Characteristic Curve for Ilford Delta 100 in ID-11 (1+1)
I used my LunaPro SBC with Lab attachment to measure the HD curve of Ilford 100 developed in ID-11 (1+1) for 11 minutes at 20 C with intermittent agitation (5 inversions once a minute). This is the time recommended by Ilford for normal development. Temperature was controlled using a water bath and the developer came out at 20.2 C.
The blue line shows the measured density. The red line is a best fit straight line to the measurements with exposures from -3.1 to 6.3 stops. The X axis (exposure) is shown in stops relative to the exposure indicated by my spot meter when set to ISO 100 (box speed). Densities are shown relative to film base and fog. I estimate the error margin to be about +/- 0.1 density units (1/3 stop).
The graph is close to a straight line for exposures from 3 stops below the indicated exposure to 6.5 stops above the indicated exposure at ISO 100 (in other words, from Zone II to half a stop above Zone XI if shot at box speed using my meter). Outside this region it flattens quite rapidly.
I used an A3 sheet of paper on my enlarger baseboard as the test target. Both the enlarger and the camera lens (Bronica 75mm f/2.8 PE) were stopped down to f/5.6 to reduce light fall-off. Exposure times ranged from 1/16" to 25 minutes 36 seconds in roughly half-stop intervals. 15 frames of 120 film were exposed. Each frame was exposed twice. During the second exposure, half of the test target was covered with black card so that the resulting frame was exposed to two different light levels. Measurements were made from within designated circles equidistant from the center of the test target that had been tested to receive identical illumination from the enlarger. The exposure indicated by my spot meter was 3.2 seconds at ISO 100, which indicates that the light level was about 3 EV higher than the point at which reciprocity failure would start to have an effect. The darkroom was kept dark (with no safe-light) during the exposures and measurements.