Letís look at an exposure example relating the numerical values for exposure with Zone System designations. Because the Zone System keys the luminance range to the exposure meter, the highlight luminance is a touch less than 100% reflectance. In fact, every reference point is shifted by a RD of 0.02. Not that this is a big deal, it just makes comparison between the two example more difficult. Iíve added a 18% reflectance guideline.
Because this is a no flare model, the exposures will be identical to sensitometric exposures. In camera Zone System testing, while using an optical system, produces minimal to effectively zero measurable flare. So, this example can be thought of as a close representation of Zone System in camera testing.
2 Quad - Exposure example - Zone 1.jpg
We can see that the metered exposure is correct for a 125 speed film, and 1.0 logs below is the speed point and the exposure there will produce a film speed of 125. Like with the 2.20 luminance range example, the shadow exposure falls below the speed point.
From the flare example, we know that the shadow will be brought up to around the speed point under normal shooting conditions, but this isn't factored in with in camera Zone System testing. The idea with ZS testing is to align the exposure Δ 1.20 logs below the metered exposure point with the speed point which is Δ 1.0 logs below the metered exposure. This usually means increasing the camera exposure (under rating the film).
The next example shows an exposure increase of 2/3 stop. This brings the shadow exposure up to the speed point. It also raises up the metered exposure and where it will fall. Instead of being at 0.064 lxs, it is at 0.102 lxs. This means the EI setting on the camera for a 125 speed film will be at 8 / 0.102 or EI 78 (80). If some experimental error exists, the EI could easily be 64 or half the film speed.
2 Quad - Exposure example - Zone 2.jpg
Notice how the negative density range in the adjusted exposure example is at the traditional Zone System 1.25. This is what one would expect from a sensitometrically exposed 2.10 exposure range, but not from a 2.10 subject luminance range shot through an optical system incorporating flare. One way to tell the target 1.25 negative density range isnít appropriate is to check to see how it fits with the photographic paper.
3 Quad - Exposure example - Zone System.jpg
The paper LER that is considered to be in the middle of the range for grade 2 paper printed with a diffusion enlarger is 1.05. The LER is determined between the points 0.04 over Pb+f and 90% of the paper Dmax. Clearly the 1.25 negative density range doesnít fit a grade 2 paper. But many Zone System practitioners swear that they use something close to 1.25 for their testing range and their negatives fit well on a grade two paper. The reason for this is the 1.25 negative density range comes from a no flare test and the prints are from negatives made in a camera with flare which reduces the negative density range.
Last edited by Stephen Benskin; 05-14-2013 at 10:49 PM. Click to view previous post history.