Re: Optics question (not alt)

Is it the circle of illumination or the circle of definition (or a
different circle) that increases when you stop the lens down to the
prime aperture?

Looking at some lenses I have which barely cover certain formats, I can
see mechanical vignetting at the widest apertures occuring as I peer
through the back of the camera through the cut groundglass corners
towards the lens. The aperture has a truncated elliptical shape (much
like the profile of an American football if appear cut in half
vertically) at this point. I assume this causes a significant decrease
in illumination at the corners of the image, but since I can still see
the central portion of the lens, the circle of illumination must still
be there from that central portion.

Relatively, the corners are receiving much less light since the central
portion of the image is accumulating exposure from all portions of the
lens while the corners only get it from the center of the lens. As the
aperture is reduced, this imbalance disappears until the film is getting
exposure only from the central part of the lens at a smaller aperture
(like f/22 as quoted for comparison of many lenses by the manufacturer).
 At this point the entire aperture is now visible from the back of the
extended camera without vignetting, and the aperture is once again an
approximate circle.

As the camera bellows extension is increased further away from the
infinity position, the film receives a greater proportion of light from
the center of the lens while the formerly vignetted areas now lay
outside the diagonal of the film. Isn't that what is really going on
with the circle of illumination, infinity focus, and so on ?

So really, isn't only part of the circle of illumination getting
brighter and the entire field getting more even illumination as the lens
is stopped down rather than the outer margins of the circular field
actually increasing?

Joe

>>> tomf2468@pipeline.com 03/10/05 10:17 AM >>>
I'm wondering if Dan has confused the size of the image versus the size
of the circle of illumination (as a lens is stopped down).

For all practical purposes, the size of a focused image doesn't change
as you alter f/stops. The size/appearance of significantly "out of
focus" areas can change slightly. That is why it is best to changes
shutter speed or lights if you intend to stack files for density range
expansion. But, the in focus stuff stays the same, F/2 or F/32.

What does change "big time" on many lenses is the circle of
illumination. That is important to view camera users. As most lenses
are stopped down, they throw a larger circle of light. Nothing in the
circle get larger. You just get more area around the edges added to the
circle. In a fixed system (DSLR, SLR) this is just wasted light. It
hits the black areas inside the mirror box and does nothing. In a view
camera it allows more movements at F/64 than you can get at F/5.6.

--------------
Tom Ferguson
http://www.ferguson-photo-design.com
Received on Thu Mar 10 14:37:04 2005