Canon extenders

Hi,

I use a modified 600D for Astrophotraphy normally attached to my telescope but just picked up a used 300mm F4 IS USM and would like to get an extender for it, will this lens be ok with the 1.4 extender and my 600D ?

Will I notice any difference between the mk ii and mk iii versions ?

Thanks.

A 1.4 extender magnifies equivalent to 1.4 times the native focal length--which in the case of your 600D is already cropped by a factor of 1.6 from its full frame equivalent. So in terms of FF focal length equivalent field of view, you now have 300 X 1.6 or 480. The 1.4 extender will make the 300 become 420, so 420 X the 1.6 crop factor and you have a FF equivalent field of view of 672 mm. The 1.4 extender does cost you 1 stop of aperture, so you F4 becomes F5.6, but that is still within the limits for autofocus for your camera body. F5.6 is the slowest maximum aperture that the camera will try to autofocus. So 1.4 is OK. If you find a deal on a mkii take it. If prices are comparable, get the newer version.