Re: [APML]: Telephoto lenses

[Chuck Vaughn <aa6g@aa6g.org>]

Michael Covington wrote:
> 
> (1) All f/2.8 lenses should be equally critical since focus tolerance is
> determined by f-number only.  (It determines the shape of the incoming
> light cone.)

This brings up some questions which shows I don't know anything about
camera lens design. It seems unlikely to me that the front element in
the lens is f/2.8 but something much slower. That would mean there 
must be substantial focal reduction going on. I wonder if the 250mm-f2
and the 350mm-f2.8 use the same front element?

These telephoto lenses have rather small rear elements. Is there really
an f2.8 light cone coming out of there? I've wondered if it's really
a projection system. I guess that would be a diverging cone. The reason 
I think this is because the addition of the 1.4 extender makes a
larger image circle covering the same area as at f/2.8 but the camera
just uses less of it.
 
> (2) This lens is mainly marketed to sports photographers and I'll bet
> the helicoid is designed for fast focusing on a moving target -- that is,
> it moves in an out a relatively long distance for a small amount of rotation.

As always seems to be case, we astrophotographers make due with
instruments
designed for another use.
 
> (3) It's a very good lens, so an in-focus image really is a lot sharper
> than a slightly out-of-focus one.

Absolutely true!

Chuck  <aa6g@aa6g.org>