[LargeFormat] 90mm Schneider f5.6 Super Angulon vs f8

[Michael Briggs]

On 07-Apr-2003 philip.lambert wrote:
> Understood but if the fresnel screen is fixed to the rear of the ground
> glass without displacing it forward the plane of focus might be unchanged.

No, the plane of focus will be changed.   Because the index of refraction of
the fresnel lens is different from that of air, the effective optical distance
from the lens to the ground glass surface will be changed.    If the ground
glass was correctly located without a fresnel lens and you locate a fresnel
lens in front of the ground glass without repositioning the ground glass, you
will have a focus error.   The rule of thumb for glass is the effective change
in distance is about 1/3 of the thickness of the glass -- this rule is based on
the index of refraction of glass and might not be as applicable to a plastic
fresnel lens.

> I
> attached  a fresnel screen by its edges on the back of a ground glass with
> instant glue and maybe I could try attaching a screen by its edges on the
> front of the glass.  The question remains does the plane of focus change and
> experiments need to be done.  I have a Century Graphic that reached  me with
> a fresnel on the front of the screen which pushed the glass backwards and
> when I compared that one with a plain CG back there was a distinct

--Michael