"Marc Wossner" wrote in message
ups.com...
Hi NG,

Can someone please explain to me if there is a connection between the
Nyquist sampling theorem and the resolution limit of a digital image
sensor?

In imaging terms, Nyquist defines maximum possible sharpness in terms of the
spacing between pixels. More pixels equals more sharpness, all else being
equal.

I mean, does it imply something like a lowest mark as far as
pixel spacing is concerned?

Another way to think of it is that - optics aside - the most detail you can
have in an image is every other pixel being black and white, in a
checkerboard pattern. If you attempt to represent finer detail, your image
gets killed by a crazed pattern called moire.

I´m quite new to digital photography and
keep reading about this stuff but must admit that it´s by far too
theoretical for me!

There's a world of mathematical theory behind sharpness: Fourier Transforms,
Modulation Transfer Functions. Some of propeller head types us love this,
but it's a world of hurt if you go in there unwillingly. The rest of us can
ignore it because the following rules of thumb will get the job done, and
only fourth grade arithmetic is required to apply these rules of thumb to
your own images.

Rules of thumb: A razor sharp print is about 320 pixels per inch. Some
people claim they can see more than that - hah. A very acceptably sharp
print is about 200 pixels per inch. At 100 pixels per inch, ordinary people
may be aware of some jagginess in the image. Less than 100 pixels per inch
is generally considered unacceptable but look how great your monitor looks
at even less than 100 pixels per inch. I've gotten away with a 72 pixel per
inch (also know as dot per inch in the trade) it on a few occasions.
Viewing distance matters, so add to this the fact that people stand back
more from a larger print, and you can go even larger than the ppi numbers
would indicate. Check this out yourself by walking up to a billboard - it's
lucky to be one pixel per inch.

Arithmetic: You can ignore megapixels. All we need is the image dimensions
in pixels.

My older camera took (and still takes) a 2048 by 1536 image. Dividing by
320 says a razor sharp image from this camera would be about 6 by 4 inches.
Dividing by 200 gives about a 10 by 8 image. Larger than that, and the
image starts to look a bit fuzzy, though remember that fuzziness is more
acceptable in a larger image.

My newer camera takes a 3204 by 2136 image. Dividing by 320 says that I can
print a razor sharp 8x10 image. Dividing by 200 gives me a pretty darn
sharp 16 by 10 image, and a very nice 19 by 11. In fact, this camera
produces 11 by 19 prints that look wonderful at just under 200 ppi.

BTW - curvemeister class starts tomorrow, Sunday the 7th:
http://www.curvemeister.com/support/class/index.htm
--
Mike Russell
www.curvemeister.com/forum/