So now we have this meter point ten times the speed point for black and white negative film.
But we really consider the normal subject range to be what, 7 1/3 stop?
Where do we expect the normal subject to fall in relation to the metered point? How much above and how much below? Is the meter smack in the middle?
It works something like in this example. The ranges have been rounded to simplify the example. Note where Hm falls compared to the shadow exposure and remember this is the exposure for a 125 speed film - Hm = 0.0064 lxs Hg = 0.064 lxs.
For reference, here are the sensitometric exposures for a 0.00 to 3.0 density step tablet exposed at 2.048 lxs.
The filmís characteristic curve is a representation of how the film responds to a range of sensitometric exposures under specific processing conditions. Itís a picture of the film. I like to think of camera exposure as being superimposed on the film curve, and while the camera exposure can move around the curve, the film curve itself doesnít change. This approach helps me distinguish between the sensitometric exposure and the camera exposure.
There are a couple of ways to think about how the camera exposure falls on the characteristic curve. One is to have an exposure range that has one of itís points falling on a target density like in the first example.
Another way is to use the actual log-H values determined from the sensitometric exposure and link them to the actual camera exposure. While this method isnít practical for most people, I believe it can be useful just to understand how it works.
First we start with the classic camera exposure equation. This isnít any different than the H = E * t equation except that value for E comes from itís own equation. The explanation is an excerpt from a K-factor thread.
Lg can be determined using the exposure meter calibration equation. Please note the value of Eg is 8.11. Rounded this becomes 8 which is the exposure constant usually referred to as ďPĒ. This is the same constant that has the 10X ratio with the film speed constant of 0.8.
With the exposure formula in hand, we can now work out the range of film plane exposures (H) for the average scene luminance range. The range is measured from a diffused white containing some semi-specular highlights which has a reflectance of 100%. The range is 2.20 logs.
The aim exposure at Hg for a 125 speed film falls at a reflectance of 12%. The value of L define in the exposure meter standard confirms this exposure (the numbers are slightly off here probably because of rounding). An exposure at Δ1.0 logs below Hg equals the aim value of Hm = 0.0064 lxs for a 125 speed film. The shadow exposure falls approximately Δ0.30 logs below Hm.
This can be plotted.
Now we can combine the camera exposure with the film curve by aligning the camera exposures up with the sensitometric exposures. For this to work, the camera, the film curve is turned on it’s side. This can be disorienting at first, but it will soon become second nature.
For a film that has a speed of 125, the exposure of 0.0064 lxs falls on the 0.10 density speed point and 0.064 lxs falls Δ1.0 log-H above it. This is how the subject luminance range is connected to the film curve using just the film speed number and an exposure meter that wants to place everything at 8 lux, except there’s a problem with this example.
Upon a closer look, the shadow exposure doesn’t line up with the speed point. It has an exposure of 0.0034 lxs or almost a full stop less than the exposure needed to reach the speed point. Where the exposure of 0.0064 occurs, it does line up with the speed point, but it represents the shadow luminance at RD 1.92 and not the deepest shadowed measured at RD 2.20. What's wrong?