The size difference in this lens is insane. Here it’s pictured next to the 600GM, 200-600 and 100-400. The bare lens image quality blows the 600GM away and that’s a high bar to cross.
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Great comparison
I find In very good light usually a selection of lenses like your testing, the difference would be surprisingly smaller than when you’re in challenging conditions,
ie: outdoors low light low contrast busy back grounds or your wanting to dissolve the environment around the subject so it pops.
The ability to use the F2.8 as a tack sharp F stop in lower light is always handy, F2.8 with the diameter size of the lens along with the short oenght has a incredible ability to gather light, revealing amazing micro contrast and detail often not seen or revealed fully at F5.6 F6.3 in other lenses.
F2.8 brings out just a beautiful rendering, even at F4 using or not the TC.
I find my 28-300 performs so much better in Tasmania than in Queensland, the difference being quality of natural light at F2.8 you gater more of the available light that translates into more micro detail and micro contrast and exceptional rendering at any distance.
The different lenses are different tools for different purposes, each has a unique purpose, price versatility performance.
A bit like a car, they all go from point A to B but take them of road ! or hit a storm !
Night time soccer matches i find my 200-500 or 100-400 while it works the ISO levels are significantly higher as is the focus speed slower or stickiness than on my 300 F2.8 VR II at F2.8, same shooting or documenting Koala bears in tree tops during the day.
For myself In general the 300 F2.8 does very similar to what the 600 F4 does at F4, however outside in challenging light usinng te same settings I usally monitor iso levels to evaluate the difference. One could look at the MTF charts.
I think Sony has made a super impressive selection of tools covering all the bases to meet the sports action,
event, and even wild life subjects, you couldn't really want for more.
A 300 F2.8 with a built in TC being and being so small and light yet powerfully versatile would be amazing, Nikon has done it with the 400 which is simply my number one choice in the Nikon lenses, In Sony i would grab a 300 2.8 with a Tc all day preferably built in if ever available, add the 50 or 60 mp providing legs and reach to burn.
At the end of the day it’s all about Light, light that comes from the sun, a flash, or harnising any available light by the lens.
I would be interested in your findings in the natural field in challenging conditions.
What about going into the garage open th door and shoot out side if teh conditions are rough just now.LOL?
Only an opinion