Re: Vignetting due to lens or sensor?

David Ruether <d_ruether@xxxxxxxxxxxx> wrote:

"Chris Malcolm" <cam@xxxxxxxxxxxxxxxxx> wrote in message news:7a6fb1F1tlamiU1@xxxxxxxxxxxxxxxxxxxxx
Jim <jj.n@xxxxxxxxxx> wrote:

The cos^4 law was discovered a very long time ago (perhaps in the late 19th
century).

This law operates on all lenses of whatever focal length. But, the
reduction in intensity of the light is only important for wide angle lenses.
Jim

If you are referring to light intensity reduction due to the additional
relative distance from a theoretical lens center to the format corner
compared with the axial distance, this is not necessarily true with
lenses with designs that depart from strictly symmetrical simple
designs. One advantage of a well-designed retrofocus wide angle
(one with generous front and rear glass sections in addition to other
design efforts to even out the lens illumination) is that it may illuminate
a sensor quite evenly. 'Course, one can still run afoul of off-axis
light-blocking edge sensor structure (commonly, a lens front, rear, or
internal structure that shades outer parts of the image) at wide stops,
even with "normal" and long lenses (especially ones of "telephoto"
design).

But there must be more to vignetting than that, since the lens I have
with the greatest amount of vignetting is a 500mm f8 catadioptric lens
(fixed focal length and aperture, Sony version of a Minolta
design).

I'm not so sure that it is a Minolta design...;-)

Perhaps I should have said Minolta implementation :-)

It does owe one remarkable feature to Minolta's technical ingenuity --
it's the only autofocussing reflex lens on the market. That not only
buys you speed of focus, it buys you an accuracy of focus normally
unachievable on digital cameras in manual focus due to the lack of
precision focussing aids compared to the best pre-autofocus film
camera designs. That matters in lenses of such sharply critical focus
as these cats. And since it can't use the non-central AF sensors, it's
provided with a focus hold button in case you're using your shutter
half press for more than focus hold.

Not only is it long in focal length, but the rear lens is
positioned quite a distance away from the flange, well up in the body
on the lens. So the angle of approach of the rays to the sensor can't
be the culprit in this case. It's a well known vignetter.
--
Chris Malcolm

All mirror lenses have a "hot spot" in the middle, which can be
conversely thought of as vignetting...;-)

I didn't know that, but now you've mentioned it, it is much more of a
central brightness than an edge vignetting. That might also explain
why vignetting correction tools have a hard time with it :-)

BTW, adding a TC14 or
TC14B to the older (non-compact, but with barrel rather than rear
focus) Nikkor 500mm f8 results in better illumination evenness and
a remarkably sharp 700mm compact mirror (but it is slow, very
s l o w . . . . ). An image taken with this combination is at --
http://www.donferrario.com/ruether/500mm-Nikkor-plus-TC14.htm

Well, I suppose that illustrates even illumination, but at that size
of image the blurriest photo I've ever taken would look sharp!

--
Chris Malcolm
.

Loading