How does contrast filtration work to make blacks blacker?

[stormbytes]

Just sheer curiosity here...

I've been testing some films lately, realizing that my goal is to up the density in the highlights (on the negative) so as to hold back light, restraining them from getting too dark (on the print) as the shadow areas darken.

Stupid as it may sound, it just occured to me that if the highlights (on the negative) are not "dense enough" to accomplish the aforesaid, contrast filtration is used to send the darks down (on the print) "artificially".

Would someone be nice enough to explain how this works exactly? When I apply contrast filtration the only thing I notice is the light eminating from the enlarger's lense gets 'red'. I'd love to understand the science behind it.

 

[SuzanneR]

I'm not sure I understand the science, and I hope I understand your question. When you use a higher contrast filter, it allows the blacks (shadow area/thin area of the negative) to get black faster. In other words, it takes less exposure to reach black in those areas on the negative with a 5 filter rather than a 4. It affects the exposure of the highlights (or dense area of the neg) less.

Does that make sense?

 

[Dan Henderson]

variable contrast paper has two layers of emulsion: one that is sensitive to higher contrast filters, and one sensitive to low contrast filters. By using a higher contrast filter, you bias the exposure on the paper, giving the shadows more exposure and making them blacker. Conversely, when you use a low contrast filter the highlights get proportionally more exposure than the shadows, thereby decreasing contrast.

 

[smieglitz]

... realizing that my goal is to up the density in the highlights (on the negative) so as to hold back light, restraining them from getting too dark (on the print) as the shadow areas darken...

The common practice is to get enough density in the thin shadow areas of the negative to provide adequate detail and then develop the negative for proper highlight density. When you print, you should base your exposures on the the highlight areas of the print (which it sounds like you have been doing) and then change printing filters to adjust contrast as needed.

However, don't assume the higher contrast filter is just making the dark tones darker. It is also making the light tones lighter at the same time. It is the middle grays that aren't shifting much. Depending on the overall balance of light vs. dark tones in your original print, a higher contrast filter could make the result look darker or lighter overall. (But, it will certainly look less gray.) Upping the contrast with a filter moves both ends to the extremes of white or black while a lower contrast filter grays everything and tends to show details that a higher contrast will block.

Generally, if you lower the contrast after determining proper exposure time for the highlights in a print, you typically need to retest and increase the exposure, and vice versa with higher contrast filtration. This exposure compensation is for both highlights and shadow print values, not just one end as your original question implied.

The 2 paper layers in a VC paper are sensitive to either blue or blue-green light so the blue/yellow complimentary pair or bluish-green/magenta pair will affect the layers differentially. VC dichroic heads typically use the blue & blue-green difference while slide-in VC printing filters are on either the yellow or magenta side.

Joe

 

[lee]

exposure effects the shadow detail and film development controls the hightlite densities

if your neg density is not high enough then longer development time is needed when developing your film. The info you have received above is good info.

lee\c

 

[stormbytes]

Joe,

First off, I'd like to thank you for taking the time to write such a thorough and informative post. This is precisely the type of information I was looking for.

I have several questions regarding some of the points you make, and I've parsed your post to better organize my thoughts.

The common practice is to get enough density in the thin shadow areas of the negative to provide adequate detail and then develop the negative for proper highlight density.

This is the traditional approach and assumes (rightfully so) that shadow detail is essential to the photographer's envisioned final print. However as we all know, there are many variations on the traditional approach, as was encouraged by Ansel Adams himself. In my case, shooting (or perhaps "striving for") dramatic portraiture, I aim to depart from the literal and produce high contrast images that some might say border on illustrations. In my very modest experience, this is done by pushing film to the limits. Thus, shadow detail gives way to a preference for skin tones and important highlights. I've found that tailoring the dynamic range of the negative to this particular effect gives the photograph a glow that seems almost surreal.

However, don't assume the higher contrast filter is just making the dark tones darker. It is also making the light tones lighter at the same time.

Had you posted the foregoing statement last night, perhaps I would've gotten some sleep! I just spent the better part of the last few hours trying to figure out why "Print A" (print exposed for 11s @ f/8 with filtration #4) didn't have identical skin tones to "Print B" (print exposed for 22s @ f/11 with filtraion #5).

Generally, if you lower the contrast after determining proper exposure time for the highlights in a print, you typically need to retest and increase the exposure, and vice versa with higher contrast filtration. This exposure compensation is for both highlights and shadow print values, not just one end as your original question implied.

I'm using a Saunders VCCE 4550 enlarger, and I paid a hefty sum for it after the sales rep gave me a run down of it's "magical properties" of Variable Contrast Constant Exposure. Yea... "Constant Exposure" my foot!

Could you recommend a rule of the thumb so to speak that would help give me an idea of the shift in tone with added filtration? Perhaps some empiracle formula or percentage?

Also, in what proportion would you say that "upping the contrast filtration" makes the "lights lighter" to making the "darks, darker" and vice-versa ?

The 2 paper layers in a VC paper are sensitive to either blue or blue-green light so the blue/yellow complimentary pair or bluish-green/magenta pair will affect the layers differentially.

Might I ask that you elaborate on this? Perhaps with an example (step by step).

Thanks again for taking the time and for your insights.

 

[Photo Engineer]

VC papers have two emulsions in them. One is a high contrast emulsion sensitive to one color of light and the other is a low contrast emulsion sensitive to another color of light.

The color of the light is unimportant, but the industry has selected blue and green light using yellow and magenta filters to select which emulsion in the paper is exposed.

If you use one color of light, you are exposing only a low contrast emulsion of about zero grade, and if you use another color of light you expose the high contrast emulsion with about grade 4. Mixtures of light by combining filters will give you all possible grades in between. So, basically you have two graded papers on one sheet of paper, and you can mix and match grades by selectively each grade of paper in one exposure or in multiple exposures.

PE

 

[gainer]

Exactly so. The magic is not in the light but in the paper. The same filtering that your enlarger provides "automatically" will not work on paper that is not labeled "VC" or "Variable contrast." The colors chosen are a convention of the industry and IIRC they are not the same as in the first such paper. You can use either yellow and magenta filters or the complementary blue and green. Yellow filters subtract blue light thereby decreasing exposure of the high contrast blue sensitive emulsion. Magenta subtracts green.

I'm sure PE will agree that it's not as simple to make these emulsions as it is to talk about them. I'm glad we don't have to make them.

 

[Photo Engineer]

Patrick, I can now make them, but coating them or getting a good result is the big problem now. Also, getting the color separation is a problem. You need a rather complex coating setup to do it well.

PE

 

[gainer]

Is it necessary to coat two separate emulsions? I had heard that the emulsions could be mixed nowadays. I don't know what would be involved there. I imagine it might be necessary to provide some kind of barrier to keep different sensitizing dyes, etc, from intermingling. Was there not at one time a color process in which tiny globules of different emulsions were randomly intermingled in a single layer? I'm just meditating. I'm sure you know about these things.

 

[Photo Engineer]

An MG paper can be coated as either one layer or two. For practical reasons, it is best to do it in one layer, but it may be impossible due to the potential for intermingling of chemicals in that one layer.

You are correct about the color process. It was called "Mixed Packet". I have seen samples of near production quality, but that approach was abandoned due to chemical wandering problems which caused severe keeping problems. Another reason for abandoning it was the development of the mutiple hopper or slide coating machine which allowed more than one layer to be applied at one time.

Today, it is possible to apply all layers of a color negative film in one pass at one coating station rather than needing one station or one pass for each layer. IDK if anyone but Kodak, Fuji, and Agfa were using this method. Of course, today, curtain coating is the newest technology and is or was used in the new Kodak facilities.

I used the prototype curtain coater in KRL back in the 70s. It was the prototype for the new B38 coating facility.

PE

 

[Ronald Moravec]

If you are having to expose the print too long to get thru the highlights, decrease the development time. The print exposure will be shorted and shadow detail retained.

If highlights are grey, increase the development time.

If the print needs more or less contrast, the VC filters are the method if the neg is not just so. Simplified, the multigrade paper has two emulsions, one high contrast ond one low. The high is sensitive to blue light only, the low to green only. An appropiate mix of filter colors will get in between the extremes. Either a blue or magenta which both block green, will get high contrast. Yellow and green block blue, and expose the low contrast. You can make it work with green or blue, ot the sets sold that use yellow and magenta.

 

[leeturner]

One question related to something PE mentioned in one the replies. Using a dichro head is a true grade 5 possible or is it as PE mentioned around a grade 4? I've just got hold of a set of VC filters and will be comparing the filter to the suggested dichro filtration and calibrating using the results. Does this vary between enlarger brands and if max magenta is not a true grade 5 does this also hold true for max yellow?

 

[Donald Miller]

I hope that you will forgive me for chiming in here.

Joe,

First off, I'd like to thank you for taking the time to write such a thorough and informative post. This is precisely the type of information I was looking for.

I have several questions regarding some of the points you make, and I've parsed your post to better organize my thoughts.



This is the traditional approach and assumes (rightfully so) that shadow detail is essential to the photographer's envisioned final print. However as we all know, there are many variations on the traditional approach, as was encouraged by Ansel Adams himself. In my case, shooting (or perhaps "striving for") dramatic portraiture, I aim to depart from the literal and produce high contrast images that some might say border on illustrations. In my very modest experience, this is done by pushing film to the limits. Thus, shadow detail gives way to a preference for skin tones and important highlights. I've found that tailoring the dynamic range of the negative to this particular effect gives the photograph a glow that seems almost surreal

I assume that the reason that you are achieving this effect is that you are effectively mismatching the camera density range to the papers exposure scale and the net effect is that you are increasing local contrast in the process. It has been over twenty years since I got into Ansel Adams writings very heavily and I don't recall that he spoke much about local contrast. He seemed to be pretty well entrenched in the box of overall contrast.

One can achieve this mismatch that I speak of in one of several ways. The first is what you alluded to in pushing the film to it's limits...that would expand the density range of the negative beyond the exposure scale of the paper... pushing the shadows down the scale and spreading the contrast in the mid and upper tonal scale.

The same thing can be accomplished by increasing the contrast grade through filtration or using a harder grade paper.


Had you posted the foregoing statement last night, perhaps I would've gotten some sleep! I just spent the better part of the last few hours trying to figure out why "Print A" (print exposed for 11s @ f/8 with filtration #4) didn't have identical skin tones to "Print B" (print exposed for 22s @ f/11 with filtraion #5).

I think that if you review the literature on your enlarger, you will find that Saunders advises that exposure time remains constant for grade one to grade four...above grade four the times increase. I own a 4550 VCCE EXLG and this hold true on my enlarger.



I'm using a Saunders VCCE 4550 enlarger, and I paid a hefty sum for it after the sales rep gave me a run down of it's "magical properties" of Variable Contrast Constant Exposure. Yea... "Constant Exposure" my foot!

Point addressed above.

Could you recommend a rule of the thumb so to speak that would help give me an idea of the shift in tone with added filtration? Perhaps some empiracle formula or percentage?

While this doesn't follow Ansel Adams teachings, I recommend that you buy a Stouffer calibrated step wedge and determine the exposure scale of your paper at various filtration grades. Once you have determined the paper characteristics, you will know what a given negative density range will perform in what way on the paper at a given contrast grade.

Also, in what proportion would you say that "upping the contrast filtration" makes the "lights lighter" to making the "darks, darker" and vice-versa ?

While this will seem to fly in the face of what has been said by some here, my experience with the Saunders enlarger that you use indicates that once the high tonal values are established through the exposure time by the enlarger they do not shift much at all from grade one to grade four. The low values will become more pronounced as contrast grade filtration is increased. In other words the papers scale will shorten as contrast filtration is increased. The shortening predominantly occurs in the lower tonal scale. That is what my experience indicates. It has been verified by densitometric evaluation of the materials.
This same result holds true in my Durst condenser enlarger with contrast printing filters too.



Might I ask that you elaborate on this? Perhaps with an example (step by step).

Thanks again for taking the time and for your insights.

Forgive me if this seems inappropriate. On the Saunders enlarger that you and I own, the manufacturer has installed a stepless neutral density filtration in combination with the variable contrast dichroic filters. Thus as contrast is adjusted, the density presented to the light path is altered as well to arrive at constancy throughout grade one to grade four. Now if someone were to use contrast printing filters, and did not do densitometric evaluation of the filters, the seeming effect would be that both high values and low values are altered with shifts in contrast grades. This may be due more to changes in filtration density then characteristics of the papers emulsions. My tests indicate that the scale is shortened from the low values upward.

The dichroic contrast filtration on the Saunders VCCE light source begins with a yellow at the lowest contrast settings and increases contrast by the introduction of increasing amounts of magenta to the light passing onto the paper.

Good luck.

 

[dancqu]

I'm not sure I understand the science, ...
When you use a higher contrast filter, ... it
takes less exposure to reach black ...
Does that make sense?

Well, that's more contrast. The stops are in the negative.
Some would put it DR, density range. The high contrast
emulsion components of a VC paper are sensitized to
blue light; so the magenta filter.

That's exactly the case, " it takes less exposure to
reach black ". To reach black, I would add, from white.
Some emulsions are of such high contrast that a one
or two stop change in exposure will send the
density from Zero to Max.

It helps my understanding of the matter if I keep
in mind that the stops are in the negative. Dan

 

[stormbytes]

VC papers have two emulsions in them. One is a high contrast emulsion sensitive to one color of light and the other is a low contrast emulsion sensitive to another color of light.

Ron,

How's this account for the lights going lighter with added filteration?

 

[gainer]

The same way it happens when you change from #1 to #3 graded paper and keep the mid tone the same. The light gets lighter and the dark gets darker. That is one way to define increased contrast.

 

[fschifano]

One question related to something PE mentioned in one the replies. Using a dichro head is a true grade 5 possible or is it as PE mentioned around a grade 4? I've just got hold of a set of VC filters and will be comparing the filter to the suggested dichro filtration and calibrating using the results. Does this vary between enlarger brands and if max magenta is not a true grade 5 does this also hold true for max yellow?

I think that getting a true grade 5 with a dichro head is highly dependent upon the combination of the filtration in the head, the light source, and the particular paper you happen to be using. Some combinations can reach a true grade 5, while others can't.

 

[Photo Engineer]

Daniel;

Patrick said it correctly. I can't add anything more.

PE

 

[stormbytes]

The same way it happens when you change from #1 to #3 graded paper and keep the mid tone the same. The light gets lighter and the dark gets darker. That is one way to define increased contrast.

Gadget,

I understand what you're saying however, I was asking this with reference to what Ron (PE) posted, which states that different emulsions (or layers) are sensitive to different color light. If that is universally the case, then shouldn't increasing the color to which the "dark emulsion" is sensitive only increase the darks? ("making the darks, darker") or vice versa for that matter.

 

[smieglitz]

...Now if someone were to use contrast printing filters, and did not do densitometric evaluation of the filters, the seeming effect would be that both high values and low values are altered with shifts in contrast grades. This may be due more to changes in filtration density then characteristics of the papers emulsions. My tests indicate that the scale is shortened from the low values upward...

My experience is with Beseler VC dichroic and condenser enlargers, and Omega condenser enlargers. I will stick to my statement regarding the need to increase exposures as filtration is shifted to lower contrast. This is purely based on visual rather than sensitometric observations and verified each day as I help some 150 students print in an academic darkroom using the afore mentioned machines. Their experience suggests roughly 5% more exposure is needed for each 1/2 grade filter decrease. This does vary depending on their negatives, and the machine, lamp, and paper they are using. How that relates to your pushed film and equipment is anyones' guess and I won't suggest a compensation to you. As Donald has suggested, using a Stouffer wedge will give you the answer quickly.

The Saunders density solution seems great although I personally can't imagine doing sensitometry on VC papers and printing by the numbers. Mine's a different world. It is simply not how I work in the print darkroom and I have pretty much abandoned enlarging anyways in prefence to contact printing in alternative processes. I do gear the negative contrast roughly to the emulsion/paper exposure scale, but because of the nature of the equipment, materials, and conditions I'm working with, exacting print sensitometry is of limited practical value to me.

Joe

 

[gainer]

I designed an easel meter that reads a tiny spot and displays a log of the light current times a constant plus a level, both constants being variable. I can set the level for the paper sensitivity and the scale for zones ( or Zones, if you will). I can set the scale to read log of the illumination on the easel as well. I calibrate it for paper exposure by printing a step wedge. There must be one such print for each contrast grade, and for each brand of paper. Zone X is pure white and Zone I (Roman numerals don't use 0) is pure black. My setting pots have scales so I can return to any previous setting.
The probe is 2" square and 1/4" thick and on a flexible lead. The readout is a digital panel meter with a high sampling rate and the photocell is a phototransistor with nearly 0 leakage current and very rapid response. I read log units to 0.01. The log circuit is a balanced pair of differential op amps operating on a rgulated power supply. Using the pair this way provides first order temperature compensation, which seems adequate. If the darkroom temperature strays enough to cause error, I'm not in there.

I don't claim a perfect print each time, but I do find it helps me to analyze why a print is not the best it could be. It also helps me predict where there is need for dodging or burning, and how much.

Not everyone will consider this kosher, but I like it.

 

[smieglitz]

I designed an easel meter ...

OMG!

And on the other side of the coin I'm making ambrotypes and wondering if I should do some freeform alchemy with each plate. A liddle bit o' dis and a liddle bit o' dat...

Different strokes for different folks.

Joe

 

[df cardwell]

Just sheer curiosity here...

I'd love to understand the science behind it.

Hi Daniel

Here's an experiment to SHOW the difference.

Get a step wedge, either from Stouffer or Kodak. Even the good old pie-shaped Projection Print guide will work.

Contact print it with full yellow filtration. Get a nice full black, and see what the palette looks like through the range of densities.

Do the same with full magenta filtration.

And make a white light print as well as using the #2 filtration. These are your basic palettes.

Does one scale suggest the feel and look you want to achieve with your faces ?

My own preference for portraiture is to make a negative that has more shadow information, and less contrast in the highlights, than I think I'll use in the print. Why ?

I'm in a hurry when I'm with a subject. Time is precious, I can't waste a second. A look, a beat, a breathe... each might be the defining instant of our time together. If I bring all home to the darkroom, I can re-live the whole shoot, respond to it; hear their voice and feel their breath again.

Tuning the print palette with filters and developers gives me the scale and chord structure that resonates with the image. And all the information is accessible.

But that's what works for me, I know 'on the edge' shooting can be a thrill, and if that's what drives your pictures, have some more espresso and go shoot.

don

 

[Ole]

I designed an easel meter that reads a tiny spot...

For those of us with less electronical knowhow, the Ilford EM-10 is a cheap and easy substitute.

I have made a "calibration table" (really a graph) for mine, by marking off the readings which give almost-white, mid grey and good black across the filtration range. It turns out that with most papers the almost-white is constant across grades. So I measure what I want to be a good black, set the filtration according to the graph, and then adjust the light to a standard reading. 10 seconds exposure at any filtration setting gives the same highlights when done this way.