From: "Al Bond" <[EMAIL PROTECTED]> >>>>>>>>>>>> I was looking at some 5 megapixel (Canon G5) sample images to get some idea how the might compare against my 2820 dpi scanner. I know I have read in the past that, despite the large difference in file sizes and pixel counts, a 5MP camera isn't as far behind a 2820 dpi scanner as it might seem.
Certainly the sample G5 images looked reasonable but, on one image of a house, the fine roof tile details had generated a fairly obvious moire pattern. This is something I have never come across on any of my film scans. Have I just got lucky on my scans or is moire more likely to happen with digicams? <<<<<<<<<<<< Moire can be avoided in digital cameras by using a stronger antialiasing (AA) filter. The Nikon D100 has (as I undertand it) the strongest AA filter of the dSLRs, and it's images appear a tad soft as a result. One Japanese digital camera magazine had a "Bayer color pattern stress test", and the D100 was the only camera that passed<g>. (I don't own a D100: I'm thinking about buying a 300D.) Moire appears when you have strong detail above about 2/3 of the Nyquist frequency (more accurately, it appears when you have strong signals (i.e. information) at and above the Nyquist frequency; that information is required to produce sharp edges in lower frequency patterns, so (IMHO) 2/3 of the Nyquist frequency is about the limit for quality, high-contrast patterns in digital images.). The Nyquist frequency for your 2820 dpi scanner is (2820 / (25.4 * 2) ) = 55 lp/mm. Since most films' MTF response at 55 lp/mm is extremely low, it's extremely unlikely that you'd notice Moire in your scans. (And scanner optics are often barely adequate and function as an antialiasing filter.) >>>>>>>>>>>>>> > Also, does the processing affect this? <<<<<<<<<<<<<< Yes. If you sharpen a scanned image and downsample it to about 1/1.414 of the original linear resolution, you will rarely see Moire (at least that's my experience with a 4000 dpi scanner), but if you go below that, you will often see aliasing effects. For large reductions in size, it's often advisable to hit your image with a Gaussian blur first. >>>>>>>> I'm just curious to know why there seems to be this apparent difference between digicams and scanners! <<<<<<<< See above. David J. Littleboy [EMAIL PROTECTED] Tokyo, Japan ---------------------------------------------------------------------------------------- Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body
