[pure-silver] Re: Repurposing Enlargers
- From: Jean-David Beyer <jeandavid8@xxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Mon, 12 Dec 2011 18:36:39 -0500
Gerald Koch wrote:
> Camera lenses have a curved field of focus.
Surely not all of them.
> So if your subject is in
> the center of the image the edges of the image will not be in focus at
> large apertures.
This would be useful only if you could adjust the field curvature to
suite the image in question and if the subject is at the center of the
image to be made. There is no curvature that would be suitable for all
images. You can isolate the background from the rest of the image if it
is in a different plane from the center-of-interest by adjusting the
aperture of the lens, even a flat field one.
> This can be used to isolate the subject from the
> background.
>
> Lenses for BW enlargement need not be designed to focus all colors on
> the same plane. Ctein had an article on this focus shift somewhere on
> the web.
They tend to be designed to focus most wavelengths at the same point,
though. It is not worth the money to design two series of lenses for
enlarging: one for B&W and one for color. And if you use variable
contrast paper, you need at least to have the green and the blue focus
in the same plane.
Focus shift is something else entirely. It has to do with the focus
changing as a lens is stopped down.
>
> For LF use you may not see any problems due to the small amount of
> enlargement.
> Jerry
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