Photo Engineer said:Stephen;
I'm afraid that your arguments avoid the point that the human eye integrates the data in the image. In that case, due to having a toe and shoulder in the photographic image, the toe and shoulder are 'soft' and the mid scale is higher in contrast than the 1:1 slope we would otherwise expect. Therefore, it is unavoidable that the 'contrast' of a print would be about 1.5 in the mid-scale to compensate for the toe and shoulder. This is what we designed for.
PE
Ah yes, 'the human eye integrates data in the image.' When a friend who is an optical and acoustic research scientist made a similar statement and said something to the effect that the most theoretically perfect lens may not give the best image the debate became hostile and heated.Photo Engineer said:Stephen;
I'm afraid that your arguments avoid the point that the human eye integrates the data in the image. In that case, due to having a toe and shoulder in the photographic image, the toe and shoulder are 'soft' and the mid scale is higher in contrast than the 1:1 slope we would otherwise expect. Therefore, it is unavoidable that the 'contrast' of a print would be about 1.5 in the mid-scale to compensate for the toe and shoulder. This is what we designed for.
This 1.5 mid-scale contrast compensates for the loss in contrast in toe and shoulder and gives the human eye the overall impression of a curve with a slope of 1. It is, in effect, an optical 'delusion'.
As for the Munsel chart, well, it can be used as a B&W tool, but we never did. To use an analogy, you can remove an appendix via the mouth! Why bother when other methods are so much more convenient and less difficult. So, we just used the H&D curve for both B&W and color. We used the Munsel chart only for color space.
I'm afraid that our methods differ, but I am familiar with what you refer to. I'll defer to the experts that I know, and rely on the work of DeMarsh, Zwick and others that I worked with personally and who taught me the methodology one on one in the lab. I have no doubt that what you say will work, but it seems rather round about to get to simpler solutions to the problem.
PE
Photo Engineer said:Sandy, I agree. What you say is possible with a totally linear D Log E curve generated digitally. It is also possible in analog photography using appropriate chemical means to generate the same end result.
Stephen, I disagree. Just for example, a negative film has an average mid scale gradient of about 0.6 and an average grade 2 paper has a mid scale gradient of about 2.5. The optimum print from this combination yields a gradient of about 2.5 x 0.6 or a mid scale gradient of about 1.5. This is what I was referring to.
As for the Munsel scale, it is mainly only used for color, however another useful scale is the CIE Chromaticity Scale. Neither of these is used strictly for the design of B&W materials. Only the H&D curves and 'print through' scales are used in design of B&W AFAIK. Print through scales, are H&D curves of film printed onto paper and then compared with H&D curves of the paper, using mathematics to determine the effect of printing.
I did all of the above during the design phase of Ektacolor 30/37 paper and Kodacolor 400 film. I used both B&W and color print through, undercut and double undercut imaging methods almost daily. I also used the 'silver criterion' for obtaining exact results from both B&W and color materials. This takes into account any deviation in color of the original negative image from the spectral sensitivity of the print material. This latter factor is significant, particularly with todays MG papers, when the silver image is not perfectly neutral.
PE
Stephen Benskin said:Thanks Sandy for the Mark Nelson reference. I'll check him out.
Stephen Benskin said:I agree with Sandy that there really is little left of any great importance concerning traditional photography in todays reality of digital. I do believe; however, that there is value understanding FG in a digital world. The parameters of the FG method are directly tied to camera exposure, understanding FG and its relationship with the ISO speed method helps in the understanding of camera exposure. Of course, a large portion of the population won't care and just let the camera do it for them, but I figure those who frequent this forum aren't part of that mindset.
Kirk Keyes said:SPSE (Society of Photographic Scientists and Engineers) Handbook of Photographic Science and Engineering
Kirk Keyes said:Sandy - Check for the Photo Techniques article that Stephen mentioned earlier in the thread. I think he covers it in there. I think the title as something like "Black & White Film Speed: An Analysis". I think it was Mar/Apr or May/Jun 2005. (I can't find my copy of it for some reason so I'm not sure of the date.)
Or if you have a copy of the SPSE (Society of Photographic Scientists and Engineers) Handbook of Photographic Science and Engineering laying around - look in there, p. 811 of the 1973 edition. I found one really cheap online - sadly it has a label in the front that says "Library of Polaroid Corporation"...
Anyway, as it uses a graphical method I'll refrain from trying to put it into words...
sanking said:Kirk,
I have most of the literature mentioned, including the Dunn and Wakefield book, edition of 1972, and a 1973 edition of the Handbook of Photograpic Science and Engineering.
What I want to know is if Stephen, or you perhaps (?), has a computer graphing program that allows one to enter sensitoimetry data and evaluate the differences between FG and contemporary ISO standard for different ESs.
I will check his article in Darkroom and Techniques to see if there is anything about graphing programs.
Best,
Sandy
Stephen Benskin said:Sandy,
I only gave you how to calculate Delta X and not about calculating FG. Sorry, it was late.
To compare ISO with FG I use two equations.
1. Finding FG exposure using DeltaX and ISO Hm
Hfg = Hm / antilog(DeltaX) or Hfg = Hm / 10^DeltaX
example for a film with an ISO speed of 125:
DeltaD for ISO standard is = 0.80 which equals a DeltaX of 0.296
0.0064 / 10^0.296 = 0.0032
2. FG speed calculation
Sfg = 0.4 / Hfg
where Hfg = exposure in meter candle seconds at fractional gradient speed point
example for FG speed
0.40 / 0.0032 = 125
As you can see, they are the same when DeltaD = 0.80, but they begin to diverge when DeltaD changes. Also, please be aware that the FG speed calculation is not the same as the original FG equation. They modified it to work with the adjustment to flare in the 1960 ISO speed version.
Above and beyond the concept of FG as being the most consistently precise speed method, it helps explain exposure theory. The DeltaX for the ISO standard also happens explain the difference between the log-H range from the mid-exposure / meter calibration point and the where the shadows fall (log-H 1.28) and the difference between the meter calibration point and the ISO speed point (log-H 1.0). In other words shadow exposure for a statistically average luminance range, 2.2 (in which exposure is based), will fall at the FG speed point. Average flare will bring it back up to around the ISO standard. It will not fall exactly on the ISO speed point because 0.80 in the speed equation gives a slight safety factor since average flare is considered 0.34 in the calculation of speed. This is what I meant when I said the ISO speed point is a point to determine speed and not where shadow exposure falls. With the dominance of 35mm, average flare is approx. 0.40. It only makes for a slightly higher safety factor. Of course, flare isn't consistent, which means actual shadow exposure placement will differ depending on flare.
To me, knowing about the variance of exposure is beneficial. Knowing about the lack of rigidity can save many from worry and obsessive speed tests.
Steve
Stephen Benskin said:Sandy,
That's right. Now, that you mention it, I remember he had that choice. It's been years since I last saw it. I seem to remember the FG method in his program had more to do with calculating gradient than speed. Is that correct? Either way, the one thing I never liked about Davis' program is that the speeds were all relative and it didn't allow for a calibrated exposure approach.
Anyway, I hope the Delta X equations and Davis' programs will give you what you need. Please let me know what you come up with.
Steve
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?