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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? |
#2
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Focusing Scope with BLUE filter?
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 |
#3
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Focusing Scope with BLUE filter?
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. -- .~. Jean-David Beyer Registered Linux User 85642. /V\ PGP-Key: 9A2FC99A Registered Machine 241939. /( )\ Shrewsbury, New Jersey http://counter.li.org ^^-^^ 07:10:01 up 55 days, 20:40, 3 users, load average: 4.53, 4.64, 4.60 |
#4
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Focusing Scope with BLUE filter?
"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 |
#5
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Focusing Scope with BLUE filter?
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. |
#6
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Focusing Scope with BLUE filter?
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 |
#7
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Focusing Scope with BLUE filter?
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#8
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Focusing Scope with BLUE filter?
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 |
#9
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Focusing Scope with BLUE filter?
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#10
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Focusing Scope with BLUE filter?
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 |
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