How to use this app.
If your camera or a light meter doesn’t list film speed in ISO values, use this app to convert to/from the Soviet ГОСТ or the German DIN systems. You can type your values above to search the table or scroll and click to highlight a row for easy viewing.
Why different metering scales?
ISO is the modern standard for measuring film sensitivity. It incorporates the ASA and DIN systems into one. Whereas Soviet Union’s ГОСТ is a virtual offset of the ASA system. There were many more metering systems; however, these three are the main ones you’ll see on most cameras and meters. This app is built to help you keep using those old or foreign tools with modern emulsions.
Knowing your film’s sensitivity is very important for creating perfect exposures. While some films may have enough dynamic range and latitude to let you photograph without much concern for the light that enters your camera, most will fail to render any usable images if your camera settings mismatch the film sensitivity (speed).
Measuring the sensitivity of a photographic medium is not simple. The first-ever method for doing so, Warnerke (1880), involved exposing a test plate through a pre-made series of filters with a strip of burning magnesium. Naturally, there was no accurate way to guarantee that the light and the filters would remain consistent; thus, Warnerke’s invention had to make way for better tools.
DIN metering system.
In 1894, German astronomer Julius Scheiner published his own system that rated photosensitive plates by determining the minimum exposure that produced a visible result. His work was later used by Deutsches Institut für Normung (a German standardization org similar to ISO — an International Organization for Standardization) to formulate the DIN system.
The DIN system used a logarithmic scale, similar to decibels. An increase of 1° DIN was calculated as the base 10 log of the sensitivity multiplied by 10.
ASA (ANSI) and ISO metering systems.
The American Standards Association (now named ANSI) created an easier-to-understand linear scale where ASA 200 film is twice as fast as ASA 100 film. Curiously, ASA had doubled their ratings in the 1960 revision, where an ASA 100 film was relabeled into ASA 200.
In 1974, ISO combined ASA and DIN metering systems into a single standard that would list both: ISO 100/21°. This is what you’ll typically see on film boxes today.
However, the DIN value is often omitted, so conversion tables like the one used by this app are still needed for certain film cameras.
The ISO values used on digital cameras aren’t calculated the same way film sensitivity is, but they are closely related and have an identical scale.
GOST metering system.
ГОСТ is a Soviet standardization org that used an earlier-variant ASA-based metering system on pre-1987 USSR photographic equipment. Whatever’s got ГОСТ on it afterwards is identical to ASA/ISO.
Other metering systems.
Some cameras bear WES (Weston) scale, which is identical to ASA/ISO with a ⅓-stop shift down (i.e., WES 160 is identical to ISO 200).
General Electric had its own scale too. And then there also was Hurter & Driffield. More on Wikipedia.
Conversion formulae in this app.
To build this app, I used a pre-made list of whole ASA values from 1 to 8000, incrementing in ⅓-stops. They are then recalculated for DIN and ГОСТ before populating the table and whenever a new value is typed in.
The ASA to DIN conversion used this formula: 10 * log₁₀ASA + 1
The ASA to ГОСТ conversion used this formula: ASA * 0.9
However, certain ГОСТ values were rounded and standardized in a particular way that required me to create a table of 24 values that overwrite the calculations.
All the values are rounded to the nearest whole number except the manual overrides.