Well today I had a spare 5 minutes and 3 lenses, my tripod and a handy brick wall.
1. Sigma 70-300 DL Super Macro - starter lens (which I've sold and was packing it up ready for posting)
2. Tamron 28-300 XR Di LD f3.5-6.3 Macro aspherical hyperzoom (very convenient and light, if not optically perfect)
3. Sigma 100-300 f4 EX
All these photos were taken at 100mm and 300mm (the only matching focal lengths on all 3 lenses), on a 350D in Aperture priority mode at f8. Camera was mounted on a tripod with remote shutter release. ISO 100
http://omicron.uk.com …&g2_itemId=6009&g2_page=1
(both small and the full size jpg image is available)
Hardly the most conclusive test I admit.
Conclusions.
1- The 100-300 is a bit of a beast and I really should have mounted the tripod on the lens rather than the camera. Its a stonking lens though.
2- My tripod is weak and spindly.
3- The 70-300 goes soft towards 300mm, I've still enjoyed having this lens as it was my first purchase after the 350D kit and allowed my 18 months of long range photography. I hope the new owner feels the same. Great value for money
4- The 28-300 is another good value lens, better than the 70-300 but both poor against the 100-300. f6.3 max aperture at full zoom is also a compromise. However, it makes up by being light and easy to carry.
I did the same shots at the lenses maximum and minimum apertures as well, and the 70-300 and 28-300 at 70mm as well.

