how far is the toe away from medium gray?

Not sure what you are after here, but here are some observations that may help. First, ISO film speed is determined by the "speed point," which is a point when the film first begins to respond to exposure (a given density above film base + fog density). See http://en.wikipedia.org/wiki/Film_speed for more. However, that has little to do with how metering and exposing is done in practice.

However, FYI, the speed point is roughly four stops underexposed from "middle grey." That's where the toe begins. Detail in the shadows, however, are not usually adequately separated until about two stops underexposed from middle grey. (I'm assuming you are concerned with black-and-white negative film here.)

"Standard" or "recommended" development times from manufacturers are designed for average conditions. They give you a usable contrast range in average lighting conditions. You set your meter to the ISO and meter accordingly for your type of meter. Meter readings are based on mid-tones (e.g., averaging meters assume that the average tonality of a scene is middle grey). Expose as your meter indicates. You need not worry about adjusting unless you have a reason (backlighting, high- or low-key subjects, etc.)

Now, "standard" developing times may work fine for you. However, your results may be too contrasty or too flat, or your shadows may not have enough detail, etc. Unless you are doing your own film-speed tests, you only need to identify your problem and make corrections to your exposure/development scheme to compensate. Too much contrast = reduce development time (start with 10-15%). Not enough contrast = increase development. Not enough shadow detail = decrease film speed on your meter.

Of course, not all situations are "average"; some are contrastier or softer, some are largely high-key, etc. If you shoot roll film, you can deal with contrast in the printing. Sheet-film users often use more complex exposure and development schemes designed to get the desired contrast for each sheet of film (something I find impractical with roll film). Among these schemes are the Zone System and BTZS. Unless you are using one of these systems, you do not have to concern yourself with metering the shadow values.

If you feel inclined to learn more about individual film testing and advanced metering/exposure/development techniques, there is a huge amount of information on this site and others. Start with the Zone System, find a site that suits your level of understanding at this point and read, read, read, shoot, shoot, shoot. There's really no other way.

Hope this helps,

Doremus Scudder
www.DoremusScudder.com

 

1) In the zone system, the zone V on the film is 4 stops from the speed point.
2) Depending on the 'k' constant of your meter and your ISO setting, the portion of the toe with the minimum useful density is supposed to be about 4 stops down from an average reflected meter reading.
3) This relationship is independent of development.
4) If you just set your meter to 'box speed' you cannot be assured of this relationship. That is why many people test the system, to be sure.