I have been able to use liveview autofocus with a 300 f/4 with a 1.4 & a 2 stacked on my 5Dm2. It isn't very fast, but once you pre-focus, you can take pictures like normal. You can also zoom in with the liveview LCD on the back to see if things are in focus, but I think Canon needs to have a 25x and a 100x mode to make sure things are in perfect focus. You just need a really stable tripod and not touch the camera (or it is too blurry from camera shake), which is a problem when it comes to changing the focus.
i'm just wondering if there will be any image quality improvments over the second generation using a 300mm prime? Or I'm also starting to get into telescope and astrophotography, and I'm wondering if using these with a telescope will help zoom in optically.
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