> 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 * ************************************************************************* -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net