> If that were true, then TTL metering in the general case wouldn't
> work consistently for different lenses in any exposure mode at
> different lens openings.
>
When lenses are wide open, it's less of a point source though,
right?
> Consider the SLR focusing screen as a rear projection screen, with
> the illumination source being the lens. Its built-in fresnel lens
> collimates what would otherwise be relatively simple hemispherical
> scatter, to first order approximation, and directs it towards the
> ocular.
>
> The metering sensors are designed to read light intensity from that
> scattering rear projection surface, they don't intercept the light
> directly from the lens. The scatter induced by the screen's matte
> focusing surface ameliorates the variations from that would otherwise
> be seen with a clear screen.
>
> (When special purpose near-clear and clear screens were available in
> the past for cameras like the Nikon F series, charts regarding TTL
> metering errors, both for total ambient curve and for focal length
> adjustments, were included. Most of the time, they recommended not
> using the TTL metering due to variabilities like this..)
>
> Godfrey
>
In any even, I'm talking out my ass... I don't know much about
optical lens design. I could also be confusing the metering inaccuracy
I've seen with my *ist-DS with the 3rd-party, split-prism focus screen. I
don't recall if I had issues with metering inaccuracy with K/M lenses and
the stock screen... been too long since I used it.
-Cory
--
*************************************************************************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
*************************************************************************
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