When I printer color, My color head has a white light lever which I use to focus my neg. So is this method not accurate focusing? Does the focus shift when I take the color out white light mode?
Thanks I understand that,
I was curious as some cold head light systems in the past used blue and green light for printing and I was wondering if that is what you were referring too.
Originally Posted by Michael R 1974
I don't see how split grade would be any different. After all, the contrast with split grade is the same as with using an intermediate filter, so the same proportions of blue and green light must have made it to the paper, so the focus shift must be the same.
what about people who use split grade all the time (ie always mixing 0 and 5)? It seems to me this would be the worst case scenario since the the focus shift between the grade 0 exposure and grade 5 exposure would be at its maximum.
I'm trying to work out in my head if I agree or not. Your statement seems logical, but I'm wondering if the transmittance might be different outside the visible spectrum - ie the intermediate filter and the split grade are the same in the visible spectrum but maybe the net transmittance of UV range light is higher with the split grade approach? I could easily be wrong about that though I'm still thinking it through.
Originally Posted by BetterSense
Sorry about that I was thinking as I wrote it, why is a guy with this much experience asking this...
Originally Posted by Bob Carnie
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This is complicated. I think we are both wrong. Visibility of the light is not important to the density that is developed on the paper. Silver is formed on the paper in proportion to the amount of actinic light that falls on it. Assume a paper with blue-sensitive high-contrast emulsion and green-sensitive other emulsion. If two prints are made to the same contrast, it directly follows that the two emulsion layers were reduced exactly the same amount.
the intermediate filter and the split grade are the same in the visible spectrum but maybe the net transmittance of UV range light is higher with the split grade approach?
I suppose, however, that it's possible that the single-filter approach exposes the high-contrast emulsion with X seconds of near-UV light, whereas the split-filter approach exposes the high-contrast emulsion with Y seconds of deep-UV light--both exposures resulting in exactly the same density of the blue-sensitive layer. The contrast could be the same, but the total wavelength spread used in exposure could be greater using one approach compared to the other. However, I don't see how you can say either way whether a split-grade approach causes more wavelength spread for the same contrast. It seems to be that it could easily go either way.
Hey guys and gals...
All of this speculation about ultraviolet paper sensitivity is a waste of electrons.
Tungsten lamps (including halogen) do not emit any significant energy in the UV portion of the spectrum.
Quoting from the Zeiss article on the subject:
"The majority of the emitted energy (up to 85 percent) lies in the infrared and near-infrared regions of the spectrum, with 15-20 percent falling into the visible (400 to 700 nanometers), and less and 1 percent in the ultraviolet wavelengths (below 400 nanometers)."
and their spectral curves for emitters of various color temperatures:
The entire article is here: http://zeiss-campus.magnet.fsu.edu/a...html#colortemp
Last edited by Leigh B; 08-02-2011 at 07:52 PM. Click to view previous post history.
Regarding the supposed focus error when using a grain focuser with blue light...
I don't understand how this could happen if the focuser is used as designed, according to its instructions.
My focuser has a set of cross-hairs. I assume other similar instruments do also.
The instructions say to focus on those, then, while holding them in focus, adjust the enlarger so the grain is also focused.
The cross-hairs set the focus point at a distance corresponding to the base of the focuser, i.e. the surface of the paper.
The point discussed by Ctein on page 76 (quoted from another person's research and article) talks about an error of the human eye when trying to focus blue light, with no mention of any calibration system, i.e. the cross-hairs of the grain focuser.
In the situation under discussion, both the cross-hairs and the grain are being viewed in the same light. If they're both in focus, then the image is focused on the paper regardless of what gyrations the eye must use to perceive focus in the first place.
Last edited by Leigh B; 08-02-2011 at 07:47 PM. Click to view previous post history.
Leigh - I think you have missed the point. There is no doubt the grain and crosshairs are aligned visually, but it is the emulsions response to light of a certain wavelength that is at issue here.
Put another way,shooters of infrared film are aware that,after reaching good visual focus,they must then offset the lens focus to bring the IR rays into proper focus on the emulsion layer.
Sometimes,"What you see is not always what you get".
A graded paper emulsion is sensitive only to blue light (within the visible spectrum), while VC paper is sensitive to both blue and green. Neither type of paper exhibits any sensitivity at the red end of the spectrum.
Any enlarger lens designed since the 1930's is corrected for green and blue, so the focus of those two colors should be coincident.
By using blue light, both colors should be properly focused. The same would be true if you used a green filter for focusing.
Using white light means the eye will focus at its wavelength of greatest sensitivity, which is yellow (i.e. green + red). This brings red into the focus equation when it should not be.