Hi,
What I have seen from my very simple test all 35 mm lenses has not so good vignetting at f/2.8 on full frame camera. At least if I compare what good result I get from my medium format Pentax 67 165 mm f/2.8 lens.
When looking at the data of the Canon 200 mm f/2.8L lens are about the same as my Sigma APO 150 mm f/2.8 lens. And a bit too expesensive to buy just to see if there are any differences, the focal length is almost the same as the one I already have.
The last weeks I got my Star Adventurer to be much better polar aligned with help of an angled viewer connected to the polar scope. Maybe I shall have a complement with a 300 mm lens instead?
The viewfinder test:
http://www.astrofriend.eu/astronomy/...iewfinder.html
I have many times looked at the Canon 300 mm f/4 L lens. But not sure if it's perfect for astro photographing, anyone with experience from that lens? I must work at full opening, f/4. I don't like the rays I get when stopped down the aperture, maybe I can stop down with an external hole in front of the lens, but I need all light I can get with high quality.
With my new experience with remote controlling of the Canon camera from smart phone I must say: For lenses I use when travelling with astro equipment I must have a built in focus motor. A telescope will be too complicated with an external focuser and heavy.
http://www.astrofriend.eu/astronomy/...e-control.html
/Lars