Re: Congratulations Canon for the 5D "Best Professional Camera" at TIPA

On Fri, 28 Apr 2006 05:51:11 GMT, Bryan Olson
<fakeaddress@xxxxxxxxxxx> wrote:

Rich wrote:
[...]
Light fall-off is most apparent when shooting broad expanses of blank
areas that are underexposed or dark to being with. Thats why if
you shoot a picture of a deep blue sky it is more apparent than if you
give the image some more exposure.
But fall off is due to one thing only, the inability of the lens to
fully illuminate the entire frame. It can be remedied by using a lens
that has a larger circle of illumination than the sensor area and this
avoids the fall off.

Not so. Illumination at wide angles is fundamentally difficult,
particularly at large aperture to focal-length ratios. A pin-hole
camera has no particular limit to it's circle of illumination,
but illumination will fall as the fourth power of the cosine
of the angle from the normal.

I know it's a problem, but is it completely impossible to minimize or
fix?

Unfortunately, for cost, size, etc, other
considerations, camera companies only build the lenses to "just" cover
the frame which means you experience visible attenuation at the
corners. There may also be a contribution to this when using older
SLR lenses on digital cameras. But, if you were to slap a medium
format lens on a 35mm camera, you would avoid the effect because the
circle of illumination more than covers the 35mm frame..

Should you actually attempt the experiment, you'll run into a
problem: where are you going to find medium format rectilinear
lenses with the focal lengths and apertures for which 35mm format
lenses produce dark corners? In medium format, a 33mm focal
length is wide-angle; B&H sells a 24mm lens as a 645-format
fish-eye.

You can't, I wasn't specifically talking about wide angle lenses,
my example was about 50mm lenses.

Considering the lenses that are available in medium format, the
same focal length and aperture in a decent 35mm-format lens will
illuminate the 35mm frame about as evenly.

Impossible. Or the vignetting with medium format would be
intolerable.

Wide-angle illumination
is optically challenging regardless of scale. It's not a problem
particular to full-frame sensors, and medium-format lenses are
not a solution.

It probably is, but it isn't insurmountable, except when you have
set costs and sizes you are dealing with. Ever see Nikon's 6mm
fisheye? Now why would a 6mm lens need a front element
6" across? Camera companies are in competition with each other,
and the two driving forces are cost and physical size. No consumer
will want to haul around a 6lb 17mm lens, but that is what it might
take to fix the problem of poor edge illumination. Digital sensors
(I think that came up before?) probably make the problem worse.
-Rich
.

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