> The equivalent resolution of a fine film
> is about 4,000x3,000.
Hmmm, let's see Velvia is about 160 lines/mm
for high contrast, about 80 for low (1:4)
contrast -- using 80mm as the target, that
works out to 3840x5760 pixels -- so at first
glance your numbers seem about right.
The problem is that it doesn't take aliasing
errors into account. A line can appear
anywhere on film, but it won't show up as a
line if it lands on a boundary between two rows
of sensors on a digital back, and curved or
diagonal lines result in moiré patterns in
the image.
As a rule of thumb (the actual math is painful)
to reduce aliasing errors to the point of
imperceptibility, you want to sample 5 times as
many points as can be perceived. Given that,
in order to match velvia, you would actually
need 19200x28800 pixel resolution -- that means
you need a 527Mpixel sensor. Pretty big huh?
Of course you don't have to match Velvia for most
applications. You just have to match the resolution
of what you're going to print on. Small-to-medium
sized pictures in books and magazines are easily
within the reach of a 3000x4000 back, and for
newspapers it's overkill.
Digital backs are great for guys who need to get
the image out the door fast and don't need film's
resolution, or for people who just want to take
point-and-shoot type pictures and aren't critical
about quality. I doubt it will be good enough to
replace film for other uses in the next 10 years
or so.
-Don
