Why does Ilford graph their exposure from 0 to 4.5, and Kodak uses -3.5 to +1.0?
They seem to be showing similar information with differing zero points on the horizontal axis, but why? Is one "correct"? I'm trying to compare films and this isn't helping.
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Ok that helps, but Ilford's graph is "relative" to what? I'm confused.Because one uses a relative scale and the other an absolute scale (w. unit).
Thank you, I appreciate the explanation. It sounds like there isn't much to be gained from my little experiment.Kodak's graph is in actual units of log exposure in lux-seconds, and has minus signs (because one lux second is a significant amount of light). One unit in log space means 10x as much exposure.
Ilford thought the minus signs might be confusing so they added a constant, meaning their graph is in log(relative exposure), not in a specific unit like log (lux-seconds).
For amateur photographic purposes they're the same thing, just shifted by a constant. You only have to worry about the relative/absolute difference if you're trying to do calibrated absolute photometry.
Different films, in different developers, will have slightly different curves. To align the curves you would need to pick a standard density to align them (like "0.1 above film base + fog"). Then you would have to address the question of whether these curves are useful to you, or should you get a densitometer and measure your film, the way you process it, because your curve may be a little different from the manufacturer's standard.
Ok that helps, but Ilford's graph is "relative" to what? I'm confused.
That was very helpful Michael, thank you. And you are describing exactly what I am trying to accomplish: make the scales match, superimpose two characteristic curves, and see which is steeper in the midtones, which has the longer toe, which has no shoulder, etc. That's easy to do if you stick to one brand only, but I've read comments like, "Delta 400 is a good substitute for Plus-X", and to check that out it would be nice to see the curves for those two films on top of each other.As long as the scaling on both axes are aligned (ie the intervals are the same size), and the absolute numbers on the y-axis (density) are the same, then you can compare the shapes of the curves of different films developed in different developers, which is useful. What you can’t technically compare, are the emulsion speeds. In order to do that, the absolute log exposure numbers have to be the same.
So, this means since Kodak tends to plot absolute log exposure on the x-axis, you can superimpose the curves for Kodak films and compare curve shapes and emulsion speeds. Since Ilford only plots relative log exposure on the x-axis, superimposing curves for Ilford films, or superimposing Ilford and Kodak curves allows you to compare curve shapes but not emulsion speeds.
In terms of practical utility, the ability to compare emulsion speeds is less important in the case of both Ilford and Kodak curves since we know the ISO speeds of the films. In other words you don’t learn a whole lot about the emulsion speeds of these films by looking at the graphs anyway. With films of known/given emulsion speed, comparing the shapes of the curves is the useful part of superimposing them, so as long as the scaling (interval size) is the same on both axes, and the absolute density numbers are the same on the y-axis, you’re good. You can do what you did, and slide the curves back and forth along the x-axis to align them closely enough to get a visual sense of differences in curve shape. For example, does one film have a longer toe than another. Is one film more s-shaped, etc.
Thanks for the extra input, I appreciate it.Not a dumb question, a perfectly good one!
Ilford's graph doesn't state units like Kodak's does. So the exposures it relates are relative to the far left of the graph, which could be labelled '0' on the X (horizontal) axis. Ilford's graph is also logarithmic (but unstated), so if the far left of the graph is one unit the 1 tic is 10 units, the 2 tic is 100, and 1000, 10000 successively.
The origin of Kodak's graph is "ten to the minus 3.5 power" in lux seconds which might be a little different from Ilford's "one unit" of exposure. To best compare, you probably want to match up the middle straight line parts, or match the curves at a low exposure point where they give approximately the same density.
I hope that helps!
On the x-axis each 0.3 log exposure is 1 stop.
Putting stops on the y-axis doesn't make sense. It is density we are interested in on the y-axis..
The confusion there is the concept of "stops of density range". There is no fixed density/stop per se, even for the straight line portion of the curve since that depends on the gradient (slope).
It sounds to me like what you are really talking about is tone reproduction. In addition to the film curve you need the paper curve. Then you connect them. There are various graphical ways of doing this. The Lloyd Jones "windmill" diagram is popular but Kodak later had a different way of showing the same thing.
Using anything other than Density along the Y-axis can be problematic because while a Density of 0.30 is equal to one stop, the average gradient for a normal Subject Luminance Range is about 0.58. A stop exposure for a film developed to an average gradient of 0.58 will have a Density difference of .16 or .07 for each stop exposure.
f/Stops along the X-axis would be confusing. F/Stops are part of an equation.
That is why I talk about stops, _not_ f/stops - just not to mess with exposure calculation. A stop is just double the amount of light. For sure aperture _stops_ are working the same way, for some pretty good reason cameras have pre-set aperture stops, but a stop is still just a double the amount of light. It works great in darkroom printing, my timer + analyzer both work on stops and it is just a joy to work with stops as unit. It is so logical and universal unit.
Sorry about that. I accidentally combined what Michael and you wrote. I need to take a closer look to what you wrote about film / paper matching before possibly responding.
Density and f-stop have mathematical correlation for sure?
Not a constant one. If it was constant, the curve would be a straight line. At the top for example, additional light doesn't build any more density. I could add 2 or 3 more stops of light and it wouldn't build any more density.
Why try and correlate density to stops anyway?
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