Ilford MG IV RC sensitivity: blue or green?

zumbido · Feb 27, 2012

So I'm looking at this:

http://www.interfoto.fi/ilfordphoto/pdf/paperit/mgivrc.pdf

I always hear that b+w paper is "blue-sensitive", but the spectral sensitivity chart here seems to say that while it *is* blue-sensitive, blue is actually in a local trough and that it's *more* sensitive to violet and green, in particular a pronounced hump there at just about the usual range for green LEDs ( ~515nm ).

Am I just too ignorant to interpret this correctly, or is it true that Ilford MG is more sensitive to violet and green than blue?

ic-racer · Feb 27, 2012

The blue sensitive emulsion is blue sensitive. The green sensitive emulsion is green sensitive.

zumbido · Feb 27, 2012

OK, interesting. I didn't know there were two separate layers. I find the green sensitivity in those charts surprising, because my personal experience with paper negatives and Harman direct positive is that most greens end up almost as featureless as red. Have to ponder why that is, if it's not a feature of the spectral response.

MattKing · Feb 28, 2012

Brian:

The variable contrast papers achieve their contrast variation by using two layers - a green sensitive layer and a blue sensitive layer. If the exposing light is low in green and high in blue, contrast will be relatively high. If, however, the exposing light is high in green and low in blue, contrast will be relatively low.

Most printers effect these changes by using filters with complementary colours - adding yellow ("minus blue") to decrease contrast or adding magenta ("minus green") to increase contrast.

The green sensitive layers tend to be more sensitive.

Simon R Galley · Feb 28, 2012

Could'nt have put it better myself....

Simon ILFORD Photo / HARMAN technology Limited :

zumbido · Feb 28, 2012

Yeah, while I was laying awake last night it occurred to me that I did know, at one point, about the separate green and blue layers wrt VC and the filters. I've gotten so tunnel-visioned on the paper negative, staying away from the enlarger, that my brain is wiped clean. Thanks all! Going to be experimenting with targeted blue and green LED arrays for more efficiently lighting my indoor pinhole work.

Pauken4 · Jun 18, 2012

Simon R Galley said:
Could'nt have put it better myself...

HI Simon:

Shouldn't that mean that when prinitng negatives with a blue tinted base with a neutal (2.5) MC filter, there would be proptionaltely less yellow light transmitted and thus more contrast in the final print?

desertrat · Jun 18, 2012

Michael R 1974 said:
It is often helpful to refer to an additive color wheel. The closer together two colors are, the more they allow eachother to "pass". Blue is not as effective at absorbing green light as colors in the magenta range. The blue tint in the film base would also have to be quite saturated/"strong".

I've made contact prints of X-ray film negatives using VC paper and getting contrast filtration from the color head of an enlarger. The X-ray film base is pretty strongly blue, but it seems to have little effect on the contrast I get with recommended filtration. The contrast filters seems to work mostly as normal.

Thomas Bertilsson · Jun 18, 2012

If you're referring to the Foma 120 films, there isn't enough blue in that film base to have a noticeable effect. The same is probably true for other films with a blue tinted base, although I personally know of no others.

Pauken4 said:
HI Simon:

Shouldn't that mean that when prinitng negatives with a blue tinted base with a neutal (2.5) MC filter, there would be proptionaltely less yellow light transmitted and thus more contrast in the final print?

ic-racer · Jun 18, 2012

Thomas Bertilsson said:
If you're referring to the Foma 120 films, there isn't enough blue in that film base to have a noticeable effect. The same is probably true for other films with a blue tinted base, although I personally know of no others.

What? Don't you know that just a faint tinge of yellow stain in a pyro negative provides a magical and otherwise unobtainable sacrosanct transformation of tonality when printed on mutigrade paper!

Ilford MG IV RC sensitivity: blue or green?

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