billnikon wrote:
No extender improves image quality. Since you have a Tamron, I would go with the Tamron (but personally I would not add an extender to any zoom lens. BUT, your focusing speed will decrease and your image quality at 400 mm will suffer.
An extender will not improve, and in fact reduces image quality. But everything is a trade-off. In this case - for me - does the cost, and weight + bulk advantage in the camera bag, of a TC vs. another lens give me an advantage?
I tested this quite easily - could I reduce weight and bulk if I used a 100-400+TC rather than carry a 150-600? Also for me, my default lens is a 24-105 so I would have a continuous focal range of 24-600+ with 2 lenses. In fact I sold a good 150-600 Tamron after buying the x1.4 TC for the 100-400 as IQ of Canon +TC was better than the excellent Tamron (and anyone could argue 'so it should be given the total cost'.)
As to IQ, that was also easy to test providing you can get access to the kit.
- I checked side by side image quality centre and corners at 100% of the 100-400 at 400. This is baseline and I expected to be best.
- I then repeated the test with the lens +TC combo at approx 400 (so I could compare like for like with the bare lens and lens+TC.)
- then checked at max lens + TC (640mm)
- also checked a crop of the bare lens with lens+TC enlarged to similar size
- for curiosity as I had the lens I compared with the Tamron 150-600 (v1)
Results were pretty much as you would expect - bare lens was best,lens+TC results at similar focal length were not as good, and at max were also less sharp but much better than I expected and perfectly good for display or printing at 100% or less. The all important crop enlargement was not as good as the lens+TC (otherwise whats the point of a TC if an enlarged crop of the same area is as good or better!)
Initially, the main surprise was that the Canon 100-400+TC was better than the Tamron at 600 - again on reflection this is not surprising given the huge cost differential and that the Canon was a new v2 lens and the Tamron an older v1.
I would also mention with my camera/lens combinations there was no noticeable impact on focusing because even with TC the maximum aperture was withing the autofocus range (a x2 would not have been and would have hunted or probably failed to focus.)
I would emphasise my testing though careful, was empirical and subjective, but I would always advise before anyone spends shed loads, to test the actual equipment they intend to purchase!