At 6:27 AM +0000 12/30/01, olympus-digest wrote:
>Date: Sun, 30 Dec 2001 08:28:23 +0800
>From: "C.H.Ling" <chling@xxxxxxxxxxxx>
>Subject: Re: [OM] XA cosine^4 falloff and vignetting
>
>All XA has f2.8 lens, only the XA-2 to XA-4 has f3.5 lenses.
OK. I do recall a salesman giving me this choice in the 1980s, but it's been
fifteen or twenty years, and I may be mixing things up. The salesman was
probably also telling me about the XA-3 et al.
>You mention you don't see vignetting in film, do you mean negative or slide? I
>wonder anyone can see vignetting in negative, the contrast ratio is simply too
>low in negative.
No film was involved, of any kind. I used the standard viewcamera trick of
looking through the lens with the camera back open, to see if anything got in
the way. The eye tries to see what the film would have seen.
With XA, the trick is to keep the shutter open long enough. For this. I set
the film speed to ASA 25, worked in dim light, and looked multiple times.
Now as I think about it, a bit of black tape (or even a finger) over the
photosensor would work as well. This gives you four seconds of look time. The
photosensor is the annular sector above the lens and below the viewfinder
window on the front of the camera.
Hmm. I may well see some vignetting, using the finger over the photosensor
approach. If this is so, the corners will be darker than cosine^4 would
predict. Cosine^4 will apply until vignetting begins.
If present, vignetting will affect slide and print film equally, and should be
visible in negatives, albeit easier to see in slides or prints.
Joe Gwinn
>- ----- Original Message -----
>From: "Joe Gwinn" <joegwinn@xxxxxxxxxxxx>
>center of the lens (the rear nodal point, to be precise) to the film plane
>(in "image space"), with respect to the optical axis.
> >
> > Note that theta is not necessarily the angle from lens center to the
>object out in the world (in "object space"). A fisheye is the best example:
>The angle to an object in the world far exceeds the angle to the image of
>that object.
> >
> > The XA came with two possible lenses, both of 35mm focal length, the
>cheaper being f/3.5, the more expensive being f/2.8. Looking through the
>f/2.8 lens from the film side, I don't see any vignetting, which is
>plausible given how short the lens in physically. I don't have a f/3.5 to
>look at, but I would expect the same lack of vignetting, for the same
>reason.
> >
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