Hello there,

Just a quick question: What (if you know) is the approximate dpi of colour and B&W negative (35/2.25/and 4X5) film ? I know this would be dependant on film speeds so lets use 50, 100 and 400 ASA as examples. Also, do you have the same info for prints?

Thanks,

jl

-- John Lambie (j_lambie@hotmail.com), January 03, 1999

Answers

A complex question to answer as it is usually the optics that determines the resolution for fine grain, correctly exposed film. Under ideal circumstances, you can exceed the equivalent of 5000 dpi which implies you scan at more than 10000 dpi to avoid aliasing.

For all but the poorest quality optics, you want to scan at 2000 dpi or higher.

For prints, a really good contact print can exceed the equivalent of 400 dpi (use about 1000 dpi for scanning) but the typical snapshot is less than 200 dpi (use 400 dpi for scanning). Only the veryu best of prints will be compromised by scanning at 600 dpi and since such scanners are quite inexpensive....

Even if you needs are for lower resolution, always scan at the higest resolution and then resize using an algolritm that integrates (most do now!). This will remove noise fromn the image and preserve as much as possible the correct colour and texture of the original image.

Note that the resolution tests for digital cameras are "fudged" by measuring vertical and horizontal resolution, the best cases. The worst case is at an angle to both, in the case of square pixels at 45 degrees. The resolution degrades by about 30% in this case relative to best case.

Unless you do a lot of scanning or live a distance from a photofinisher than does scanning, consider paying to have you negatives scanned.

Now you know why can sell those expensive scanners with 5000+ dpi resolution :-)

bcnu John

-- John Ohrt (jeff1@connection.com), January 03, 1999.

ANOTHER INTERESTING POST! I regularly print on dye-sub and ink jet printers sizes from 5x7 to 60" by 80"(large prints output on the ink jet). When using the film scanners I scan at max resolution--when using the flat-bed I normally scan at 300ppi. No matter what the final size is (be it for dye-sub, ink-jet, or offset printing) I almost always size the final image to 300 ppi and at that the file size normally is larger than is normal. I have just found that that 300 number sticks in people's minds, so I use it. What type of output are you looking for as the final product. Most of the large digital graphics you see are output at 72 ppi (believe it or not). Walk up to a large graphic and look at it.

-- fred deaton (fdeaton@airnet.net), July 01, 1999.