Re: More camera ideas and questions

[bercov@xxxxxxxxxxx (John Bercovitz)]

> Finally, is there an easy way to figure out how far beyond the
> infinity point you have to move the lens in order to focus at a
> specific distance?

Sure, use Newton's form of the lens equation:

   x * x' = f * f

x is the distance from the front focal point to the object
x' is the distance from the rear focal point to the image
f is the focal length of the lens

Naturally I lost the message I was replying to 8-( so let's say
you have a lens with a focal length of 100.  So f * f is 10000.

If the distance from the front focus to the object is 1000 mm, 
then the film has to be moved away the lens:

x' = 10000/1000 = 10 mm

Simple, huh?  This is the equation camera makers actually use to mark 
scales on lens focussers.

If you're not familiar with the concepts of front and rear focus, get 
back to me.  They are the easiest lens points to measure.

John B