projection lenses-- 2.8 fixed vs 3.5 zoom

[P3D Elliott Swanson <e3d@xxxxxxxxxx>]

Re credentials: I don't have a Brackett, but I have worked as a theatrical
projectionist (back in the ancient days of carbon arc and changovers
every 20 minutes)...

Projection has a lot of factors that need to be considered in addition to
how much light you can whomp on the screen. If one is using a projector
for the same setup at all times, then a fixed 2.8 lens would be the one to
get. Projection lenses are expensive, but if someone uses a machine with a
couple of standard setups, then it might be worth buying a second lens set
for the alternate use (keeping in mind that with a Brackett, you need to
buy *four* lenses per setup!) 

But for the average user, there's a lot to be said in favor of a zoom even
though it might cost you .5 fstop. If you're moving the machine around for
use in different people's homes and sometimes use it for clubs or group
showings where the projection distance and room configuration can vary
wildly, the ease of setup for a zoom equipped projector is hard to beat.
It has an added advantage of allowing you to deal with whatever sized
silver screen you're likely to encounter. 

In the real world of projection, the way light output is commonly cranked
isn't so much related to the lens, as the source of light itself. People
may have seen those super Ektagraphic slide projectors with the big box
underneath. Well that's a monster power supply and cooling assembly for
its high output light system.

So, to sum up, if I was projecting slides under the same cirumstances all
the time, I'd go with a fixed lens that matched the projection
environment. For all other situations, gimme a zoom...

Elliott


------------------------------