Weird patterns on scans of long exposures

I've seen a grid pattern in drum scans from a professional lab and was told that it's normal and that I was getting the best quality possible. I attributed it to the fact that the lab scanned at 4800 ppi with a scanner that had an optical resolution of 4000 ppi; in other words, an interpolation problem. It could have been an electrical interference problem but that isn't really a very good excuse for a "high quality" lab. With my own drum scanner (same model) I have never had this problem although I only use optical resolution and have it plugged in through a voltage regulator and RF filter.

My guess is that in this case the problems are inherent in the scanner. Grids and stripes from external noise or poorly-shielded power supplies tend to wander about a bit like sand ripples. The highly regular grid in Mark's first example is almost certainly an interpolation artifact from a crude interpolation routine. The solution is to scan at simple multiples or ratios of the scanner's native resolution and do the interpolation yourself in your photo editor.

I have seen the second sort of blotchy effect in my own flatbed scans of nighttime or twilight exposures. My best guess is that it is caused by slight variations in the background signal reaching the CCD, either caused by electrical noise, or more likely, by crud on the underside of the scanner glass. Try scanning a colour filter or a simple piece of coloured cellophane and use your photo editor to remove the 'colour cast'. If you see similar patterns, it's not your film.

If you do the last trick, you may well also see hot pixels in the form of dotty lines of one or other primary colour (with my Epson 3200 it's always red). These too show up in my scans of low-contrast originals. In theory you should be able to remove them with a calibration scan or a single 'dark frame', but the scan-to-scan repeatability of these units makes that a futile exercise.

My solution has been to accept that my 3200 is an excellent proofing tool, and capable of making great prints from normal negatives and slides, but that I will have to send out for 'real' scans for the best resolution and tonal reproduction. I take a lot of my LF photos at twilight or after sundown, so it's a real frustration. In B+W one can increase contrast with N+ development, but in colour a change of contrast means a change of film, and thus a change in feel. Sometimes cheap tools just don't work.