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

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.

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.

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. 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.


--
--Bryan
.

Relevant Pages

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