Re: [Leica] 24MM ASph Filter Question

["Henning J. Wulff" <henningw@archiphoto.com>]

At 3:14 PM -0400 9/20/99, InfinityDT@aol.com wrote:
>In a message dated 9/20/99 2:04:56 PM Eastern Daylight Time,
>chris@almqvist.net writes:
>
><< Sorry, this is not a reply, but rather a request for clarification:
>
> If I look at the camera lens thru the filter, and then tilt the camera away
> from me so that the inside  of the lens partly disappears, but not because
> it is covered by the filter but because it is covered by the outermost
> rings of the lens, then to my mind there is no vignetting caused by the
> filter. Right or wrong?
>
> And if there is a bit of vignetting caused by the filter when the lens is
> fully opended, will the vignetting disappear if I close the lens down a few
> stops? >>
>
>The way I check for physical vignetting is by pointing the lens toward an
>even light source (like a white wall) so all 4 corners of the finder are
>brightly-lit, stop the lens down to the smallest aperture and press the DOF
>preview.   The vignetting effects of a filter or lens hood always appear more
>pronounced in the finder at smaller stops than wide open.  With some
>lens-hood-filter combinations the vignetting as not even visible at all until
>the smallest aperture.  I also check at both minimum focusing distance and
>infinity.  Some lenses will show filter/hood vignetting at one but not the
>other.  In addition, wide zooms (not an issue with Leica) need to be checked
>throughout the focal length range.  With the M Leica you either need to shoot
>a test film, or place a ground glass at the film plane.  With the R Leicas I
>would advise the same, because their finders do not show 100%.  I have many
>lenses which I've checked on my Nikon F-series bodies (100% finders) and
>found them to vignette whereas they look perfectly fine on FM2n or n90s which
>have 93% and 92% finders (respectively) which are close to the R's finders.

Vignetting cannot be reliably checked through any viewfinder. The following
is a procedure which does work reliably:

<snip>

In any event, if you mount your lens of concern on your camera, open the
camera back and lock your shutter open with a cable release or whatever,
hold it up to a brightly lit surface such as a window, you can look through
the front of the lens and see the film aperture through it. Look for the
corners, and stop your lens down. If you have a 50/1.4 lens on your camera,
you probably have to stop the lens down 4 stops to f/5.6 or so before it is
the aperture blades that are cutting off the light coming from all
directions, rather than part of the lens barrel. A 105 macro lens with an
aperture of f/4 might need to be stopped down only one stop, or also to
f/5.6 before the same condition applies. Only at this aperture or smaller
does physical vignetting cease, and the only vignetting that still applies
is optical vignetting due to the cos^4 law and other design issues.

<snip>

You can also check for filter or hood vignetting this way. If the filter
cuts into the light path while you can still see the corners of the frame,
it vignettes at that aperture and that focussing distance (and that focal
length, if a zoom)

   *            Henning J. Wulff
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