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#1
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Cold Light for B&W?
Is there any truth to the suspicion that cold-light heads have a lot of
light close to UV which coincides with the extra sensitivity of some B&W papers in the same color-region - therefore causing lesser sharp prints? |
#2
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Cold Light for B&W?
jjs wrote:
Is there any truth to the suspicion that cold-light heads have a lot of light close to UV which coincides with the extra sensitivity of some B&W papers in the same color-region - therefore causing lesser sharp prints? I do not think so. There is a lot of uV produced by the mercury vapor arc in the tube, but the tubes are internally coated with phosphors that convert most of that to visible light with a spectrum decided by the manufacturer. Some put out a quite blue light, most put out bluish white, and some put out green (I have seen green only in dual-tube heads where a blue tube and a green tube, whose brightness is individually controlled for VC papers). I think there would be little point putting out uV light since if it is too far in the uV, the glass in the optical system would absorb it. Actually, provided the uV wavelength is long enough to get through the glass of the optical system and the gelatine holding the silver grains, if the light were truely monochromatic, I would imagine the prints could be sharper because of the shorter wavelength than normally used. For best results, you would want a lens designed for monochromatic light of the same wavelength. And, most important, you would need a way to see it so you could focus it. And all this is probably inconsequential, since you need only around 10 line-pairs/millimeter at the paper to make a sharp image. So if you really care, stick a #47B filter in your enlarger and use white light. Also stick a #47B in your focusing magnifier when you focus. It may be hard to see. I have such a filter for my magnifier, but it is too dim to focus by. -- .~. Jean-David Beyer Registered Linux User 85642. /V\ Registered Machine 241939. /( )\ Shrewsbury, New Jersey http://counter.li.org ^^-^^ 09:55:00 up 2 days, 1:30, 3 users, load average: 4.32, 4.24, 4.18 |
#3
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Cold Light for B&W?
jjs wrote:
Is there any truth to the suspicion that cold-light heads have a lot of light close to UV which coincides with the extra sensitivity of some B&W papers in the same color-region - therefore causing lesser sharp prints? I do not think so. There is a lot of uV produced by the mercury vapor arc in the tube, but the tubes are internally coated with phosphors that convert most of that to visible light with a spectrum decided by the manufacturer. Some put out a quite blue light, most put out bluish white, and some put out green (I have seen green only in dual-tube heads where a blue tube and a green tube, whose brightness is individually controlled for VC papers). I think there would be little point putting out uV light since if it is too far in the uV, the glass in the optical system would absorb it. Actually, provided the uV wavelength is long enough to get through the glass of the optical system and the gelatine holding the silver grains, if the light were truely monochromatic, I would imagine the prints could be sharper because of the shorter wavelength than normally used. For best results, you would want a lens designed for monochromatic light of the same wavelength. And, most important, you would need a way to see it so you could focus it. And all this is probably inconsequential, since you need only around 10 line-pairs/millimeter at the paper to make a sharp image. So if you really care, stick a #47B filter in your enlarger and use white light. Also stick a #47B in your focusing magnifier when you focus. It may be hard to see. I have such a filter for my magnifier, but it is too dim to focus by. -- .~. Jean-David Beyer Registered Linux User 85642. /V\ Registered Machine 241939. /( )\ Shrewsbury, New Jersey http://counter.li.org ^^-^^ 09:55:00 up 2 days, 1:30, 3 users, load average: 4.32, 4.24, 4.18 |
#4
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Cold Light for B&W?
"jjs" wrote
Is there any truth to the suspicion that cold-light heads have a lot of light close to UV which coincides with the extra sensitivity of some B&W papers in the same color-region There is no need for suspicion. Tubes not made for polycontrast printing produce light at the bluish end of the spectrum. Special tubes are now made for use with polycontrast filters and these do not have a blue spectrum. - therefore causing lesser sharp prints? There was some hysteria about this 5-10 years ago, caused by an article in one of the Photo/Darkroom/Techniques magazines. The claimed effect has never been duplicated. It turned out to be yet another whale of a tale. -- Nicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. Remove spaces etc. to reply: n o lindan at net com dot com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/ |
#5
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Cold Light for B&W?
"Nicholas O. Lindan" wrote in message hlink.net... [...] - therefore causing lesser sharp prints? There was some hysteria about this 5-10 years ago, caused by an article in one of the Photo/Darkroom/Techniques magazines. The claimed effect has never been duplicated. It turned out to be yet another whale of a tale. Ah, so very good to know! Thank you, Nicholas! (I still use tungsten bulbs. ) |
#6
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Cold Light for B&W?
"jjs" wrote in message ...
Is there any truth to the suspicion that cold-light heads have a lot of light close to UV which coincides with the extra sensitivity of some B&W papers in the same color-region - therefore causing lesser sharp prints? Perhaps you've confused actinic light (violet) with visual focus. Light sources rich in violet may yield blurry prints because you can't see that actinic light as vividly when you focus, and since lenses focus each wavelength a little different, you may end up with a differntial between the visual focus (yellow-green) and the actinic focus (violet). So, the anser is yes, but not for the reasons you may have imnagined. |
#7
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Cold Light for B&W?
"jjs" wrote
I still use tungsten bulbs. The article referred to was one of the many written by Ctein for, amoung others, Phototechniques. In a very few words, it is at end the light that reaches the paper and the color correction of the lens which determine the exact best focus. Tungsten sources are not excluded. The problem is due to the print paper's extrem sensitivity to the deep blue and long UV portion of the spectrum. To focus, visible light is required but it is the deep blue and UV which account for a large part of the exposure. So, no problem with a well corrected lens. To help, add a between light and lens UV filter; UV, as in Skylight, Haze. My personal read on the subject is to keep it in mind. Check the print against what was seen when focusing. The paper makes a difference. Many poo-poo Ctein's work with this subject but I think it a valuable contribution; very methodical painstakeing time consuming research. A good read but be very focused. If from a practicle view point it has no application with this or that individual the article is informative and "if the shoe fits" put it on. What you see MAY not be what you get. Dan |
#8
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Cold Light for B&W?
In article ,
"jjs" wrote: Is there any truth to the suspicion that cold-light heads have a lot of light close to UV which coincides with the extra sensitivity of some B&W papers in the same color-region - therefore causing lesser sharp prints? One unit I personally tested was the ZBE variable contrast Cold light lamp house. Tested it using Forte paper, very flat mid- tones sort of ran them all together. -- To win one hundred victories in one hundred battles is not the measure of skill. To subdue the enemy without fighting is the measure of skill. Sun Tzu |
#9
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Cold Light for B&W?
On Fri, 6 Aug 2004 09:39:51 -0500, "jjs"
wrote: "Nicholas O. Lindan" wrote in message thlink.net... [...] - therefore causing lesser sharp prints? There was some hysteria about this 5-10 years ago, caused by an article in one of the Photo/Darkroom/Techniques magazines. The claimed effect has never been duplicated. It turned out to be yet another whale of a tale. Ah, so very good to know! Thank you, Nicholas! (I still use tungsten bulbs. ) The article was by Ctein. Theoretically it's correct but nobody I know was able to determine the practical degree of the problem. Regards, John S. Douglas, Photographer - http://www.darkroompro.com Please remove the "_" when replying via email |
#10
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Cold Light for B&W?
On Fri, 6 Aug 2004 09:39:51 -0500, "jjs"
wrote: "Nicholas O. Lindan" wrote in message thlink.net... [...] - therefore causing lesser sharp prints? There was some hysteria about this 5-10 years ago, caused by an article in one of the Photo/Darkroom/Techniques magazines. The claimed effect has never been duplicated. It turned out to be yet another whale of a tale. Ah, so very good to know! Thank you, Nicholas! (I still use tungsten bulbs. ) The article was by Ctein. Theoretically it's correct but nobody I know was able to determine the practical degree of the problem. Regards, John S. Douglas, Photographer - http://www.darkroompro.com Please remove the "_" when replying via email |
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