In article <5aivf7$b9c@strange.dfwmm.net>, triston@dfwmm.net says...

>I have a Canon AE-1 Program camera and I once tried to calibrate the
>meter using the sunny f/16 rule. With a Canon 50mm f/1.8 FD lens, I
>pointed the camera toward the northern sky (about 45 degrees above the
>horizon) in the middle of a sunny day. I change the ISO setting on the
>camera's film speed dial until the meter reading agrees with the sunny
>f/16 rule -- in my case it was f/16 and 1/125sec for ASA 100 film.
>But when I mounted a Sigma 35-135mm f/3.5-4.5 zoom on the same camera
>and pointed it to the same area, the meter indicated f/11 for 1/125sec
>in other words one stop open. I tried 50mm, 100mm and 135mm focal
>lengths with the zoom and the reading was the same (f/11).
>Could anyone explain to me why there is such discrepency?

(Um, I think I would ignore JC17FL's response - I do not think it is
correct. The transmission differences in lenses that are honestly
rated is there, but minimal... And the maximum aperture does not
affect the meter reading...;-)
First, why did you choose the Northern sky as a reference?
The rule-of-thumb, as I heard it, is to place the (clear!) sun
nearly at your back, and meter an average scene (buildings, folliage,
sky etc.) all within the meter view. BTW, in my Northeastern US
location, the rule of thumb becomes the "Sunny f11 1/2 Rule"...;-) For
slide shooting, a 1/3rd or 1/2 stop reference error is quite important.
An error of even 1/4 stop applied to all slide exposures will cause
slides to generally appear slightly too dark or too light.
As to your exposure descrepency between your two lenses, it may be
due to lens illumination roll-off near the frame edges at the short
end of the zoom's range being metered by a full-frame camera meter.
The lens loses about 2/3rd stop in (center of the frame) speed as it
is zoomed to the long end, and it probably still has noticeable
illumination roll-off (though less) even at the long end of its range.
The 50mm Canon lens probably has much more even illumination, so it
appears "faster" at a given f-stop when used to meter with your camera.
BTW, many variable-aperture zooms, due to illumination problems, are
effectively nearly constant-aperture in use...
Hope This Helps