That was for sensitometric exposure. Camera exposure is a touch more complex. Actually the hardest part for me keeping track of the correct terms (and conversion) for the units of measurement, especially for luminance.
Printable View
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?
Haa, I'm not teasing like I know the answer and am quizzing. I know these threads are easier to follow as a storyline playing out... but I don't know and so will have to sketch it out to see if I can figure. Does the bottom of the 7 1/3 stop average range equal the speed point. I don't think so but don't know for sure. I think the speed point only equals the bottom of the Normal range for Zone System.
This is about the relationship between the subject luminance range, camera exposure, and sensitometric exposure. I'm working on something that will support my answer to your question. The range above the metered exposure point is approximately 0.92 logs and the range below is 1.28 logs (not including flare).
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.
Attachment 68594
For reference, here are the sensitometric exposures for a 0.00 to 3.0 density step tablet exposed at 2.048 lxs.
Attachment 68595
I see the bottom is practically zero. So adding "about a stop" for flare would bring the bottom of the "standard subject range" about to 0.10 density. Not that the low end of the range "hits" the speed point, but that flare effectively makes your film have the density associated to the speed point.
At the approximate fractional gradient point actually. Think about it. What is ΔX for the contrast parameters outlined in the ISO film speed standard? 0.29 logs. Hm is Δ1.0 logs from the metered exposure point. And the average log-H range below the metered exposure point is Δ1.28 log-H. 1.28 - 1.00 = 0.28 logs. This is not a coincidence.
The film’s characteristic curve is a representation of how the film responds to a range of sensitometric exposures under specific processing conditions. It’s 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 doesn’t 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 it’s points falling on a target density like in the first example.
Attachment 68598
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 isn’t 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 isn’t any different than the H = E * t equation except that value for E comes from it’s own equation. The explanation is an excerpt from a K-factor thread.
Attachment 68599
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 “P”. This is the same constant that has the 10X ratio with the film speed constant of 0.8.
Attachment 68601
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.
Attachment 68606
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.
Attachment 68615
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.
Attachment 68618
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?