On Sep 13, 2006, at 2:55 PM, Toralf Lund wrote: > I think push processing is a very good analogy. You can't strictly > speaking change the ISO on a digital camera - the sensor has a fixed > sensitivity. ...
... at a specified voltage input/output level. If you change the voltage levels, you are changing the response curve and thus the ISO. Changing ISO is not the same as multiplying the pixel values from a sensor with a fixed voltage level, which is basically what the exposure compensation slider in a RAW converter does unless the programmer who designed it includes additional processing of differential values. Given a reference film emulsion and density curve with a given developer, push processing with the same developer and emulsion means reducing the exposure so as not to block up highlights while extending the development. This changes the gamma curve and allows you to treat overall exposure as a rise in sensitivity, but you're shortening the dynamic range of the capture. It is directly analogous to modifying the voltage environment of the sensor and has similar results except that the digital sensor, within its range of acceptable voltages, can be more pliant about change and more controllable about side effects. With film, you're locked into the inflexible world of chemical reactions and can't apply mathematical transformations to correct a problem which develops. Godfrey -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
