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

RE;Richard Knoppow wrote:


Its supposed to prevent mis-focusing due to chromatic
aberration in the enlarging lens. ...
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.
Focusing using blue light is supposed to eliminate the
error caused by the chromatic aberration of the lens.




I can't see anything there to argue with. I would state
the
case a little differently. The blue filter is there to
assist in
focusing the image on blue only sensitive materials. A
Graded paper is the usual blue only sensitive target.
If the lens is corrected for only the blue, as I believe
is the
case for some process lenses, then use of the blue filter
is almost manditory. Dan



I doubt that you will find a lens that is corrected for
only one color.
That amounts to no correction of any kind, or to perfect
correction at
any color as long as you expose it to only one narrow
color band. If you
do the experiments you will find that the blue filter
gives no better
focusing for blue sensitive materials than no filter. It
is not the blue
sensitive material that does the focusing, but the human
eye. It is a
demonstrable fact that the resolving power of the human
eye is very poor
at the wavelength of blue to which graded paper responds.
It is
customary to achromatize lenses for blue and green for
that reason.


The choice of color for achromatizition depends a lot of
the age of the design and intended purpose of the lens. Very
old designs were achromatized for blue and green but more
modern lenses are often achromatized for blue and red. The
deviation from focus beyond the two colors depends on the
closeness of the match between the glasses used. Blue
correction is also better in later designs than in earlier
ones because glass types with a better match for anomolous
dispersion became available.
An example of a fairly early design corrected for blue
and red is the Taylor, Taylor, and Hobson Speed Panchro
lens. This was a Biotar type designed for professional
motion picture cameras using panchromatic film. many earlier
motion picture lenses were corrected for orthochromatic film
and did not perform well at the red end of the spectrum. The
Speed Panchro dates from the early 1930's and is another
result of the change to sound in movies. This, for various
reasons, required a change to panchromatic film.
Anomolous dispersion is the result of using glass near
its cut-off wavelength. At this point the dispersion begins
to increase very rapidly. In most of the pass band of the
glass the dispersion changes fairly regularly with
wavelength but at the far blue end it can change rapidly
enough so that the glass type being used to correct it can't
keep up. So, the chromatic aberration becomes very great. I
think this is the problem Kodak had with their early
enlarging lenses (pre-WW-2). In their set up instructions
for their auto-focus enlargers they recommend not focusing
visually when using these older lenses because their
"chemical" focus an visual focus does not co-incide. Later
lenses, like the Kodak Enlarging Ektar and Enlarging
Ektanon, do not have this problem. Kodak did NOT recommend
focusing with a blue filter but, rather, experimentaly to
find the best focus.

I agree about the loss of sharpness of the eye at the far
ends of the spectrum.


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Richard Knoppow
Los Angeles, CA, USA