well i have been to the zoo today and it seems to be an issue with varying apertures, i cannot put my finger on what is causing it tho... some over expose other under (under i can understand with a hugely bright background behind the subject!)
as an example i have attached 2 photos, both taken with the sigma 18-50 3.5-5.6 that i was just carrying around it seems to be this lens that suffers a fair bit..
anyway.... pictures...
Model: PENTAX K20D
ISO: 100
Exposure: 1/60 sec
Aperture: 5.6
Focal Length: 50mm
Model: PENTAX K20D
ISO: 200
Exposure: 1/20 sec
Aperture: 8.0
Focal Length: 50mm
im sure its operator error as its such a learning curve from my old camera which was vey much point and shoot... but the prism is most definitely blacking out with both of my sigma's on not badly... just maybe enough to cause over exposure..
Thanks
Steve