In article <4hvfi9$r7o@nuntius.u-net.net>, stephensaj@cheshire-cc.btx400.co.uk says...

>I recently bought an EOS50e (Elan IIe) with a Sigma 24-70 lens, and >have so far run a few films through it (Fujichrome and Velvia). A lot >of the frames are too dark, generally those with sky on them.[......]

Several things come to mind. I normally rate Velvia at 40, rather than
50. Fuji Sensia and Provia 100 (if that is what you use) render a blue sky too dark for my taste (gone is that wonderful luminousity of the
Fuji 100 and Fuji 50 sky renderings). Many cameras have a slight error in their film speed settings, and even 1/4 stop error is obvious in
a set of slides as a general tendency for the slides to be too dark or too light. If you use ASA 40 for Velvia, and rated speed for most
other films, and a "+ 1/3" compensation does not bring good results,
I would return the camera, return it for exchange, or send it in for
warranty repair.
Hope This Helps