RE: [Leica] digital versus film

[Henning Wulff <henningw@archiphoto.com>]

At 10:25 AM -0800 12/17/02, Randy Jensen wrote:
>A simple calculation I did (based on some leaps of faith, of course) is that
>to match slow film (ISO 50 or so) you would need an equivalent megapixel
>amount of about 28MP.  This is how I came to that number.
>
>With a decent lens and film such as Fuji Velvia you can expect a lpm of
>about 45.  Multiply this by the long side of a 35mm neg (36mm) and you get
>1620 lines across.  This is theorhetically the maximum number of lines you
>could discern from the far left to far right of the neg (or slide).
>
>Nyquist theorem dictates that you need to double sampling resolution to
>record detail.  But this only works for things that are in phase, such as
>audio.  That's why CDs sample audio at 44.1kHz.  People can hear up to 20k
>(at best) so to record detail that high you need to at least double that
>(and there's debate that 44.1k isn't high enough, but that's WAY off topic).
>
>Anyway, for resolution of an image, doubling is not nearly enough.  You need
>to multiply it by 4 (can anyone say "Reciprocity Law"?  lol.  It's like
>doulbing in both directions (2x2=4).  This link hints that it may be more
>like a factor of 6, but I'll be conservative with 4:
>http://clarkvision.com/imagedetail/sampling1.html
>
>If you multiply by 4 my 1620 lines of resolution with film, you get a CCD
>sensor that must be 6480 pixels wide (4x1620), which would make it 4320
>pixels on the short side (to keep the 24x36 aspect ratio).  Total pixels:
>4320x6480=27,993,600 (about 28MP).  If it truly is closer to 6 (instead of a
>factor of oversampling by 4), this number rises dramatically.
>
>This ONLY refers to straight resolution.  There are other factors,
>obviously, in favor of both sides.
>
>My observations for the morning....
>
>Randy

Actually, the resolution figure you want to start out with is 40 or 
more line _pairs_ per mm, giving about 160 pixels per mm, or in the 
order of 4000x6000. Going higher than doubling isn't really necessary 
for resolution in either the purely vertical or horizontal direction; 
it is, however, for the diagonals. 2 times the previous number should 
be enough. So you wind up with 24Mpx2 or 48Mp. This could give you 
very good MTF performance up to around 40lp/mm, and a little more as 
you get closer to the purely horizontal and vertical directions.

One of the reasons digital looks so 'sharp' is that it's MTF is very 
good up to the resolution limit, so that things that need up to 
20lp/mm to image well are rendered extremely 'sharp' by cameras like 
the 1Ds, while film has a gradual roll-off where some contrast is 
lost already at lower frequencies, but on the other hand at very high 
frequencies there is still information left.

In practice what this means is that images from digital cameras and 
backs (and to some extent, scans) will look very good up to a point 
(a certain value of resolution), and then the MTF will drop off and 
there will be no more detail. Each generation of sensor chip pushes 
this 'drop-off' further into higher frequencies.

If we make a print of a very high quality, low ISO 35mm negative 
taken with a very good lens using a very good enlarging lens, etc, we 
can produce 'resolution' and detail well beyond what any 35mm based 
digital camera can produce. It's just that there will also be 
evidence of grain, and a roll-off in MTF that will not be as evident 
in the digital image, and usually we don't produce negatives that 
good. So in that case the digital image will look better because it 
has little or no grain, and the sensor has a high MTF in the region 
that is most important to us in judging a 'sharp' image, namely the 
10-20l/pmm region for the 35mm format.

Film still wins a lot of the numbers games, and will for quite a 
while into the future. If you are judging just by looking at images 
taken under normal circumstances - handheld, non-optimal shutter 
speeds, moving subjects, slight off-focus, etc, - digital often comes 
out ahead even now because it doesn't show grain and has very good 
MTF in the most important region.

As usual, there are of course many, many other factors such as 
longevity and archiving, colour balance, dynamic range, post 
processing and transmission ease, life-cycle costs of equipment vs. 
film & processing costs, and others that help determine which to use.

We're in a good time right now. We have choices, can make decisions 
and acquire images in various ways. The difficult thing is often 
getting the information together to make an informed choice, and only 
allowing such prejudices and emotions into the equation as _we_ 
determine; ie, make informed choices about the prejudices etc. as 
well.

- -- 
    *            Henning J. Wulff
   /|\      Wulff Photography & Design
  /###\   mailto:henningw@archiphoto.com
  |[ ]|     http://www.archiphoto.com
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