Here are some photos which I took with my Canon 200 mm F/2.8 Mk I and 2 tele-converters (1.4 x TC and 2 x TC) recently. This set up allows me to use focal length 560 mm. You have to use also filter for Sun photos because with this magnification, you could burn your eyes or camera sensor (when shooting via live view) very easily. I bought a filter (Baader Astro-solar filter) in telescope store.
Here you can see Canon EF 200 mm F/2.8 Mk I with Canon 1.4x TC & Canon 2x TC.
Here are the results and below you can see partial Solar Eclipse from 20th March 2015.
Click on photos to see bigger version.
It is really nice that you can see also solar sunspots, I took this shot today (16th April 2015). Sun has really huge sunspots. These have their numbers and on the photo you can see Sunspots number 2321, 2322, 2324 and 2325. If you are interested in more details, you can visit NASA web page:
And another shot, this time Moon with the same set up and 560mm focal length.
See more photos on my FB page http://fb.com/jarolimzacek