EF first tests

 I'm satisfied by my initial tests with the Viltrox EF-L adapter and Canon 100-300/4.5-5.6 USM lens.

Tests show nice color contrast and sharpness, and - so it's a typical sample that verifies what reviewers have said. It focuses at a decent pace but definitely not to L-native standards - but that's OK for now, and better than my no-AF adaptations of Minolta AF, Sigma SA and Pentax K lenses. Manual override is straightforward, and the S5 internal stabilization provides four stops with acceptable results (1/10s at 200mm is consistently decent, 1/8 or less would be a crapshoot). Focus is sufficiently silent if somewhat pokey, and AFc is not available at the USM grade - I'd have to check online to see if newer, higher-grade lenses can manage it. Weight isn't an issue for me, which is quite nice and sadly rare for L-mount telephoto lenses at any price. The new Sigma 20-200 and Lumix 28-200 lenses are lighter, but both stop at 200mm and are slower than this lens!

Viltrox EF-L adapter summary: 

• Viltrox EF-L (first edition, not Pro)
• adapter weight is 134 grams
• AFc is not available with USM sample lens
• camera screen shows "use AF-S" when set to AFc
• O-A switch - needs to be A with 100-300!

Canon EF 100-300 f/4.5-5.6 USM summary:

• Tests show nice color, contrast and sharpness (noted in reviews)
•  some chromatism away from center at longer focal lengths (also noted)
• manual override is straightforward - just rotate ring at base of lens
• 640 grams with adapter; weight is very nice
• no IS within, but S5 internal stabilization provides four stops
• f/4 only to about 120mm but f/5 to 180mm!