Resolution & publication quality: Digital cameras

[Una Smith]

Dave Carmean wrote:

>Most publications print at 300dpi-

No, sorry.

It is a common mistake to confuse printer resolution and halftone
screen size.  Many scientific journals that publish high quality
photographic illustrations use a halftone screen size of 300 lpi
(lines per inch).  American Journal of Botany is one of them.  I
am told they do this so that published images will look good when
photocopied.  The printer resolution (dpi) used by these journals
is far higher, often in the range of 2700 dpi.

The rule of thumb for optimal halftone reproduction is to use about
2 pixels per line.  Hence to print an image at 4x5 inches in any
journal that uses 300 lpi halftones, you need 600 ppi (pixels per
inch).  For an 8-bit deep grayscale image, the "image size" will
be 8*600^2*4*5 = 57.6 megabits = 7.2 MB (megabytes).

An image size of 7.2 MB is significantly larger than the output of
most digital cameras.  And remember, that's for pixels only 8 bits
(1 byte) deep.  To get a 7.2 MB grayscale image, you'll need a
color digital camera that produces 7.2*3 = 21.3 MB images.  Such
cameras exist but, as Stinger has already mentioned, they are very
expensive.

Another thing to watch out for:  when reporting the output image
size, many digital camera companies count each color channel as a
separate pixel, so their "3 MB" cameras produce color images that
are a mere 1 MB.

There are many great websites about digital photography (and even
more bad ones) but, to get the real nitty gritty essential details,
nothing beats rec.photo.digital.


        Una Smith               una.smith at yale.edu

        Department of Ecology and Evolutionary Biology
        Yale University
        New Haven, CT  06520-8106

        http://pantheon.yale.edu/~una/