400mm 2.8 vs. 600mm f4

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GregWCIL
GregWCIL Senior Member • Posts: 1,001
400mm 2.8 vs. 600mm f4

Just wondering if anyone has compared the 400 f2.8 to the 600mm f4 using teleconverters. On the surface, it would be attractive to have the extra stop of light the 400 gives for early and late lower light (sans converters)  If you need extra reach, I wonder how the 400 with a 1.4 (would make it f4) compares auto focus speed and sharpness-wise to the 600. Then the tougher test would be the 400 with 2x vs. the 600 with 1.4.

Like I mentioned, if sharpness and focus speed were similar, the 400 would offer better low-light performance at its native 400mm. I currently have a 70-200 2.8 and I really appreciate the speed (both in light and focus speed) it offers compared to my 20-500 f 5.6.  But it seldom has the reach I need for wildlife - especially birds.

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