Focusing Scope with BLUE filter?

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?

In article ,
"2" wrote:

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?

Peak or Omega-"Peak" focuser:

The blue filter is supposed to aid in grain focusing fine grain black
and white film. Never found I really needed it.
--
The sometimes insomniac.

www.gregblankphoto.com

2 wrote:
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?


Since paper was traditionally sensitive only to deep blue, the idea was that
if you focused on what the paper saw (instead of what the eye sees: peaks in
the green), and you had poorly color-corrected enlarging or focuser lenses,
the blue filter would give you better focus.

For my top of the line Omega focuser with the long mirror, it comes with a
blue filter, but it is too dense and I cannot see to focus with the filter.
My enlarging lenses are very good, and the focuser eyepiece is probably very
good, so I do not use the blue filter.

In fact, I am so nearsighted that I can focus almost as well without the
focuser.

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"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.


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

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.

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

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.

PATRICK GAINER wrote:

If you do the experiments you will find that the
blue filter gives no better focusing for blue sensitive
materials than no filter.


Well nearly 100% of us keep our fingers crossed and hope
that is the case. I'm hoping my 105mm f5.6 Nikor has a blue
focus Exactly in the same plane as that more visible portion
of the spectrum with which I focus.
I use most powerfull over the counter reading glasses and
keep both eyes open. Then I stop down. Dan

wrote:

PATRICK GAINER wrote:


If you do the experiments you will find that the
blue filter gives no better focusing for blue sensitive
materials than no filter.




Well nearly 100% of us keep our fingers crossed and hope
that is the case. I'm hoping my 105mm f5.6 Nikor has a blue
focus Exactly in the same plane as that more visible portion
of the spectrum with which I focus.
I use most powerfull over the counter reading glasses and
keep both eyes open. Then I stop down. Dan



If you feel like testing that, the best way to measure effects of color
on focus is to use the head movement rather than the lens movement. I'm
assuming that you have a crank or wheel to move the head. Set the focus
of something like a hair between two pieces of glass in the negative
plane. Measure the height of some part of the enlarger head above the
easel. Shut your eyes and move the head a little and refocus. Do this
about ten times starting from above and below the point you first set.
You will see some more or less random variations. Record them. Now
change to either red or blue light and repeat the series. I think you
will see random variations increase with either red or blue light,
showing a higher probable error of focus, and you may also see a shift
of the mean focus distance with change of color. Is it the lens or your
eye? Make a print of the hair at each of the mean distances you found,
with and without the filters you used.

PATRICK GAINER wrote:

If you feel like testing that, the best way to measure
effects of color on focus is to use the head movement
rather than the lens movement.


OR, lift the plane of focus some varying minute amounts.
I've had machinist training and may have just the instrument on
hand for measureing the amount. Glad you reminded me. Had
it in mind a few years ago to check for my lens' focus in
the deep blue and violet portion of the spectrum. Dan