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Film Speed: ISO to DIN Conversion Explained

Film speed labels like ISO 100/21Β° combine two historically separate rating systems into one number β€” here is what each half means.

ISO film speed is actually two numbers in one

A film box printed with "ISO 100/21Β°" shows the arithmetic rating (100, inherited from the old ASA scale) alongside the logarithmic DIN rating (21Β°), two different scales describing the same sensitivity to light.

ASA/ISO arithmetic scale doubles for each stop

On the arithmetic scale, doubling the number means double the light sensitivity: ISO 100 to ISO 200 is one stop more sensitive, ISO 200 to ISO 400 is another stop, and so on.

DIN logarithmic scale adds 3Β° per stop

On the German DIN scale, each one-stop increase in sensitivity adds 3 to the DIN number rather than doubling it, so DIN 21Β° to DIN 24Β° represents the same one-stop jump as ISO 100 to ISO 200.

A few common ISO/DIN pairs

ISO 25 = DIN 15Β°, ISO 100 = DIN 21Β°, ISO 400 = DIN 27Β°, ISO 1600 = DIN 33Β°. Each doubling of the ISO number corresponds to a +3 jump in the DIN number.

Why two systems merged into one label

ASA (American) and DIN (German) were competing national film-speed standards through most of the 20th century. When the ISO standard was created, rather than picking one, it combined both numbers onto the same film box label, which is why older cameras and light meters may reference either system separately.

Why DIN's logarithmic scale exists at all

A logarithmic scale compresses a huge sensitivity range into small, easy-to-print numbers and lines up naturally with how camera exposure itself works in stops, which is a doubling/halving relationship. That made DIN convenient for engineers even though it reads less intuitively to casual film shoppers than a simple doubling ASA number.

Frequently Asked Questions

Do modern digital cameras use DIN at all?

No, digital camera ISO settings use only the arithmetic (ASA-derived) scale. DIN is essentially a legacy reference seen on classic film stock, older meters, and vintage camera manuals rather than in current digital practice.

Is a higher DIN number more or less sensitive film?

Higher DIN means more sensitive, same as higher ISO/ASA β€” the two scales always move in the same direction together, they just use different math (multiplying versus adding) to represent each one-stop increase.