View Full Version : .1 over B+F ? Do you set your speed point differently?

18-Dec-2011, 16:20
When you do your speed testing do you stick to .1 over B+F or do you choose .15 or .2 or some other value and why?

If so what difference does/will it make?

Stephen Benskin
18-Dec-2011, 19:36
Here's something from Applied Photographic Theory by P. Kowaliski which I found interesting.

“Speed, or sensitivity, of a light-sensitive system is in general defined by a parameter derived from the minimum exposure yielding a record satisfactory or just useful for the chosen purpose. The parameter, characteristic of the material and its process, is therefore defined by the magnitude of exposure corresponding to an appropriate density value on the characteristic curve. This density level must be chosen according to the practical purpose, and therefore varies with the material and the process; the importance of a correct choice is well demonstrated by the comparison of the characteristics of two different material (Figure 2.2): at a net density of 0.60, for instance, above fog and bas density and the density level where the two curves intersect, the speeds are equal for both materials. At a net density of 0.10, it is however material A which is four times as fact as material B, whereas at a density of 1.0 the speed of material b is twice that of material A.”

Like it says in the quote, an effective speed point is dependent on how the material is to be used. For example, the speed point is different for aerial film than it is for pictorial. A good speed point also shouldn’t be arbitrary. It should also have some relevance in defining the important limitations of the photographic material and achieves consistent results with the greatest variety of film types.

While the fixed density point of 0.10 over Fb+f is the speed point of the ISO standard, when the processing parameters of the standard are followed, there is a fixed relationship between the fixed density point and the fractional gradient speed point which represents the minimum gradient that will produce an excellent print. This not only defines the limits of exposure, but also incorporates the concept of image quality to exposure.

An arbitrary speed point does not offer this correlation. However, what most people don’t realize is that the speed point doesn’t necessarily represent where the shadow exposure is suppose to fall. The importance of it’s placement is defined above. Once the limitations of the exposure placement is defined, it’s then possible to determine where to place the exposure.

Take for instance the fractional gradient method, which was the ASA standard prior to 1960. It’s speed point falls approximately one stop to the left of 0.10 over Fb+f. If you calculate film speed with the intention of the shadow exposure fall on the fractional gradient point, speeds using the fractional gradient method would be one stop faster than with the fixed density method of 0.10. However, film speeds prior to 1960 were one stop slower. That’s a two stop difference.

What makes this possible is once the speed point is determined, the exposure at that point is divided into a constant. In the paper Safety Factors in Camera Exposure, C.N. Nelson discusses the reduction of a safety factor for the 1960 standard.

“The reduction in the safety factor could be accomplished simply by changing the constant in the ASA formula for deriving the ASA exposure index from the ASA fractional-gradient speed of the film. The present formula, which gives a safety factor of about 2.4, is

Exposure Index = Fractional-Gradient Speed / 4 Es

Or Exposure Index = 1/ 4Es

where Es is the exposure in meter-candle-seconds at the fractional-gradient speed point and 1/Es is the ASA fractional-gradient speed. If the constant of ¼ were replaced by a constant of ½, a new type of "exposure index" would be obtained which would provide the proposed lower safety factor of about 1.2.

There are several reasons, however, for adopting not only a new constant but also a different speed criterion. The fractional-gradient criterion was originally chosen because it has the desirable feature of giving speeds that correlate closely with speeds obtained by practical picture tests.9-11 It has the objectionable feature, however, of being somewhat inconvenient and difficult to use. Consequently, a simpler and more convenient criterion, such as that based on a fixed density above fog density, is often desired. Fortunately, as shown by the recent data of Nelson and Simonds,12'" a good correlation exists between fractional-gradient speed and speeds based on a density of 0.1 above fog, provided the development condition's are controlled so that a fixed "average gradient" is obtained. This average gradient is measured on the portion of the .D-log E curve of the film lying between two exposures, E and 20 E, where E is the exposure at a density of 0.1 above fog. The specification of a fixed average gradient in an American Standard would be justified by the fact that such a specification corresponds to the common photographic practice of developing negatives so that they print satisfactorily on a "normal" grade of photographic paper. Thus the adoption of the 0.1 fixed-density speed criterion in combination with a suitable development specification would offer the advantages of convenience and practical significance.”

So in my opinion, it’s better to use a speed point method such as the ISO standard's or the one I use, the Delta-X Criterion, which is an easily applied fractional gradient method, then if desired make the necessary adjustments for the placement of the exposure.

18-Dec-2011, 19:40
I estimate 0.3G with the "W-speed" method.
Why? The minimum gradient method is based on reasonably sound panel testing of photographs and the "W-speed" estimation of 0.3G can be done automatically on a spreadsheet. http://www.apug.org/forums/forum37/90074-w-speed-action.html

A comparison of various film speed methods is found in Emulsion Speed Rating Systems by G.S. Allbright 1990