Film Speed

Within limits, "film speed" is an individual value and should be the result of testing with one's own equipment and procedures. Photographers may have widely varying ASA settings for their favorite films, sometimes depending on the equipment being used. Kodachrome 64 is often exposed at ISO 80, and Tri-X, which is rated at ISO 400, is frequently exposed at 320 or even 200. The variability can often be attributed to individual preference or even intentional light meter calibration deviation (but that's another story),

For a full discussion of this topic, and how to establish a personal film speed or "Exposure Index", I recommend Ansel Adams The New Ansel Adams Photographic Series volume The Negative.

ASA, ISO, DIN
Throughout the history of photography, there have been many rating systems; Weston , Kodak , Scheiner (American or European). There have been many attempts to classify "film speed", even though there is no single way to do so. The objective is to provide the photographer a way to estimate the exposure necessary to achieve the desired negative or transparency. The problem is that film emulsions and developers have widely varying characteristics, making comparison quite complex.

Of all of the various speed rating systems for film, three may still be commonly seen

ASA
(American Standards Association) Most common film speed rating in the U.S. until the conversion to ISO. Only the name has changed.

 

DIN
(Deutsche Industrie Norm). Based on a logarithmic scale wherein each increase represents 1/3 stop. ISO 800/30 on a recently purchased roll of ISO (ASA) 800 film indicates that the DIN rating is 30. The DIN values are included in table below for comparison with ISO values.

ISO
(International Standards Organization). Most common film speed rating in the U.S. Doubling the value, doubles the film speed. The table below shows the actual computed theoretical values in the scale, while in practice the values are rounded for simplicity.

EV (Exposure Value)
The relationship between EV (Exposure Value) and light intensity is apparently defined as EV 10 = 10 candles per square foot (I say apparently because I can't find anyone who seems to know for sure). If each whole EV represents a doubling or halving of the light value, then the EV table looks exactly like the ISO table (surprise) and the intermediate values can be similarly derived

Most exposure meters intentionally give erroneous readings. The manual for the Pentax Digital Spot Meter shows a "K factor" 1.4 and EV 10=13.3 c/sq.ft. This means 100 candles per square foot will read ~EV 13 instead of ~13.3. This is apparently done to provide a margin of "overexposure" for black and white film??? It amounts to about 1/3 of a stop and may be the reason your Kodachrome 64 looks better at ISO 80 than at ISO 64.

f/stop values
Did you ever wonder where f/5.6, f/6.3, f/8, f/11, etc. came from?
The scale used for f stop settings starts with 1 (focal length = diameter of the opening) and doubles in value as the opening gets smaller, cutting in half the amount of light allowed to the film. The basis for the scale is one in which each element has twice the value of the previous member, i.e., the powers of 2 (1, 2, 4, 8, 16, 32, etc.) as shown in the ASA Scale. The scale is therefore a "geometric" progression and the mid-point and 1/3 points cannot be derived by division.

So how do you calculate intermediate stops and ISO values?

It happens that if a linear scale is established, and intermediate steps inserted therein, these values can be used as exponents for 2. The resulting values produce the geometric scale seen as the ISO Scale. Merely taking the square-root of each of these values produces a scale of f/stops. Tricky process for a non-math type, but the result provides valid intermediate values for ISO calculations and f/stops. A piece of cake for a computer spreadsheet. (I remember doing this sort of thing on a Monroe mechanical calculator, ugh!). Those of you who used early European cameras will recognize f/6.3 which is 1/3 stop smaller than f/5.7 (f/5.6)

Curiosity Question #1.
If
f/5.6 is really f/5.6568 (square-root of 32), and f/11 is f/11.3137.(sqrt of 32)..
and if
f/11.3137 is rounded to f/11,
shouldn't f/5.65 be rounded to f/5.7 instead of f/5.6?

AND shouldn't f/22.6274 be rounded to f/23 instead of f/22?