I do wildlife video with 5D Mark IV and mostly Canon 70-300mmL using Kenko 1.4-2.0 extenders that work very well in auto (F11) or manual f13-16. Coming from the Betacan-DVcam era where I could use the old dependable FD series L lenses with extenders for close ups, I really miss the long lenses for close ups, modern ones are impossible to pay with with my resources, non talking about weight. After a lucky search on the web I found and 40+ year old Celestron C90 F:11 NEW,no scratches. optics as coming out of the factory, no fungus, no, no no, so I bought it, put a T to EOS adapter and gess what....it works great, focusing manually with the ca;mera aid gives me the close ups I was missing for a long time, put a 5% sharpness and that's it...
I am interested in knowing what is the real focus lengh on this lens, I made a trial with photo and video to try to establish a comparison with the 300mm+ 2.0 extender. There is a relation between focus distance and size of frame but I am a bit rusty at my 72... I need to crop a 486 square cm image (300+2,0) up to 198 sq cm to get the same image size I get with the 1000 F11, a factor of 2.88 in terms of area. Can I deduct with this the real focal lengh of the celestron? The 1.75 crop factor works the same on both lenses, of course.