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   Posted by chris on 10/29/02 at 3:22 AM

Subject:   calibrating my film speed using a scanner

Message Posted

I regularly process B&W film and decided (after many years) to determine my personal film speed and development times after reading articles about the zone system on the web. My initial experiment is related below and comments would be welcome. This is because, apparently, the manufacturers iso film speeds are over optimistic for a majority of films.

The vast majority of zone system/ film calibration articles that I have viewed make use of a dark room and enlarger/printing to assist in calibrating the film speed. None mention the use of a film scanner/software/photographic inkjet printer. However, as I own a film scanner, I decided to see if it would be possible use the scanner (and inkjet printer) as substitutes for darkroom enlarger/printing.

For my first attempt I disabled the auto-exposure compensation (set at zero) and disabled the auto-focus before scanning a blank (Zone O) negative (the scanner was told to expect a b&w negative). The resulting scan was not what I expected- it was light grey with lots of noise. I was hoping for a darker scan result, if not 100% black.

It seems to me that the scanner is trying to second-guess what it is being given to scan and perhaps trying to convert the blank image to zone V. If I offset a negative (with an image) in the scanner, so that the rebate is in the middle of the scan area resulting scan (same settings as above) gives 100% black where the rebate is. This is what I expect. In fact, every time I scan negs or slides this rebate area is black.

Anyway, I decided to modify my procedure as follows. I took an old negative (with image) and cut a small hole in the middle of it to form a window area. This neg was then placed in front of the blank negative and this time the blank area of the negative (through the window) was scanned much darker. This is almost what I wanted.

I then proceeded to gradually darken the brightness in a sequence of scans using scanner software. I needed to reduce the brightness of the scan by 90% to get a (Zone I) black on printing paper (this was 98% black in ‘Elements’). 100 and 99% black were zone O.

I then proceeded to scan the negatives from my roll of film (these negatives were all under exposed (-4 stops) to give zone I at different film iso settings). Unfortunately, every scan gave me the same 98% black when viewed in elements- even though every negative was scanned in exactly the same way as the first blank (Zone O) negative.

Possibly, having the auto-exposure set a zero (and reducing the brightness by 90%) is giving too much light through the negatives and “bleaching out” any subtle differences between the zone I negatives. Perhaps I should scan with the exposure reduced and then potentially increase the resulting image brightness by 10%?

Alternative approaches could be:
scan the B&W negative as a transparency
adjust the scanner gamma instead of the brightness

Any other thoughts would be welcomed.

Then again, am I wasting my time with exercise?


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