"PATRICK GAINER" wrote in message
...
Richard Knoppow wrote:

"2" wrote in message
...


I picked up a focusing scope which has a deep blue filter
that flips into the eyepiece. (In fact, it's in there as
the default position.) What's the purpose of it? To
enhance
viewing contrast? Focus UV?



Its supposed to prevent mis-focusing due to chromatic
aberration in the enlarging lens. The idea is that
printing
paper is sensitive mostly to blue light and the eye to all
colors. In the early days of photography, when lenses were
not very good and plates sensitive nearly exclusively to
blue and UV light focusing by the visible image would
often
result in blured pictures. A lens was said to have visible
focus and _chemical_ focus, the latter being the focus for
the blue light the early plates were sensitive to.
Focusing
using blue light is supposed to eliminate the error caused
by the chromatic aberration of the lens.
Even some pretty good enlarging lenses made up to the
end
of the 1940's can have enough chromatic to make a
difference. Most modern enlarging lenses of good quality
are
very well corrected for chromatic and have _chemical_ and
_visual_ focus which are the same. Add to this that modern
variable contrast papers are sensitive to a range of
colors
from green to blue so that the enlarging lens has to be
well
corrected to get sharp images.
There is another problem which can be caused by the
filter. Some eyes have a significant amount of chromatic
and
some spectacles are also not very well corrected, so, you
will have to adjust the eye lens of the grain focuser
_with_
the filter in place to eliminate problems which may be
caused by the eye itself.
For most modern lenses the filter will make no
difference.




I did some research on this subject some years ago and
presented the
results in Photo Techniques. All human eyes have chromatic
aberration.
However, the idea that using a blur filter to focus for
blue sensitive
material does not work. It introduces two problems. First,
the eye's
maximum resolution is at its most sensitive wavelength,
which is the
yellow-green. Using any other wavelength for focusing
introduced a
greater random focusing error than did white light.
Second, the eye's
focal length varies with color. Combined with the reduced
resolution,
the effect was more random error with either red or blue
focusing as
well as a shift in focus of as much as 10 mm at the
baseboard in a 10X
enlargement.

You can demonstrate the shift in focus of your eye with
red and green
color separation filters. Focus as close to a printed page
as you can
without a filter. Interpose a filter and see which
direction you must
move the object to bring it to closest sharp focus.

The net result is that the least random focusing error as
well as the
best agreement with modern enlarging lenses will be
obtained by using no
filter. The random part of the error will be the more
important as with
proper use of a focusing crosshair the mean error will be
minimized.

I agree with this. The blue filter is based on a
misunderstanding.


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Richard Knoppow
Los Angeles, CA, USA