Theoretically it's possible that all the factors could pull in one direction to give a higher than nominal speed. But I have never come across it in hundreds of cases. It is also true that some film makers have stopped quoting rigid ISO speeds in favour of an EI (exposure index) figure. The basis for this isn't always clear, but Kodak's own publications state that a four stop subject brightness range for full detail in shadow and highlight is considered normal. If the scene you are picturing exceeds this, and almost any landscape with sky will, then the film speed will be too high to record details in the shadows as well as the highlights. In describing how to determine your true film speed, Kodak actually say that most people generally find they need a number slightly lower than the film's rated speed (Kodak Workshop Series, Advanced Black-and-White Photography). They discuss one hypothetical test, which set a photographer's personal system film speed for T-Max 100 at 50. There is nothing wrong with this and it does not imply that Kodak films are slower than others. Indeed, my own experience is that T-Max films are more accurately rated than most. Kodak are to be congratulated for their clarity in the criteria they use. It is fortunate that it is simple and instructive to find your own true personal film speed. It is far harder to describe than to carry out. Zone 0, printing as paper maximum black, or as near as practically makes no difference, means an area in the negative that has received so little light during exposure that it is simply the clear base of the film plus any inevitable fog from development - not surprisingly known as the 'film base plus fog' density. Note that this is not nil exposure - it takes a certain degree of exposure before anything begins to show on the film after development. Once this point - the inertia point - is passed, density begins to build up on the film as exposure to light increases. There comes a point where enough density occurs after development that it will just visibly register when printed on paper. In other words, if you find the shortest possible printing exposure at which a negative of film base plus fog density produces paper maximum black when processed, and then use the identical exposure and print processing for the second slightly more dense negative, then we would just be able to see that the tone produced was not maximum black but a discernibly different very dark grey. This is how zone 1 is described: the deepest possible grey discernible from black, and it is theoretically set at a negative density value of 0.1 above film base plus fog. This is easily measured with a densitometer, but a simple practical test with our own equipment is probably more accurate and useful. Since each zone above zone 1 is achieved by doubling the exposure (i.e. increasing by one stop), we can move back down from the meter indicated exposure for zone V in the same way to reach the theoretically correct exposure to produce zone l for the claimed film speed. If the speed is right, it will produce a just 'unblack' grey when printed identically with a zone 0 film base plus fog negative from the same film. If the claimed film speed is too fast, it will print as black, and if it is too slow, it will print as too light a grey. The test procedure is simple and often a revelation. Set the camera up outside in even, unchanging light (not always easy - try a north- facing wall on a bright, overcast day). Don't use flash or artificial light, they are unreliable for these tests. To avoid lens extension, keep the camera focused on infinity and fi11 the frame with an even-toned target - it could be a plain concrete wall or a piece of grey coloured card, though not a bright colour because meter cells vary in colour sensitivity. Remember, don't focus. Meter the surface in your usual way, taking care not to let your own shadow influence the reading and to meter as closely as you can to the lens axis. Use a middling manual shutter speed because these tend to be more accurate. Set the indicated exposure - this is set for theoretical zone V. Try to get an aperture and shutter speed combination that will allow you to stop down five stops without changing shutter speed and also avoiding full aperture. Don't worry about the speed too much, but do avoid full aperture. Now close down four stops and the exposure will be at theoretical zone 1. So if zone V was 1/15 at f/4, stop the aperture down to f/16 to reach zone 1. Make an exposure at this setting, which ought to be correct, but probably won't be. To find out if it is the correct zone 1, we shoot some more frames bracketed in as small aperture increments as possible. This might be approximate half stops by setting the aperture. ring between click stops or you may be lucky in having 1/3 stop click settings, or maybe you can override your shutter speeds (in manual, not auto) by 1/3 stops.

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