Doug Kerr
Well-known member
The table below gives three lists of values of the ISO speed that may be of interest:
The values in the first columns ("Base list") we so chosen that they all absolutely and precisest follow these two "rules"
• If we start with any value, and then go down in the list by 3 values, the second value is exactly 2 times the first value.
• If we start with any value, and then go down in the list by 10 values, the second value is exactly 10 times the first value.
There could be many such lists, I constructed this one to include the value 80.
I had said earlier that such a list could not have more than 30 numbers. I was in error. Such a list can have up to 59 numbers
The next column has the list of values of camera sensitivity that are permitted by ISO 12232. It follows the base list other than (and the differences are highlighted in pink):
• Where the base list value in non-integral, the ISO 12232 value is rounded to an integer.
• The base list values 1280, 2560, and 5120 are replaced in the ISO 12232 list by 1250, 2500, and 5000, respectively. This is presumably in the interest of having only "tidy" values.
The third column has the list of values assigned to the "clicks" of the film speed dial of the Sekonic L-398 exposure meter (as given in the manual for that meter). It follows the ISO 12232 list other than (and the difference is highlighted in pink) the value 1250 is replaced by 1200. I assume that was done in the interest of having only values with no more than 2 non-zero significant figures. This facilitates labeling on the dial, which is done with the multiplier "M" (for 1000) where applicable. The L-398A uses the same list, but on the dial the SI multiplier "k" is used.
In this list, only the values shown in bold are actually labeled on the dial.
Best regards,
Doug
The values in the first columns ("Base list") we so chosen that they all absolutely and precisest follow these two "rules"
• If we start with any value, and then go down in the list by 3 values, the second value is exactly 2 times the first value.
• If we start with any value, and then go down in the list by 10 values, the second value is exactly 10 times the first value.
There could be many such lists, I constructed this one to include the value 80.
I had said earlier that such a list could not have more than 30 numbers. I was in error. Such a list can have up to 59 numbers
The next column has the list of values of camera sensitivity that are permitted by ISO 12232. It follows the base list other than (and the differences are highlighted in pink):
• Where the base list value in non-integral, the ISO 12232 value is rounded to an integer.
• The base list values 1280, 2560, and 5120 are replaced in the ISO 12232 list by 1250, 2500, and 5000, respectively. This is presumably in the interest of having only "tidy" values.
The third column has the list of values assigned to the "clicks" of the film speed dial of the Sekonic L-398 exposure meter (as given in the manual for that meter). It follows the ISO 12232 list other than (and the difference is highlighted in pink) the value 1250 is replaced by 1200. I assume that was done in the interest of having only values with no more than 2 non-zero significant figures. This facilitates labeling on the dial, which is done with the multiplier "M" (for 1000) where applicable. The L-398A uses the same list, but on the dial the SI multiplier "k" is used.
In this list, only the values shown in bold are actually labeled on the dial.
Best regards,
Doug