Archived posting to the Leica Users Group, 1998/11/22
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Erwin Puts wrote, in part: >You can not test for accurate film speed by using the greycard and exposing >it as a mid gray. Erwin, Isn't that a bit of a sweeping statement, especially when the laboratory aim density (LAD) method uses a grey card exposure? Are you referring to the ISO definition of film speed? I guess that the LAD method has limited use for still film because of the lack of published aim densities (Kodak only give a range for their neg still films, eg Portra 160NC: red 0.70 to 0.90 for grey card) but it is fairly common in the movie industry, for which the manufacturers give more precise info (eg the100 ASA tungsten film Eastman 5248: grey card density above base fog - red .79, green .84, blue .92. A third of a stop changes the density measurement by about .05 to .07. Often the three colours each suggest different film speeds within a range of two-thirds of a stop, rarely do they all agree on the same third of a stop). If manufacturers published aim densities for their still films, wouldn't this be a valid appproach for determining the number you set your meter to? Kodak do publish spectral-dye-density curves for grey card exposures and D-min. The information is there, it only needs to be interpreted. The appeal of the LAD method (to me, if no-one else) is the way it corresponds to the commercial method of determining printing exposure by concentrating on the mid-range. I would value any opinions on this one. I admit that I could be barking up the wrong tree or simply just barking (slang for mad). I admit that I do not use the LAD method for still film. I tend to decide on what number to set on the film speed dial for colour film by simple experience rather than test, though I might start off with a transmission-illuminated step-wedge test (able to give a wide brightness range at third-stop intervals) when first using a film. Of course I don't call that number the accurate film speed. Regards, Malcolm PS, we use the local film between the sprocket holes for measuring base density. Couldn't your method use that for comparing grey card with base fog (assuming that the grey card filled the frame)?