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Focusing Scope with BLUE filter?



 
 
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  #11  
Old June 13th 06, 03:39 AM posted to rec.photo.darkroom
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Default Focusing Scope with BLUE filter?


"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.


--
---
Richard Knoppow
Los Angeles, CA, USA



  #12  
Old June 13th 06, 03:39 AM posted to rec.photo.darkroom
external usenet poster
 
Posts: n/a
Default Focusing Scope with BLUE filter?


"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.


--
---
Richard Knoppow
Los Angeles, CA, USA




 




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