[pure-silver] Re: making smaller prints with 8x10 enlarger
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Wed, 13 Oct 2004 20:27:38 -0700
----- Original Message -----
From: "Leigh Solland" <solland@xxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Wednesday, October 13, 2004 5:54 PM
Subject: [pure-silver] Re: making smaller prints with 8x10
enlarger
> Ryuji Suzuki wrote:
>
>> technically, "longer lens" and "telephoto
>> lens" do not mean the same thing.
>
> What is the difference?
>
> Leigh
>
A telephoto lens is one where the back focus, that is the
distance from the back of the lens to the film when focused
at infinity, is closer than a "normal" lens of the same
focal length. The source of the image of a lens is the rear
or second principal point. If one measures the angle made by
the light rays on the film for a point at infinity focus
they will coverge on the rear principal point. The telephoto
effect is gotten by mounting a regular lens in front of a
strong negative lens. Most of us are familiar with the
effect of looking through a negative lens to make objects
look further away. Artists sometimes use negative lenses to
get a better idea of perspective. The telephoto lens works
on exactly the same principle. The negative lens makes the
front part of the lens seem further away so it is focused
even though it is physically closer than the front lens
would be otherwise. The distance the lens must move to focus
to closer distances remains the same.
The reverse system is used for "retrofocus" or reverse
telephoto lenses commonly used on single lens reflex
cameras. Here, the negative lens is in front of the normal
lens. The effect is to increase the distance the lens is
from the film for infinity focus. In an SLR the back of the
lens must clear the mirror so this trick is used to allow
the use of wide angle lenses on such cameras. A normal wide
angle lens would project into the mirror box.
Both telephoto and retrofocus lenses are harder to
correct than normal lenses because the designer can not use
symmetry. However, when carefully designed they are capable
of excellent performance.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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