Re: Are primes brighter and sharper than wide open zooms
- From: David Littlewood <david@xxxxxxxxxxxxxxxxxx>
- Date: Fri, 30 Sep 2005 10:08:51 +0100
In article <1128038151.161411.315790@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, Alan Meyer <ameyer2@xxxxxxxxx> writes
David Littlewood wrote:
As you suggest, you should look at the number of groups rather than the number of elements.
In practice, the number of cemented pairs in any lens is usually much smaller than half the number of elements. 1 or 2 pairs is common in fixed focal length lenses (and quite a few have none). Zooms on average have a couple more - but even here, some have none (e.g. the EOS 28-80 f/3.5-5.6II and 35-80 f/4-5.6 III both have 10 elements in 10 groups).
David,
I'm confused about this.
If two lenses in a group were cemented together for their entire surfaces, why would they be made as separate lenses instead of as a single lens?
I thought "groups" referred to lenses that moved together in focusing, not lenses that had no air between them.
Alan,
This is a standard technique (actually developed 250 years ago by John Dollond, whose name lives on in a British firm of opticians) for reducing longitudinal chromatic aberration. A positive (convex) lens of low dispersion glass is cemented to a weaker negative (concave) lens of high dispersion glass. The net effect of the strong positive lens and weak negative lens is to give a positive (converging) lens; the effect of the difference of dispersion is to cancel out the difference in the converging power with different wavelengths (colours) of light. Such pairs are known as "achromatic doublets". The reason for cementing them is to reduce surface reflections; the cement used has a refractive index as close as possible to that of the glasses used.
Dispersion refers to the difference in bending power (refractive index) which all glasses show. Light of shorter wavelength (blue) is bent more than light of longer wavelength (red). Although all glasses (indeed all interfaces between different transparent materials) show dispersion, it does vary according to the exact composition of the glass. Lens designers now have a wide range of glasses available with different combinations of refractive index and dispersion, especially glasses of very low dispersion, which has made possible great improvements in design.
The term "groups" is slightly ambiguous. In lens statistics, it is normal to state "X elements in Y groups", which means that some (actually X-Y) pairs of lenses are cemented together to form permanent pairs. However, in zoom lens design, it is common to refer to "groups" of lenses (or doublets!) which are moved in synchronisation as the focal length is changed - or indeed when sets of elements are moved together for internal focussing.
So, in essence, you were right, for one of the meanings, but unaware of the other one.
David -- David Littlewood .
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- From: David Littlewood
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