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# Thread: Are the film specification sheets accurate?

1. May I ask, to what practical application will the OP put a perfect interpretation of these spec-sheet curves?

It's not meant to be a silly question, as all that consistency necessary thirty years ago - when film was the starting point for many analogue image-production processes - seems to have been removed. Now aren't we are using consistency in our own personal processes to achieve what we want at the level of an individual print (or small, personally repeatable, batches), not as the start of a long chain of analogue processes where the published result depends on perfect global consistency at every step?

2. Originally Posted by Photo Engineer
Retro...

The sheets can be used that way. The vertical height of the curve is the response in LogE. The "Y" axis is thus the speed at that wavelength. However, the speed is the integral of the entire curve. And so the speed that you measure at one wavelength is infinitely small regardless of the "Y" value unless combined with all of the other values.

This all is related to calculus, I'm afraid. And, the curves have to be smoothed out because they are bumpy due to the dyes themselves.

It is easier for a B&W film than for a color film, due to overlap. Thus, some green light produces red exposure. How do you estimate that? You need to do spectral curves of each dye formed at each wavelength and then begin to calculate what must be done to improve color reproduction.

To do that, you must also include a factor for the human eye vs the measuring system, and you must therefore consider the half band width of the imaging dyes.

If I can find one, I might post a typical picture used for this type of measurement.

PE
Photo Engineer, thanks for the response,

I actually am familiar with calculus, so feel free to discuss more in terms of it, I actually prefer more technical explanations.

Could you work out an example? Lets say I have a spectral power distribution where the spectral irradiance is as follows: between 450 and 451 nm, the spectral irradiance is 7 erg cm^-2 s^-1 nm^-1 and at every other wavelength the spectral irradiance is zero. What would the exposure be for the blue layer in lux-seconds for Kodachrome 25? I would like to see an example worked out so I can find the error I think is present in my calculations. You can just give me the approximate value (no need to actually integrate over that 1nm range, just approximate using a riemann sum of 1 rectangle to keep calculations simple)

I noticed that the exposure in the spectral sensitivity curves for kodachrome 25 is a radiant exposure
but the exposure on the H-D curve is a luminous exposure. Ive accounted for that in my conversion by involving the luminous efficiency function, I think this may be where my mistake lies, theres a detail(s) Im failing to account for.

3. Originally Posted by MartinP
May I ask, to what practical application will the OP put a perfect interpretation of these spec-sheet curves?

It's not meant to be a silly question, as all that consistency necessary thirty years ago - when film was the starting point for many analogue image-production processes - seems to have been removed. Now aren't we are using consistency in our own personal processes to achieve what we want at the level of an individual print (or small, personally repeatable, batches), not as the start of a long chain of analogue processes where the published result depends on perfect global consistency at every step?
Some end-users can interpret, to some degree, in a manner meaningful to them. If given enough time I could probably understand the specs, but nothing I do (or want) requires the vast majority that information. Still, it is nice to have, so long as the person can interpret, and it can be interesting just to read and learn.

Along those lines, consider ingredients listed on food. Most do not read them. Some do, but only look for certain things and do not take full advantage of the information (which is not terribly useful descriptive in the first place).

However, I am lactose-intolerant, and have learned there are many words that basically boil-down to "dairy" product/lactose. Just because a label does not say "milk" or "dairy" does not mean it is not in there in some capacity; the laws in the U.S. don't require that in the way many believe. Fortunately, disclosure is required for ingredients that can be deadly.

Only by reading and interpreting can I discern what I need to know, even if it is of no interest to someone else.

Admittedly, this does not affect the majority of analog food users, but I am glad the information is there and that I can understand it well enough for my purposes.

4. In practical use, one can assume that, under the relevant light-source and with any "Professional" film, you will end up with a neutral grey and optimum colour representation.

One then measures the light you are actually working in (assuming that it is a continuous spectrum source, else you'll need to test, spec-sheet or no), goes to the filter diagram and check how many mireds you need to shift and choose the appropriate filtration.

Working out the modification from first-principles, based on wavelengths and amplitude of radiation, seems somewhat inconvenient.

5. Retro...

What you ask is a rather large problem to me at present. At EK, it was all done by computer. And, our work was directed to getting the total speed which was reported back to us. And so, in the third image, this is a material with only blue speed, so the green speed would be reported as zero. In the second image, the green speed would be reported in some given speed value related to the illuminant used for exposure (thus for your example, the illuminant must be specified). You did this, but we used a different energy standard.

In the first image I show 6 types of spectral sensitization. In the final graph I show a concentration series of one dye on a film. In all of these cases, the energy was held constant so that the output was the same on the "X" at each wavelength. Speed can be calculated for each dye at each concentration, and by varying the distribution of energy, one can then calculate the speed under daylight or tungsten (and etc.).

The next to last scan is an exposure to light of different wavelengths and overlapping, thus giving us a human eye view which is compared to the densitometry to give us values that relate color reproduction to dye set to spectral sensitivity to illuminant (working backwards from the actual image). There are about 25 or so of these exposures in a set.

In any event, although I have examples, I have no computer program to do this for me. I have given some real data but that is as far as I can go.

Sorry. And sorry for the scrambled order of the uploads. APUG did that!

PE

6. Alright Photo Engineer, I appreciate your help.

7. I just have an interest in understanding the technical details of photography and how film works on a quantitative level. Im not actually going to use this in practice when I go out shooting film. Just for the interest of it.

8. Fair enough, but you end up working backwards in most cases. You work from a neutral step wedge and a color chard and work back to what does what you want. Often this is empirical and this is a key point.

PE

9. Data Sheets are written by engineers for engineers.

I stated that above.
Well, today I glanced through one of my german books: "Applied Mathematics for Photographers and Photo Lab Technicians" 3.ed, 1971

On its densly printed 200 pages it covers stoichiometry and mixing equations, calculations based on the lens formula, calculation of wages, expenses and profits. But no word about logarithms...

The understanding of logarithms, among other things as indicated above, is a prerequisite of understanding data Sheets.

10. One of the first things I do when interested in a new film application is check the data sheets. And I'm certainly no engineer or math geek.
There are a lot of nuances one can recognize when comparing one set of data to another without having to quantify either the input or output.

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