Another project from the depths of the lah-bor-a-tree on the bayou.....
From the intro: "I’ve started getting intrigued with “Spectral Density Curves” for films. Is this topic the missing “secret” for film behavior? Is it the background for the H&D curves that we spend so much time looking at and deriving - inadequately informed - conclusions? Is this the criteria why some films seem so wonderful and others suck? "
A look at the 20 35mm films from all of the major companies.
Thanks and a tip of the hat to D F Cardwell, whose comments got me thinking. And wondering.
I also think spectral sensitivity is a pretty important factor that most people ignore. It's the main reason I don't care for T-max films--their response is too linear, like B&W video.
I also think spectral sensitivity is a pretty important factor that most people ignore. It's the main reason I don't care for T-max films--their response is too linear, like B&W video.
What we all need to remember is that the same film and a different developer combination can generate a dramatic and vastly different density / exposure relationship from each other.
T Max 400 in Xtol or Pyrocat at regular dilutions and processing is as you said, a straight 45 degree line.
Dilute Pyrocat with T Max 400 utilizing stand or semi stand process generates an extended long toe and an emphasized S shoulder to such a degree that you would swear that this could not be the same emulsion. I am sure that there are many other examples that fit this model.
As a result I am convinced that photographers that really know their materials can make quality photographs with virtually any film and developer combination. Looking at a graph is "interesting" but prints are the only thing that really matter.
Interesting thoughts, and yes, the spectral sensitivity curves are certainly something we don't spend enough time looking at. I try to raise this issue whenever we discuss filter factors and tonal contrast / separation.
P.S. I'll just add, for what it's worth, that after some initial forays into UV photography, I think panatomic x and type 55 are some of the most UV-sensitive films still in use. I suspect that their sensitivity remains high well down into the 300s. I'll be doing some tests at ~325 in the coming weeks.
Michael, you're talking about the H&D curve, which shows the relation between exposure and density, and I'm talking about the spectral sensitivity curve, which shows the film's sensitivity to different colors of light. Developer choice doesn't change the spectral sensitivity of the film.
Last edited by David A. Goldfarb; 07-23-2008 at 02:20 PM.
Spectral sensitizers can be development and fog inhibitors, desenstizers to blue light, and have a number of other properties. It is the total combination of emulsion, sensitizer and other addenda that make up both the wavelength that a film is sensitive to and the characteristics of that film.
A case in point is a sensitizer that can be a green sensitzer for one emulsion and a red sensitizer for another. It is based on the emulsion, not the dye. A second dye can be a red sensitizer that leaves residual blue sensitivity, and another can repress blue sensitivity. And a third dye can be a super sensitizer giving huge boosts to an emulsion speed but imparting no special spectral sensitivity.
Nothing substantive can be derived from the data you refer to other than the exact sensitivity of the given film itself.
Much data on this is found in the chapter of Mees and James to which Paul Gilman contributed.
I agree with David and I think what is seldom appreciated is that:
- spectral (wavelength) sensitivity can be a major player in tonal separation and contrast
- stated filter factors are quite arbitrary around the edges of the sensitivity spectrum; you have to convolve the lighting spectrum (~colour temp) with the film sensitivity spectrum to get reliable numbers
- no matter what the manufacturer's stated toe and knee look like, you don't know from the H&D curves what actual highlight and shadow curves you're going to get unless you again consider the actual lighting spectrum used for your shot and the film sensitivity spectrum. A pan film can deliver an image that is tonally contrasty or it can be blah flat... with the same developer and the same dilution and exposure time and everything, just by shooting through a colour filter. Extreme example: rollei R3 unfiltered, Rollei R3 red filtered. Please ignore the clouds in the sky, I am not talking so much about the dehazing effect of a red filter, but rather how the wood and grass tones are rendered, which has nothing to do with haze or Rayleigh scattering.
David I don't know if this is what you meant, and I don't mean to put words in your mouth, but when you said tmax's response is too linear for your taste, I took that to mean too linear in overall wavelength sensitivity, which is an issue I see in these R3 test shots. My feeling is that tones simply don't separate well at all unless a filter is deployed.
If a filter selects out that wavelength range that coincides with a big change in the derivative of the wavelength sensitivity curve, then, voila, more tonal contrast. I suppose this is because small changes in the tone/colour across the subject give rise to large changes in actual density in the neg. Requires more thought.
I seem to have more success generating tonal interest and good separation with traditional-grained films, and the tones clump up for me with tmax and the deltas. Maybe it's just me :s
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Originally Posted by David A. Goldfarb 
