SPLITGRADE LED coldlight source

8/17/2019 SPLITGRADE LED coldlight source

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DARKROOM AUTOMATION1412 Dorsh Road

Cleveland, OH 44121-3840

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http://www.darkroomautomation.com

Application Note

THE WORKINGS OFVARIABLE CONTRAST PAPERS

AND LOCAL GAMMA

If you have ever wondered why some images just look better on some papers than on others,and why VC papers so often give blah results, then this paper may be of help to you.

This paper shows how VC paper works and also shows some of the drawbacks and pitfalls ofits use. The concept of local gamma for printing papers is introduced and is shown to be auseful tool for picking the right paper for the subject matter. The information is of generalinterest and not limited to those working with Darkroom Automation produces.

VC paper is made with 2 or 3 emulsions with the following characteristics:

1. All emulsions have the same intrinsic contrast2. The first emulsion is sensitive only to blue light3. The second emulsion is sensitive to both blue and green light4. The third emulsion, if present, is sensitive to blue and cyan light5. Each emulsion can only produce part of the density required for maximum black6. The densities of the emulsions add together to produce maximum black

That first point is worth stating again: All emulsions have the same intrinsic contrast - the samerange of illumination that takes them from white to their DMax. There is a commonmisconception that the paper is made from a blue sensitive high contrast emulsion and lowcontrast green sensitive emulsion - this is not the case.

A two emulsion VC paper will serve as an example of how these papers work. The curvesshown are all idealized. Real papers are not nearly as well behaved. The numbers on thecharts, while representative, are arbitrary.



The HD curves of the green and blueemulsions, when exposed to whitelight, are shown in the figure to theright. The resulting black HD curve isthe addition of the densities of the twoemulsions.

Since the ‘green’ emulsion is sensitive

to both the blue and the greencomponents of white light its overallsensitivity is higher: it takes less whitelight exposure to produce the samedensity as the blue layer.

Each layer can only produce a densityof 1.0 OD on its own. When thedensities of the two layers add thepaper density gets to a max black ofaround 2.0 OD.

If a green or yellow filter is placed inthe light path the amount of blue lightwill be reduced, effectively lowering theprinting speed of the blue sensitivelayer. More white light is needed toexpose the blue sensitive layerbecause most of the blue has beenremoved by the filter, as shown in thegraph at right. This shifts the positionof the blue curve to the right,

increasing the spread between the twoemulsion curves. The resulting papercurve will now accommodate anegative with a stop more contrast.

If a blue or magenta filter is placed inthe light path the complimentary actionoccurs: now the green light is reduced,reducing the effective sensitivity of thegreen-blue sensitive layer so it is equal

to that of the blue layer. The curve forthe green layer shifts to the right so thatit is on top of the blue curve. The bluecurve remains where it was, as shownin the white light graph shown at the topof the page. The resulting high contrastpaper curve now needs less negativecontrast to go from black to white onthe print.

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-2.5 -2.0 -1.5 -1.0 -0.5 0.0Paper exposure in stops

Green + Blue Sensitive Blue Sensitive Result

VC Paper Medium Contrast - White LightGreen+Blue Layer Sees More Light

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P a p e r D e n s i t y

-2.5 -2.0 -1.5 -1.0 -0.5 0.0Log10 Paper Exposure


VC Paper Low Contrast - Yellow FilterAttenuate Blue Sensitve Layer

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P a p e r D e

n s i t y



VC Paper High Contrast - Blue FilterAttenuate Green+Blue Sensitve Layer



A pathologic condition exists in VCpaper when too much yellow filtration isadded to produce very low contrast.

In this case, the yellow filter reducesthe sensitivity of the blue layer so muchthat the two emulsion curves have agap between them.

The resultant curve has a flat spot andas a result all detail is lost in the print’smidtones. When this happens with a 3emulsion paper, like Ilford MGIV, thereare two flat spots in the resultant curve.

LOCAL GAMMA - 2 EMULSIONS

Because the response of VC paper is the response of a set of paper curves, each with theirown toe and shoulder, VC paper exhibits some strange effects when looked at from theperspective of local contrast. Some refer to local contrast as contrast between adjacentelements in a photograph where the contrast difference between them may be large. In thispaper, as seems to be the morecommon usage, local contrast meansthe contrast of objects that are close toeach other on the tonal scale, and maybe better termed ‘Local Gamma’.

Local Gamma can be found from theHD curve by taking it’s derivative - ameasure of how fast the tones in theprint are changing against how fast thetones in the negative are changing - andseeing how this changes with varyingprint density. The graph at right showsthe local contrast for the idealizedcurves above.

The contrast of VC paper changes themost in the mid-tones as filtration ischanged. The toe and shoulder portions of the curve stay constant until the highest contrastgrade is reached

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-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0Log10 Paper Exposure


Too Much Yellow FiltrationMidtone detail dissapears

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L o c a l G a m m a

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Print DensityHigh Contrast Medium Contrast Low Contrast Too Low

Local GammaFictional 'ideal' VC paper



THE REAL WORLD - 2 EMULSION PAPERS

The curves for a real 2-emulsion paper aresimilar to the idealized examples presentedbefore.

The HD curve at right looks very wellbehaved, with no obvious artifacts or flatspots.

However, when local gamma/contrast isexamined the graph clearly clearly showswhere the end of one emulsion’s HD responseoverlaps the beginning of the next emulsion’sresponse. The characteristic ‘blip’ in the localcontrast is clearly visible.

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R e f l e c t i o n D e n s i t y O D

-1.4 -1.3 -1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0Log 10 Exposure

HD Curve2-Emulsion Paper, Grade 2

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G a m m a

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Print Density OD

Local Gamma2-Emulsion Paper, Grade 2



THREE EMULSION PAPERS - THE THEORY

Modern VC papers, Ilford MGIV and Kodak’s late Polymax, added a third emulsion to thepaper. The third emulsion is described as sensitive to blue but a bit less sensitive to green -for this discussion we will refer to it as the ‘cyan emulsion’. The workings of the threeemulsions in controlling contrast are as before: green light exposes one emulsion, cyanexposes two and blue exposes all three.

The following HD graphs, though still idealized as mathematical functions, have beenmathematically fit to be close to the actual curves of MGIV paper.

The 00 curve shows the attenuation of the blue emulsion in relation to its position in the grade2 curve. As the contrast is increased the cyan and green curves are brought towards the bluecurve.

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Green + Blue Cyan+Blue Blue Result

VC Paper Filter #53-Emulsion Model

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-2.5 -2.0 -1.5 -1.0 -0.5 0.0Paper exposure in stops



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VC Paper Filter #3.53-Emulsion Model



LOCAL GAMMA - 3 EMULSION PAPERS

When the local gamma, the slope of the HDcurve, is plotted against print density thefollowing graph appears. If local gammacurves are available for a wide variety of

papers it is possible to select the paper who’scontrast response matches the importanttones of the subject of the print. Note that atvery low contrast the contrast - and detail - inthe print will disappear in the skin (ZVI) andshadow (ZIII) tone. Very little control isavailable in the highlights and shadows andthe grades don’t show any real differentiationin print tones from white to skin tones (ZVI)until very high contrast grades are reached.

The graph also illustrates why there can be so much value in using a #5 filter to burn inhighlights - it is the only way to raise highlight contrast.

Normal contrast prints will have most of their detail in the shadows with only half that contrastin the skin tones. It is likely the reason that portraits on VC paper are lackluster whencompared to older work printed fixed grade papers designed for portraiture.

Another way to look at local gamma is as afunction of subject luminance or negativedensity.

This curve, although interesting, is of lessvalue. It does, however, show the value offitting negative development to scene contrastfor high contrast subjects - if low contrastpaper is used then there are pronounced gapsin image detail. Expanded development forlow contrast subjects may be of less value,and better results may be obtained from highcontrast printing - although high contrastfiltration accentuates grain and scratches.

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L o c a l G a m

m a

0.0 0.5 1.0 1.5 2.0 2.5Print Density

#00 #1.0 #2.0 #3.5 #5.0

Local Gamma3-Emulsion Model

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L o c a l G a m m a

-2.5 -2.0 -1.5 -1.0 -0.5 0.0Log10 Exposure

#00 #1.0 #2.0 #3.5 #5.0

Local Gamma3-Emulsion Model



THE REAL WORLD - 3 EMULSION PAPERS

In the real world things are not nearly as prettyas they are in the Platonic word ofmathematical objects.

The graph at right shows the HD curve forIlford MGIV MG RC paper when exposed towhite light. The curve shows thecharacteristic break in the curve where thecyan and blue emulsions kick in and add theirdensity.

The local gamma shown on the right indicatesthat MGIV in white light isn’t quite optimum inthe overlap of it’s three curves. It shows ahorrid dip in contrast at ZVI - right where skintones should be, and puts most of it’s effortinto revealing detail in the shadows.

A good pick for low-key portraits, but don’t tryto emulate Hurrell using this paper.

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R e f l e c t i o n D e n s i t y O D

-2.0 -1.9 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

Log 10 Exposure

HD Curve3-Emulsion paper - white light

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G a m m a

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Print Density OD

Local Gamma3-Emulsion Paper - White Light

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