. Design of an apocromatic lens corrected at three wavelengths is easier. (achromats are corrected at two wavelengths)
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01/13/2005 09:17:17 PM · #8 |
Originally posted by ElGordo:
These newer low dispersion glass types permit using fewer lens elements. There are several advantages as a result: 1. less reflected light in the optical path. Each surface reflects some of the incident light causing a reduction in contrast among other things. 2. Fewer elements means lower production cost (but probably higher engineering costs). 3. Low dispersion glass has the potential to reduce fringing (not associated with normal CA) and improve edge definition as a result. 4. Design of an apocromatic lens corrected at three wavelengths is easier. (achromats are corrected at two wavelengths) |
I knew that.
ROFL
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