Just like the image contrast at the film plane will be lower due to flare, so will the image contrast on the paper be lower due to enlarger flare. Although the paper LER for grade 2 is right on 1.05, an NDR of 1.05 will not at all compensate for any loss due to enlarging flare. So, bumping the aim NDR up to 1.2 compensates for at least a half stop loss of contrast due to flare losses at the enlarging stage---------a full stop loss of contrast at the enlarging stage would require an NDR of 1.35. But I've no idea exactly what the flare loss is from the enlarger, but bumping the aim NDR up by some amount to help compensate for it is probably wise.
Numbers don't mean anything on their own. To clarify everything above, all you need to do is put it all into context, and this is done by applying a little theory. First let's look at the paper LER. As with the film speed point, the 90% of D-max of the paper is just a point of measurement. Remember what I said about needing to define the limiting points of quality on the film curve? It's the same principle here. Loyd Jones found that after 90% of a paper's D-max perception of tonal separation drops off (it's never about density. It's about tonal separation). He never suggested that this is the usable range of the paper. In fact, the graph below is from the Theory of the Photographic Process and it illustrates this concept using a tone reproduction curve. The curve is generated by comparing the resulting print with the original subject.
Originally Posted by Rafal Lukawiecki
The average luminance range is 2.20 or 7 1/3 stops, but this doesn't represent the entire luminance range of the average subject. There usually are small areas of specular highlights and "cavity" shadows. The graph brings this out to a luminance range of 3.0 or 10 stops. So it's easy to understand how the ISO LER for a grade two paper is 1.05 even though a paper can handle a negative with a greater negative density range. The ISO LER of 1.05 only represents the 2.20 part of the luminance range.
Now if the paper can handle a 1.20 density range (paper D-max and D-min), and the film is processed so that the 2.20 range equals a density range of 1.20, then there's nothing left over for the specular highlights and cavity shadows. So how can Ralph suggest a negative density range of 1.20 for a 7 stop luminance range? The reason has to do with how the curves are measured.
Gradient is defined as rise over run. The luminance range is the run and the negative density range is the rise. 1.20 / 2.10 = 0.57. The standard model uses a luminance range of 2.20 and a negative density range of 1.05. The difference is that it factors in a flare factor. The subject luminance range is reduced at the film plane by veiling flare. Whatever the actual range of the subject might be, the determination of the degree of processing is based on the optical image at the film plane. Average flare is around 1 to 1 1/3 stops. Kodak uses 0.40 or 1 1/3 stops of flare in their calculations. 1.05 / (2.20 - 0.40) = 0.58.
Both methods produce almost identical gradients. The gradients of 0.57 and 0.58 are ideal for producing quality negatives printed on grade two glossy paper using a diffusion enlarger. As gradient is input to output, if the gradient values are the same, the same input will produce the same output. How then can the theoretical output numbers not be equal? Simple. One uses flare in it's calculation and one doesn't. Mathematically, In order for the no flare method to produce the target gradient, the rise must be higher. For a gradient of 0.57 / 0.58, a density range of 1.20 is where a seven stop illuminance range (note: not luminance range) will fall, but we are not processing the film for a 7 stop illuminance range but for around a 6 stop illuminance range. At around the 6 stop illuminance range, the negative density range falls around 1.05.
There is a chart in Photographic Materials and Processes that has the aim LERs for printing with a diffusion enlarger and a condenser enlarger. For printing on a grade two paper with a diffusion enlarger, it has the aim LER as 1.05. For a condenser enlarger it has it as around 0.80. Mind you this is for the same paper. As the aim numbers are for negatives printing with enlargers, we can reasonably conclude that the aims take into consideration some degree of flare. The reason why the diffusion enlarger LER aim is the same as that derived from a contacted test is because there is a slight Q-factor with the diffusion enlarger that tends to compensate for average enlarger flare.
And I have not even thought much yet about the impact of enlarger flare.
Last edited by Stephen Benskin; 09-19-2012 at 09:10 PM. Click to view previous post history.
Stephen's post illustrates why I say it is important to follow your teacher.
He shows the two different calculations that come up with almost the same result.
1.2 NDR is the right target when working with the techniques described in "Way Beyond Monochrome," while I found 1.0 NDR to be the right target the way I work.
The Kodak Data Book, Kodak Papers (first 1955 printing), says you should select a paper with scale index 0.2 greater than the Density Scale of Negative and offers an explanation: "The reason for this appears to be that the method of measuring scale index takes into account the total available scale of the paper, while in practice it is desirable to use less of the extreme shoulder portion of the paper curve."
In the data book, Density Scale of Negative is effective density scale at the exposure plane so there is no doubt that you are left to figure out your own enlarger flare.
The data book gives a chart, keep in mind this is effective density scale:
Grade 2 paper, Scale Index 1.3, Density Scale of Negative 1.0 to 1.2
Further the data book says the densitometer would match a contact print. Diffuse enlarger lowers scale 10% or more, condenser enlarger with a diffuse bulb raises scale 15-25%, point source condenser maybe raises scale 60%
Essentially my point is that targetting a negative density range to fit the paper is ok for subject brightness ranges within a fairly narrow range, but when you approach high contrast scenes you need to be more "intelligent" about how you use the zone system (or any other system of applied densitometry). Aiming to have a certain density range in all your negatives is, in my opinion, a naive approach to any system of controlled exposure and development. In some cases you may find even though your negative fits nicely onto the paper, you've actually lost more than you've gained. When you are making a photograph and metering the various shadows and highlights, remember to also think about everything in between and how you envision the print. Don't just do the calculations. The final print is what counts. Make sure your negatives contain all the information you need to get there.
Originally Posted by Rafal Lukawiecki
I think just a few sheets with your new setup will get the sensitometry exposure in a useful place. With the 10-stop range on the Stouffer wedge, really any exposure placement that gives you 7 stops' worth is useful. I like to get the toe down near base+fog on one of the last few steps, that gives a long run and graphs the toe nicely. Absolute accuracy is only required for the ISO tests the manufacturers have to follow. For home and pro photographer, consistency such as 1/10th stop is remarkable - 1/3 stop would be better than we need.
Instead of trying to home in on a full statistically valid test with 25 sheets, I think you can get by with a single 5 sheet test. Maybe put two or three exposures on each piece of film - then you can average the readings. Spot check yourself occasionally for consistency. Run a test sheet every once in a while with your regular film runs. I update my Time/CI chart with the results (dots and date stamps creates a "scatter" diagram that tells how good the predictions are).
Then run another 5 sheet test using a compensating developer. You will then be armed with two curve families, one family using a standard developer and one family by compensating developer. You will be able to look at the compensating developer family curves to choose times to develop N-3 or N-2. Even N-1. Look at what shape you would like the whole curve. As Michael R 1974 reminds us, you may be better off developing longer and printing on Grade 0 - choose the most attractive curve - not necessarily the one that fits Grade 2.
Flare is an issue we debate, that's why there are 300 posts on it. I'd focus on identifying "it" and checking where my procedures correct for "it" so that I know I correct exactly once. If a correction for flare is buried in your methods, it's still a correction. So the results can be identical. Just don't want you to correct for flare twice.
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Not printing on grade 0. Still perhaps a grade 2, but with dodging/burning and other print manipulations to bring in the high and low values. Compensating development needs a watchful eye...
Flare has been the toughest variable for me (well, aside from Merklinger's focusing stuff...). I'm working my way through it and I think I've more or less got it - at least in principle - but still going back and re-reading the recent discussions we've had on this with Stephen.
The negative density range as derived from the paper LER is more of an aim than an absolute. There isn’t a perfect correlation between the two. Popular photo books aren't big at caveats and tend to simplify this relationship which can lead to the conclusion there is. Serious sources have never claimed a perfect correlation. Jones, who developed the method, in one of my favorites of his quotes wrote, “The procedure followed in obtaining a relationship between DR and LER may seem forced and artificial. This we grant, and it must be borne in mind that the print quality obtained by its use will not be the highest quality. But what other course is there to follow? Either we must make the best of a somewhat imperfect relationship or face the prospect of having no criterion whatever for choosing the paper contrast grade.”
His conclusion shouldn’t surprise anyone. It’s not that psychophysics isn’t an effective science. It’s just that there are too many variables involved with the determination of print quality to fully express in a few digits. Here’s a graph from some of Jones' tests.
The graph shows the DR of the negatives, horizontal lines, printed on a grade two paper that were judged as high quality prints. They don’t all fit neatly within the defined limits of the print. The type of subject matter as well as the amount and distribution of the tones are a couple of factors that makes it impossible to predetermine print quality with any precision.
But of all the various methods Jones explored, this one tends to produce better results in the greatest number of circumstances. It gets you close enough to work with. It's statistics. I don’t know about everyone else, I only develop in one stop increments, and I don’t believe most scenes are considerate enough to have luminance ranges in perfect one stop increments. Even if there was a perfect correlation between DR and LER, it would be unrealistic to believe it could be achieved in most cases because of all the variables involved. Flare along would make this impossible.
There is another inconsistency with the DR / LER relationship that most books don’t discuss and that Michael has observed. The DR / LER relationship chances with the scene’s luminance range. Jones writes, “for the soft papers, the density scales of the negative (DR) should in most cases exceed the sensitometric exposure scale of the paper (LER), whereas, for the hard papers, the density scales of the negatives should in most cases be less than the sensitometric exposure scale of the paper (LER).” In other words, it’s not only acceptable, but preferable to have a slightly contrastier negative for contrasty scenes and a slightly flatter negative for flatter scenes.
Sure it's not perfect, but what other course is there to follow?
Last edited by Stephen Benskin; 09-22-2012 at 03:58 PM. Click to view previous post history.
New Test, New Data for Kodak 320TXP and Ilford HP5+
I have just completed a second test of 320TXP and a first one for HP5+, a film I am quite familiar with, yet which I have never tested formally. I am attaching WBM spreadsheets for both of them. As I have been reading about sensitometry, both here, thanks to your detailed posts, and in books, I am getting a better feel for the tone reproduction process, and the data that supports it. I am, at last, starting to gain a clear understanding of my materials, which is a wonderful feeling.
I will plot the curves myself, and I will try to arrive the the gradients/CIs by hand, as I am not sure if the 0.17–1.37 DR method is the one I can easily relate with my book reading. I am also calibrating my enlarger light source-paper-developer combination, and soon I will be able to use an target DR based on that analysis (plus 10-15% for average flare, I suppose).
In the meantime, have a look, if you are still following this thread, at the numbers below. These tests were performed using a contact technique, with a flash above an opal plexi diffuser (shown on this thread). I've exposed 3 sheets for each film and developing time, and I am amazed that the consistency of the flash exposure was so good. Sheets do not vary by more than 0.02 at any point (sheet-to-sheet), and, in fact, they usually read the same density for most steps (sheet-to-sheet), except for the steps 1-2, where they may vary by 0.04 at most. A test sheet exposed with an ND filter varies only 0.04 across extreme corners, so the illumination was OK for the purpose of this test.
It has not all gone very well, though. I have clearly underexposed 320TXP by a 1.3 stops, pity... Also, I have mistimed the 5.5 min development batch, as I had an incident with my timer. I think that batch is closer to 6 min, and I have tried to correct for that on the "Summary" page of the spreadsheets.
Overall, it would seem, using the WBM spreadsheet method only, that the dev times for HP5+, within 30 sec of what I am used to, are a very reasonable: N-1 at 8.5 min, N at 12 min, and N+1 at 16 min, using XTol 1+1 20˚C. I am very pleased to see those results, even though I have reduced agitation from 4 to 3 full inversions every 30 sec (plus 30 sec at the beginning). I suppose that accounts for the difference.
As for 320TXP, I do not have experience of that film, so I am not sure how to trust the dev times, bearing in mind the lack of full data due to underexposure. The spreadsheet seems to suggest: N-1 at 5 min, N at 7 min, and N+1 at 11 min. All of those use the target gradients suggested by WBM, which I remember Stephen agrees with, too, concerning the aspect of flare.
I am curious what aim DRs will I get when I measure my paper ES.
Many thanks to all for guiding me.
Hi Rafal,in those excel files you have omitted all the other worksheets which plot the curves. I am curious to see them.
I went through exactly what you did with WBM and spent ages getting it worked out and understood too ! I also had big flare issues that I solved . My last spreadsheet and film speed curve can be found in this thread here on APUG
I just downloaded the attachment, and it shows 8 sheets, with the individual curves. I'm not sure why they hid from you. I can PM the files to you, if you would be so kind.
I'll review your thread now.