----- Original Message ----- From: "mike wilson" <[EMAIL PROTECTED]>
> Hi, > > Anthony Farr wrote: > > <snip> > > > allowed. I don't know, but contrast control by preflashing may not even be > > necessary if the whole range of the slide can be captured. Film to film > > copying needs contrast control, unless duplicating emulsion is used, > > because the gamma of the original slide being about 1.8, and the gamma of > > the camera's slide film also being about 1.8 gets a resulting gamma without > > contrast control of 3.6, or basically unuseable. > > Why would you want to (or be able to - does the *istD have multiple > exposure facility?) preflash with a DSLR? > > mike > > THE SHORT EXPLANATION: Most photography is concerned with recording scenes from real life. A slide film is nearly twice as contrasty as real life. To copy a slide you need either special low contrast emulsions that are sold only in large amounts, or you need to use some method of contrast control if using a normal film. With black & white the development can be modified to alter contrast, but that method raises too many problems with colour. The most common method of contrast control for colour copying is "preflashing", aka "prefogging". THE LONG (LONG) EXPLANATION: Preflashing is an overall brief exposure to light either before or concurrrent with the main image-forming exposure. It's important to know that it's an overall fog being applied, not a pre-exposure to the image being recorded. Two slide duplicators that I can name immediately have this function, the Bowens Illumitran and the Elinchrom Dia-duplicator. The advantage of these units is that ordinary camera film can be used in place of special low-gamma copying emulsions. For the purposes of my (then) employers I could make duplicates with better archival properties with Kodachrome than with Ektachrome duplicating film. First an explanation of terminology. Gamma (also written as the infinity symbol, IIRC) is an antiquated expression of film contrast introduced (I think?) by Edward Weston, but because it's easy to write and say it survives in name, although now it mostly refers to "contrast index" (CI). Another method which I was once familiar with (and was the one my college used) can't be written in plain text without special characters, but is said as "G-bar" and is literally a letter G covered by a horizontal bar. The only significant difference between the three is their definitions of where the straight-line portion of the characteristic curve starts and ends, the slope of which is the measure of an emulsion's contrast. The results of these methods are so close that it matters very little which one is being quoted. I don't know if digi sensors measure their contrast in terms of gamma, I think the bit-depth is the ruling factor, but their contrast/latitude characteristic is close enough to film to consider them as approximate equals. The problem with duplicating is that photographing an original slide is not like photographing real-life. The real world has a gamma of 1, consumer slide films have a gamma of about 1.8 to overcome contrast losses in projection. Professional transparency films have a gamma of about 1.4, being intended not for projection but for print reproduction. Duplicating a slide with a normal slide, with no contrast control, will build contrast to an unacceptable amount. You also need to know about the "inertia point" of a photosensitized material. Essentially, a few photons of light will strike a silver halide grain or a photosite before it will move from a completely unexposed state to a state where it begins to properly record light exposure. Digital performance in this regard is unknown to me, but it is still true that the small amount of light visible to the eye in the shadows of a normal scene nevertheless produces no record of exposure in the corresponding photosites of a digi sensor (apart from a little electronic noise which I think is an artifact of the analogue to digital conversion). Whether or not this point is called the inertia point in digi-speak, that's the term I'll use for the sake of simplicity. The purpose of preflashing is to nudge the film/sensor to the brink of its ability to record exposure, i.e. the "inertia point". If no extra exposure is received the result will be as black as if no light at all was ever recorded. But here's the beauty of the technique - the very least extra exposure will immediately begin to be recorded without inertia. The result is that the film's/sensor's capacity to record shadows and dark tones is dramatically expanded, plus there's a moderate boost in overall sensitivity. Preflashing beyond the inertia point will reduce overall contrast, at the expense of a loss of maximum density in a slide (elevated d-min for negatives), or in digital terms a clip into the darker tones. In duplicating terms this means that the range of densities in the original slide can be captured by the duplicate, even using "normal" emulsions. Some photographers have dabbled with preflash in general photography as well, to expand a films latitude without resorting to pull-processing. Better to ask an *istD owner if multi-exposure is possible, but preflash is still possible without that feature. Too much information? Sorry about that, but you did ask :-) regards, Anthony Farr
