Tortie wrote in post #19529784
Oh my! 14 to 19 feet is quite the distance! With smaller birds or BIF, I'll be curious to see if this is bothersome (I'm usually pretty far too begin with).
You might be able to use macro extension tubes between the lens and camera to make the lens focus closer. I've done that occasionally (I always have macro ext tubes with me, they're so handy for so many things).
For this bush-tit image I needed to make my 500mm focus a little closer and used a 36mm extension tube...
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© amfoto1 [SHARE LINK] THIS IS A LOW QUALITY PREVIEW. Please log in to see the good quality stuff. Sometimes you just have to make do with what you've got on hand. Here I had to use both a 1.4X teleconverter on a 300mm lens and an extension tube to make the combo focus closer for the image below...
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© amfoto1 [SHARE LINK] THIS IS A LOW QUALITY PREVIEW. Please log in to see the good quality stuff. In this case I added an extension to a 70-200mm lens to get higher magnification...
Canon doesn't yet make any RF-mount macro extension tubes (and when they do, they will probably be expensive). Fortunately, some third party manufacturers do. Kenko and Vello each offer sets of two tubes: Kenko's set includes 10mm and 16mm. Vello's set includes 12mm and 20mm. Better yet, Fotodiox offers two sizes that are sold separately: 15mm and 35mm.
As long as these telephotos are, you would need a lot of extension to make significant difference in how close the lens will be able to focus. I don't know how to calculate it, but would probably try the 35mm tube on either the 600mm or the 800mm. It will have more effect on the 600mm.
All the extension tubes mentioned have the electronic connectivity to support autofocus, aperture control and image stabilization.
While the extension tube is installed, you won't be able to focus to infinity for more distant subjects.