Nelson touched on this in Safety Factors in Camera Exposure. Lens coating shifts it back 0.05 log-H units. This simply introduces a small safety factor. In fact, I believe this might be what is at least partially responsible for the ASA / ANSI standard's note in the forward about a built in 1/3 stop safety factor.
Originally Posted by albada
From Safety Factors, "As point out by Jones and Condit, however, the shadow point should coincide with the speed point only when the flare factor is 4. When the flare factor is 2.5. the deepest shadow can be placed about 0.05 to the left of the speed point because a lower slope on the toe of the curve becomes usable when the shadow contrast in the camera image is increased by the reduction in camera flare. Consequently, the "first-excellent" point in Fig 3 is considered to lie 0.05 to the left of the speed point. The "first excellent" point, therefore, lies 0.37 in log-H units to the left of the shadow point, c, representing the exposure obtained from the use of the exposure meter and the ASA exposure index. The interval of 0.37 is the logarithm of the safety factor. These calculations, therefore, lead to the conclusion that the safety factor is 2.35" (for the fractional gradient method)
So basically, yes I believe the factional gradient value could be lower with coated lenses. You can also see that 1.80 was being used when considering the change from fractional gradient to the fixed density / Delta-X method.
Last edited by Stephen Benskin; 03-08-2013 at 07:27 PM. Click to view previous post history.