In article
>How much brighter would the ground glass of a 4x5 be with the addition
>of a fresnel glass? What, exactly, is one?!?! I would look it up
>myself, but it's not in my books.
If you are using a longish lens on the 4x5, the Fresnel lens
may not be worth bothering with, but with normal, and shorter
lenses, it is, and it will noticeably brighten the edges/corners
of the viewing screen. There may be a slight loss in focus ease,
but the brightness/illumination-evenness improvement is worth it
for me. (BTW, you can buy Fresnel lenses from Edmund Scientific
quite cheaply. Something about 12' in focal length, and with fine
circle-spacing should do - and you can cut larger sizes to fit by
scoring the plastic lens using a matte knife and ruler, and
breaking the lens along the score. A little sanding cleans up the
edges.) Be careful of the faces (especially the ridged side) when
handling the lens. Place the Fresnel (ridged side in, for protection)
against the rear surface of the camera GG.
Oh, a Fresnel lens is a collapsed condenser lens... Imagine laying a
magnifying glass with one flat surface and one convex surface down
on a surface (you want a "+" diopter type, not a "-", like those
used in RV rear windows...;-). Next, imagine cutting the lens into
concentric thin circular pieces, all with the same thickness between
cuts. Since the essence of the lens focal length is contained in the
shape of the outer two surfaces (for our purposes), you can cut down
the material inbetween the top and bottom surfaces of the concentric
rings, and collapse the surfaces together. The original convex surface
retains its original curvature, but in a stepped format, allowing the
overall lens to be flat and thin while retaining its original FL.
Hope This Helps