An interesting point about the speed point is that it's not necessarily where you want a specific exposure to fall - the shadows for B&W negative material and the metered exposure point for color reversal film for example. The purpose is to have a point that has meaningful information about the material. One of the functions of in the determination of the speed point is that it defines the limits of the material being tested. When the B&W ISO parameters are met, the minimum useful exposure that will produce a high quality print falls approximately one stop to the left of the speed point density of 0.10. If the processing is different, greater or less, then the relationship of the minimum useful exposure point and the 0.10 density point changes. In contrast conditions outside the ISO parameters, a different method should be applied such as the Delta-X Criterion.
Originally Posted by Rafal Lukawiecki
ISO Speed Graph with Delta X equatioin.jpg
Film speed is then determined using the speed point, but as I said, exposure doesn't necessarily fall at that point. The ISO film speed is just a different approach to determining the fractional gradient speed point. It is basically the Delta-X Criterion. As we know, the fractional gradient speed point falls around a stop lower than the 0.10 density point, yet when this was the standard, film speeds were one stop slower than film speeds produced using the current standard. This sounds contradictory to our understanding on how speed works. Shouldn't film speeds increase the further left the speed point falls? One of the points missed in almost all general purpose photography books is that the speed point is only part of the determination of film speed. There is also the speed constant.
Film speed is part of the exposure equation used in exposure meters. This is why the ratio between the speed point and metered exposure point is so important. For black and white the speed the constant is 0.80 and for color reversal it's 10. This means that there is 1/3 stop difference between the color reversal speed point and the metered exposure point for a given film speed. With black and white film it's a 3 1/3 stop difference. The speed constant for color reversal films used to be 8. Without changing the speed point, the change in the speed constant caused a 1/3 stop increase in the film speed. Changing the constant instead of the speed point preserves all the important information about the material that the speed point is associate with.
This can be done with the black and white speed constant too. WBM uses a density of 0.17 as the speed point. With normal development, this produces approximately a 2/3 of a stop decrease in film speed compared to the 0.10 speed point, depending on the shape of the film curve. There's nothing wrong with wanting a little extra exposure as a safety factor or perhaps to achieve better results with your personal metering techniques. The problem with the 0.17 speed point is that it isn't in good agreement with the limits of the material. If you want the extra film speed, it's better to simply use the 0.10 with a speed constant of 0.50 instead of 0.80, or for those using relative film speed, adjust the EI by 1/2, 2/3, or whatever amount works best for you.
Last edited by Stephen Benskin; 09-01-2012 at 11:42 AM. Click to view previous post history.