What is Film Speed?
“Expose for the shadows. Develop for the highlights.”
So, how much exposure is needed for the “shadows”? ….It depends on the film speed!
- Film “speed” depends on –
- the film’s sensitivity to light intensity and color, AND
- to a lesser extent, the developer and development procedure used.
- Your individual film “speed” may also be affected by –
- the sensitivity, color response, and accuracy of YOUR light meter,
- the quality and condition of your camera lens, and
- the accuracy of the diaphragm and shutter of your camera.
- whether you are “exposing for the shadows” or using an auto-exposure camera, gray card, incident light meter, etc.
- The film’s characteristic curve or “personality”.
This is not intended as a full discussion of the topic and primary references should be consulted for complete understanding. I have found Fred Picker’s Zone VI Workshop especially helpful in clearly outlining the steps necessary to find (1) your personal film speed, and (2) your “normal” film development time. Both are very helpful and will ultimately save a lot of time and money!
Finding your individual film speed (often called “exposure index” or EI) for a film-developer combination is a little like finding the “home” key on a typewriter, or “middle C” on a piano. It gives you a reference around which you can maneuver.
|Kodak Publication No. F-5, KODAK Professional Black-and-White Films states that “..densities less than about 0.10 density units greater than gross fog level usually print as black.” This gives rise to the notion that the part of the negative which is to print darkest (other than pure black) in the print, should have a density of 0.1 above film-base plus fog (fb+f), or in Zone System terms, ..enough exposure to provide a Zone I density of 0.1 above fb+f. This is the Zone System equivalent of “Expose for the shadows” in the the ageless rule that heads this page.|
Since exposure meters are supposed to interpret what they read as middle gray (Zone V), a Zone I exposure can be made by metering any uniform surface and providing 4 stops less exposure.
To determine one’s own film speed rating for any film-development combination, it is necessary only to …
- Meter a uniform surface using your best estimate (or manufacturer’s rating) of the film speed.
- Starting with the meter’s recommendation (Zone V), make a series of exposures, reducing the exposure one stop per exposure for at least six frames. I like to put a Zone V exposure at the end to mark the end of the blank frames and double-check the Zone V value.
- Develop the film according to the procedure you are testing.
- There should be frames with no density other than fb+f (film base plus fog), followed by a frame in which additional density is clearly identifiable. Theoretically, this frame should have a density of 0.1 above fb+f, but in the absence of a densitometer, your guess is probably adequate.
- If the frame four stops from the Zone V frame is too light (or clear), your speed (ASA) setting is too high (you think your film is faster than it is).
- If your “Zone I” frame is too dark and/or there is density in the lighter frame next to it, your film speed may be set too low and you are getting more exposure than you need.
If further adjustment is needed, you may either repeat the process with a new estimated film speed, or estimate how much you would have to change your estimated film speed to give the fourth (Zone I) frame a density of 0.1.
|It is an hour after sunrise. The sky has light clouds with a little blue sky. The sun is hidden behind a cloud. With a Pentax Digital Spot Meter, the clouds read EV 151/3 – 161/3 while the ground under the edge of the deck reads 61/3. That is a 10 stop range, in subdued light! If you desire tonal separation in the clouds AND definition in the leaves on the ground, you need a film that can record more than 10 stops in a reasonably linear way. Interestingly, in bright afternoon sunlight, the range between bright cumulus clouds and forest shade is about the same, from 7 – 171/3.|
Photographing a Kodak Gray Scale in either sun or shade exposes the film in only about a 5 stop range. This is adequate for short scale color reversal films like Kodachrome, but color negative and black-and-white films typically have a much longer scale and exceed the range of light reflected from the Gray Scale. If you want your negative to have both Zone I and Zone IX exposures, a single photograph of the Kodak Gray Scale won’t do it. Adding the exposures from BOTH sun and shade does, however, approximate (EV 8 – EV 171/3) the 9+ stop range of light available in a normal outdoor scene.
A Kodak Gray Scale was examined with a Pentax Digital Spot Meter. Two sets of readings, one in bright sun and the other in shade, resulted in the EV readings shown above. The slight difference in range is probably due to the way light is reflected from the glossy paper in diffuse and direct light. The illustration above shows that the black square is read by the meter as EV 8 in the shade and EV 112/3 in the sun, while the white square is read at EV 13 in shade and EV 171/3 in the sun.
Solutions have been suggested through the years in which two cards are mounted, one to the front surface of a box in direct sunlight, and another in a recessed shaded area inside the box. This allows a single photograph to reproduce the full range, but retains the problem of getting uneven illumination to the “shaded” card, since light inevitably “bounces around” inside the box.
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