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Old September 28th 05, 04:42 PM
David Littlewood
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In article . com,
Siddhartha Jain writes

Given two lenses, one a prime (say 28mm) and the other a zoom (say
28-75mm) and both with an aperture of f2.8 -
- Will the prime be brighter than the zoom because it has fewer lens
- Will the prime be sharper wide open than the zoom at 28mm?

Ofcourse, a lot will depend on the particular makes and models being
compared but is there a rule of thumb?

- Siddhartha

Other things being equal, yes to both, though with well-designed modern
lenses the difference may be small.


The conventional f-stop designation of aperture is a purely geometric
measure and takes no account of the actual transmission properties of
the lens. In reality, not all of the light going into a lens will come
out at the other end, and some of the light that does come out will be
scattered, and hence will reduce image quality.

Attenuation takes two forms: absorption and reflection/scattering.
Absorption is purely proportional to the depth of glass; a typical
figure for normal optical glass would be 10% for a total glass path
length of 100mm. Most photographic lenses would fall far short of this,
though some big lenses may get there. This attenuation is entirely
proportional to the length of the light path through glass; thus a zoom
with 12-15 elements is likely to experience more absorption than a fixed
focal length lens with 5-10 elements.

The other form is reflection from glass-air interfaces. This is
unavoidable, but can be reduced very greatly by coating. The percentage
of reflection depends on the refractive index of the glass, but for
typical n=1.50 optical glass (uncoated) the percentage is about 4%.
This, remember, is at each glass air interface, two per lens element.
Thus a compound lens with 15 elements will have 30 interfaces, and will
only transmit (0.96)^30 or about 20% of the light. (In fact another
20-40% will get to the film or sensor as scattered light - giving an
image of appallingly bad contrast). A single layer coating will reduce
the reflection to about 1-1.5%, and modern multi-coating reduces it to
around 0.3-0.5%.

This still gives a transmission factor of about 83.5% for a 20-surface
(10-element) system, against 91.4% for a 10-surface (5-element) system.


There is not the same direct relationship between complexity and
resolution as that above between complexity and transmission. However,
the compromises required to balance zoom ratio, overall size, mechanical
complexity and cost at the same time as controlling the seven distinct
varieties of lens aberrations mean that in almost every case the zoom
lens will have lower resolution than the fixed focal length lens of
similar quality of design and manufacture. You can see this from the MTF
function curves published by most lens manufacturers. Having just
checked some of these myself to answer your question, I am actually
quite impressed by how small these differences are; a couple of decades
ago the differences would have been much greater. (Be aware when
comparing MTF curves that they usually show wide open and f/8 data; as a
zoom will usually have a smaller maximum aperture than the comparable
fixed focal length lenses, you should avoid comparing these - best look
at the f/8 curves for a fair comparison.)

David Littlewood