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| p.1 #12 · Which of the following 3 would be sharper / focus better around 400mm? |
A lot of folks go though the process of comparing the pluses and minuses of those three options. On one hand, your question about which will be sharper at 400mm could be the determining factor, though I also think that some practical/functional questions might be at least as important.
In truth, you can get a good 400mm photograph from any of those options - not one among them fails to be "sharp." (Some would point out that if your only criterion is shooting at 400mm a 400mm prime would likely be worth consideration.) With that in mind, I'd consider framing the question a bit differently.
If you intend to shoot a whole lot at 300mm (though you did not mention this) and only occasionally have to extend to get a bit longer at 420mm, then the 300mm prime plus 1.4x TC could make sense.
If your primary need is for a large aperture zoom in the 70-200mm range and you will only occasionally need to shoot at the longer focal length, then the 70-200 plus 2x TC could make sense.
If you want flexibility to quickly cover any focal length in the 100-400 range without changing lenses, the 100-400mm zoom could be your best choice.
If you only or primarily are concerned with shooting at 400mm, then a 400mm prime could be a better choices.
To play this out a bit... if your main goal is being able to have a 400mm lens, I would not recommend any solution that requires you to use a TC. The main issue is that adding/subtracting a TC from your lens is an extra bother that makes sense for extending the primary functionality of your lens to allow a secondary functionality, but (usually) not the idea solution for solving your primary need. Instead, I would consider the 400mm prime or, more likely, the 100-400mm zoom. With the latter, one lens will cover all of your needs - and I note that you mentioned f/5.6 as your primary aperture - with no need to switch lenses or add/subtract a TC.