[filmscanners] Re: Moire - Scanners vs Digicams

["David J. Littleboy" <davidjl@gol.com>]

From: "Al Bond" <al@greenspace.freeserve.co.uk>
>>>>>>>>>>>>
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
davidjl@gol.com
Tokyo, Japan


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