Well, I slept on swapping 80-400VR with 70-200VR and using TC's to give some of the reach back and..... I haven't swapped but added a 70-200 f2.8 to the collection.
I have performed another test to ascertain whether 70-200VR with 1.7TC (1.5 stop light loss) is better or worse than 80-400 zoomed at 340mm.
The test...
I used a cushion from my sofa and propped it up at chest height on my dutch dresser in the dining room. Tripod was used with the camera set to f6.3 1/400, this setting is typical in aviation my photography. Using TC's the camera reports the effective aperture not the master lens aperture so as I used manual exposure mode, TC test was set to f11 which is 1.66 stops from f6.3. Both lenses were set to the same aperture, VR used at all times which was needed as my tripod is inadequate for a 400mm heavy lens so some shake did occur stabilized by VR system in both lenses.
Results
500 x 500 pixels cropped from the centre of the image around focus point. No resizing, sharpening or processing of anykind has been added.
Pic 1 340mm using 70-200VR f6.3 (effective f11)
Pic 2 340mm using 80-400VR f6.3
Pic 3 200mm f6.3 70-200VR so TC performance can be realised.
Conclusion
Using a master lens of outstanding sharpness TC's can be used with acceptable loss of quality. Compared with 80-400, 70-200 and 1.7TC at the same aperture shows much improvement.
I am now sleeping on whether I should sell 80-400VR but I am worried about light loss premium of TC's
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