[Leica] PIXEL PITCH SIZE

[bryanwi at bryanwi.com (Bryan Willman)]

>From astronomy refering to telescopes - aperture wins.   (A cheapish 16"
reflector will put the wump on even very high end 12" refractors for all
but special cases.)

For terrestrial imaging (what we call photography) "imaging plane size
wins, mostly".

Footnotes - historically, a good lens on a 4x5 view camera could make an
image that was superior to any image made by a smaller camera, if you
could manage to capture it with 4x5.

But of late, I find images from 1st rate 35mm FF cameras (5dM2 and of
course the M9) to be very competitive with samples from medium format
back cameras (I don't own one.)   Well scanned film from large format
seems to be the only thing with an obvious advantage and even that is
narrowing.

So our old notions of how large an image we can make from a 35mm frame
seem to be changing in the digital era.

I'm sure cameras like the S2, or the coming full frame 645 cameras, will
have advantages in quality - whether you can see them in prints smaller
than, say, 20x30 or even 40x60 will likely determine the fate of those
formats.
 

-----Original Message-----
From: lug-bounces+bryanwi=bryanwi.com at leica-users.org
[mailto:lug-bounces+bryanwi=bryanwi.com at leica-users.org] On Behalf Of
Jean Louchet
Sent: Monday, 01 March, 2010 04:50 AM
To: lug at leica-users.org
Subject: Re: [Leica] PIXEL PITCH SIZE

George,

You are right to point out the fact that on most serious cameras the
pixel
size is somewhere between 5 and 10 microns. There is a physical
explanation
to this.

ZVisible light wavelength is spread between (roughly) 0.4 and 0.7
microns.
Lenses do have both geometrical optical aberrations and diffraction that
limit their resolution.


Again in very rough terms, geometrical aberrations are greater at large
apertures, inversely proportional to the inverse of the f-number.
Technology
may help reduce them (better glass with higher refractive indexes and
less
dispersion, more complex designs, aspheric elements etc.) to a certain
extent - we all know Leica are very good at this.
On the other hand, diffraction can't be avoided, it is a physical law.
Parallel light coming through a hole (here, the diaphragm) in a
"perfect"
lens will draw on the film or sensor, not a single point as we woulod
like
it, but a "diffraction pattern" that looks a bit like the circles when
one
drops a stone into a lake. The main spot, the central diffraction
pattern
has a diameter equal to 1.22*f*lambda/d where f is the focal length,
lambda
the wavelength and d the diaphragm diameter. As the f-number "n" equals
f/d,
the diffraction diameter is 1.22*n*lambda.

As lambda is not a single value but is spread between 0.4 and 0.7
microns,
the diffraction pattern is really ugly with colour fringes on its sides
and
the effects of diffraction are still visible and annoying inside a
circle
with diameter approx. 2*n*lambda. As lambda is around 0.5 microns, this
means the diffraction pattern diameter will be about 2 microns at f:2l;
4
microns at f:4,  etc.

As the actual blur pattern is the addition of the effects of diffraction
to
those of geometrical aberrations, and as it is very expensive to correct
geometrical aberrations,in most lenses the blur diameter is a convex
(parabola-like) curve with its minimum (= best sharpness) a couple of
f-stops above full aperture- the good old Nikon 1.4/50 had its optimum
at
the centre of the image, at about f:5.6. Top lenses like the recent
Leica
ones have their optimum very close to full aperture (and this is at a
cost!). All in all, one can safely say that the best blur circle
diameter of
a given lens, measured in microns/micrometers, is equal to about twice
its
maximum aperture.

This is why with a top-of-the-range lens with aperture 2.8 it is wise to
choose a pixel size around 6 microns. Quid erat demonstrandum :-)

By the way, some P&S cameras are sold as 12 Megapix. Their sensors are
often
about 4.5 x 6 mm large . This means the pixel size is 1.5 microns, which
is
totally ridiculous - even worse since most of them use high-factor zooms
that go with even more aberrations and low maximum aperture. Actually
there
can't be more than 0.7 MPix useful in these cameras! All the rest is
just
redundent data.

On the other hand, a pocket camera like the D-lux 4 uses a lens with a
very
small zoom factor (hence low aberrations) , a high max aperture (hence
low
diffraction) and ... I don't remember its sensor size but here
advertising
10 MPix looks like it makes sense.

If I had to draw a conclusion, it is that what matters most of all is
the
sensor size. Megapixels don't mean anything if the sensor is too small.

Jean

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