It's my frame of reference, I guess. I once lugged a Nikon D810 with a 200-500 attached. So yeah, this lens seems pretty small and light. Compare it to any FF 300mm f2.8 lens and see.
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The 40-150mm F/2.8 is not comparable to a FF 300mm F/2.8 regarding both DOF control and critically total light gathering. A FF 300mm F/5.6 would gather the same total light, have the same DOF control and assuming you had to bump ISO the same noise. The Sony FE 75-300mm is equivalent to a m43 37.5-150mm F/2.2-2.8 unfortunately from my perspective slow lenses on Ff tend to be at a lower level than their faster cousins
Or you could go the full hog and get a 25-600mm F/2.8 { across the range } zoom with a free camera chucked in , it has a much smaller sensor but hey that doesn't matter , does it
Here is the Sony FE 75-300mm, Oly 40-150mm and Pany FZ330
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Jim Stirling