What is the Relationship between Film Speed and Camera Exposure?
I’m sure almost everyone has experienced this at one time or another. You think you understand a certain concept until you have to explain it to someone, and then you realize there are gaps or questionable assumptions in your thought process. I believe this is a frequent occurrence with teachers early in their careers. From my experience, after the flop sweat has dried, it can prove to be as much of a learning experience for the teacher as for the student. The following concept might just fall into this category.
The classic saying for exposure is the exposure meter wants to make whatever it is pointing at middle gray. In a recent thread there was a discussion about the scene luminance range and the range of Reflectances from a 12% and 18% Zone V model. So, I would like to think we’ve determined what the scene is and what the meter “keys” on.
In order for the exposure meter to work properly with films of differing sensitivities, the speed of the film is entered into the meter’s exposure calculator. There’s been copious discussions on film speed on this forum, so the basics of film speed should be relatively well understood.
In a paper by D. Connelly, he writes, “It is evident that the relationship between the sensitometric measurement of exposure (film speed) and the photographic exposure (camera exposure) requirement must be determine.” There needs to be a way of connecting the various photographic elements together. You determine the film speed and that value somehow has to relate to the exposure meter and camera exposure in order to achieve good repeatable negatives.
Here are the questions to consider, and please keep in mind it's the sensitometric measurement of exposure. What is the relationship that Connelly writes about?
How can it be determined?
How does the film speed work with the exposure meter to determine the exposure placement of whatever the meter is pointing at?
Also keeping in mind that exposure meters are designed to produce one data value for the scene, what should that exposure be for a given film speed?
I know it seems pretty straight forward, but it involves some fundamental underlying principles. I've uploaded Connelly's paper for a reference. The above quote comes from the section on Film Speed on page 187.
Last edited by Stephen Benskin; 12-31-2011 at 12:52 AM.
I'd say that paper presents alot of interesting and practical information for those attempting to design a system for dimwit camera operators. However, it seems to me to be irrelevant if one determines one's own EI, especially if one is shooting only negative film.
I'm no Nikon fan, but their matrix metering system seems to be designed by some knowledgable folks to address many of the significant exposure issues illustrated by Steve's posts on exposure and film sensitivity.
Here are the questions to consider, and please keep in mind it's the sensitometric measurement of exposure. What is the relationship that Connelly writes about?
How can it be determined?
In my head there are only two variables, the volume of light and the physical properties (speed) of the film in a given process.
The only thing that seems to be missing from the equation is a constant.
That constant can only be determined one way, by experimenting to see what effect a/any given volume of light has on that film.
The actual reference point though to key a meter to seems to be pretty arbitrary. Any given density on the negative could be used as long as it is repeatable.
Originally Posted by Stephen Benskin
How does the film speed work with the exposure meter to determine the exposure placement of whatever the meter is pointing at?
Also keeping in mind that exposure meters are designed to produce one data value for the scene, what should that exposure be for a given film speed?
Light meters measure the intensity of light, we already know (well we could look up) the speed of light, add in the time a shutter is open and the area through which the light can pass and we have what we need to define the volume of light; at this point we can effectively count how many photons we will get.
The only thing missing is the constant we "found" experientially for a given film's speed.
Last edited by markbarendt; 12-31-2011 at 09:09 AM.
Mark Barendt, Ignacio, CO
My aspiration of late is to become more Bohemian; "a person with artistic or intellectual tendencies, who lives and acts with no regard for conventional rules of behavior."
Step 1:
What are the equations used to determine B&W and Color Reversal sensitometric speed?
Well, I don't think there can be one formula for all films. Even for one film there are different developers and times and temps etc., and different development will affect the density/exposure curve.
I am guessing that the formula would relate some standard optical density (minus the film base density, usually very small) to the exposure.
So, suppose that you have a known light intensity on a standard grey card. We would stipulate that a standard exposure will yield x% of the maximum optical density on the film under standard development. That will determine the nominal box speed. (astute analogue photographers will of course note that this point actually moves around a bit depending on how you develop)
In other words, you'd simply look at the log density / log exposure chart, pick the special point that you want to correlate to standard grey under standard (say, sunny 16) light and exposure, and off you go. There will be a log relationship.
Of course, every standard has its limits, and in this case the standard grey point would say nothing about what happens to the higher and lower tones. But still I suppose it'd be reasonable to subtract the high density from the low density, say grey corresponds to x% of that range, etc.
If I'm understanding the question right, once the ISO value is determined, the camera computes exposure with no other consideration for the film.
This is a good generalization of it. I have some reservations about the "no other consideration part", but will discuss that later. What I'm asking about is the specifics of the relationship. How does the film speed value interact with the exposure meter and why? So far everyone is being very general or just offering an overview. How about nailing it down?
No one has given the film speed equations for either B&W negative film or color reversal film. I believe this is a good place to start, because we need to know where on the characteristic curve the film speed point is and where the metered exposure point is in order to determine what their relationship is. Most of this can be found on pages 187 and 188 of the Connelly paper. One small note: There has been a change in the equation for color reversal film speed since this paper was published which I believe offers further insight into way in which film speed and exposure placement work.
Maybe the questions are too broadly phrased. Let's break it down. And I'm not asking for reference links.
Step 1:
What are the equations used to determine B&W and Color Reversal sensitometric speed?
For B&W negative films, arithmetic ISO speed is determined by:
ISO Speed = 0.8 / (Exposure st speed point)
The speed index result is rounded off to the nearest 1/3rd f-stop.
For color reversal films, the the exposure needed to get to the shoulder point (S), which is about 90% maximum density is determined. Then the exposure needed to get to the toe (T) which is 0.2 density above the minumum density is measured. These two exposures are averaged, that is: ((S+T)/2), and this value is the Speed Point. The reciprocal of the exposure at the Speed Point is then multiplied by 10 and the result is the arithmetic Speed Number.
The preceeding was taken from "Sensitometry for Photographers" by Jack Eggleston. Stephen recommended this book to me many years ago, and it is certainly worth getting if you are at all interested in the subjects at hand.
Kirk
For up from the ashes, up from the ashes, grow the roses of success!
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Originally Posted by Stephen Benskin
