Doug Kerr
Well-known member
The exposure time (shutter speed) scale of the Sekonic L-398 exposure meter is marked with radial lines (the "major divisions") at what is almost certainly intended to be 1/2 stop intervals (ration of 3 between adjacent lines. In between each pair of lines are two dots (the "minor divisions"). One cannot directly align the dots with the markings on the adjacent aperture scale, but is is almost certain that they are intended to represent intervals of 1/3 stop (in the working of the "slide rule".
The labeling on the major marks generally follows the 1:2 progression as we go in the increasing direction. There is however a departure from that, as we see in this partial list of shutter speeds (I will show the denominator of the speed, as is done with the labels on the meter scale for times that are less than one second):
2 4 8 15 30 60
After 8 we might expect to see 16 (not 15), But these labels are from what I called the "preferred values" for shutter speeds. Thus, on one die of thew "disjuncture or the other, there is a discrepancy between the marked time and the time represented (in the operation of the "slide rule") of about 6.7% (small potatoes in this context).
The series of labels follows the list of "preferred values" for exposure time.
The manual for this meter has a figure that shows all the assigned values for this scale (not just the values labeled on the major divisions". Only one of the two dots between two consecutive lines has an assigned value.
But if we consider all the marks that have assigned values, we find that:
• Values separated by a distance of 3 dots have a ratio of assigned values of 1:2 (except for the bump as we pass from 1/8 s to 1/16 s)
• Values separated by a distance of 10 dots have a ratio of assigned values of 1:10.
This is of course the same seemingly-paradoxical situation we earlier encountered in the sensitivity ("film speed") scale labeling. And I earlier discussed "how this can be".
Again the totality of labels follows the list of "preferred values" for shutter speeds. Here is part of the list. The "o"s indicate marks for which there is no label (all dots); the values in bold are on "major division" marks.
4 5 o 8 10 o 15 o 25 30 o 50 60 o 100 125 o 200 250
So again we see the shadow of the obsolete DIN system, in which a distance of 10 "permitted values" has a value ratio of 1:10.
There is a quite similar situation on the adjacent aperture scale. The details are different there because the f-number appears squared in the metering equations.
Best regards,
Doug
The labeling on the major marks generally follows the 1:2 progression as we go in the increasing direction. There is however a departure from that, as we see in this partial list of shutter speeds (I will show the denominator of the speed, as is done with the labels on the meter scale for times that are less than one second):
2 4 8 15 30 60
After 8 we might expect to see 16 (not 15), But these labels are from what I called the "preferred values" for shutter speeds. Thus, on one die of thew "disjuncture or the other, there is a discrepancy between the marked time and the time represented (in the operation of the "slide rule") of about 6.7% (small potatoes in this context).
The series of labels follows the list of "preferred values" for exposure time.
The manual for this meter has a figure that shows all the assigned values for this scale (not just the values labeled on the major divisions". Only one of the two dots between two consecutive lines has an assigned value.
But if we consider all the marks that have assigned values, we find that:
• Values separated by a distance of 3 dots have a ratio of assigned values of 1:2 (except for the bump as we pass from 1/8 s to 1/16 s)
• Values separated by a distance of 10 dots have a ratio of assigned values of 1:10.
This is of course the same seemingly-paradoxical situation we earlier encountered in the sensitivity ("film speed") scale labeling. And I earlier discussed "how this can be".
Again the totality of labels follows the list of "preferred values" for shutter speeds. Here is part of the list. The "o"s indicate marks for which there is no label (all dots); the values in bold are on "major division" marks.
4 5 o 8 10 o 15 o 25 30 o 50 60 o 100 125 o 200 250
So again we see the shadow of the obsolete DIN system, in which a distance of 10 "permitted values" has a value ratio of 1:10.
There is a quite similar situation on the adjacent aperture scale. The details are different there because the f-number appears squared in the metering equations.
Best regards,
Doug
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