[LargeFormat] RE: Correction

[Richard Knoppow]

----- Original Message ----- 
From: "tap" <tyler.a.payne at comcast.net>
To: <largeformat at f32.net>
Sent: Thursday, July 21, 2005 9:40 PM
Subject: [LargeFormat] RE: Correction


> To keep the echo going, I'm here too.
>
> And speaking of lenses, I wish there were more out there 
> on the web about creating one's own lenses from scratch. 
> (I'm currently making a camera, haven't taken a large 
> format picture in my life yet, hence no expertise of my 
> own to share.)
>
> I have several crummy lens elements I bought from American 
> Science and Surplus, and I'm still searching for the best 
> way to line them up in a way that lets me experiment with 
> them, rather than attaching everything permanently right 
> from the start. My current line of thinking is that I'll 
> afix them solidly within some threaded juice bottle tops, 
> then I can interchange them with others mounted similarly 
> in the opposing threads.
>
> I suspect I'll end up buying some decent elements and 
> follow the precise instructions and focal lengths outlined 
> in the book Primitive Photography.
>
   One simple thing you can do if you have two the same or 
nearly the same is to arrange them as a symmetrical pair 
around a stop. You will have to find the optimum spacing by 
experiment. Symmetry corrects lateral color, coma, and 
geometrical distortion. You will still have to contend with 
other aberrations. Since each element in a complex lens is 
designed to fulfil a function in that lens its hard to find 
random, ready made, lenses that can be combined into a 
working lens. For one thing you must know something about 
them. Some of this can be measured fairly easily, like focal 
length. If you measure the curvature of the lens and its 
thickness you can get some idea of the index of refraction 
of the glass but you have to measure the dispersion by 
measuring the difference in focal length for two 
monochromatic colors.
   Of course, there is nothing to stop you from 
experimenting by just mounting lenses in cardboard mounts 
and moving them around while observing the image. You never 
know what you will come up with.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk at ix.netcom.com