Originally posted by Steve Belcoski Is the IQ for the 1.7x as good as the 1.4x?
It's tough to say. Purely objectively, no, as there is more image degradation with the 1.7x vs the 1.4x. However, the 1.7x gives you more magnification. Using the 1.7x TC may require a higher ISO because of the longer focal length requiring a faster shutter speed
and the aperture being darker by another 1/2 stop versus the 1.4x TC. And higher ISO will lead to more detail-destroying noise.
Also, some TC's simply work better with certain lenses and not as well with others.
Last year, I shot an osprey @ 40 yards with 2 combinations - the
Pentax A 400/5.6 +
A 1.4x-L TC (560mm total) and the
Tamron 360B 300/2.8 +
1.7x AFA (510mm total) - and the results were basically a wash. The Tamron 300/2.8 is noticeably sharper than the Pentax 400/5.6, but the Pentax combo offers more magnification.
I guess all my blather is really to say "it depends"
Like @normhead, I use my 1.7x AFA on f/2.8 glass, because I don't like the performance on slower lenses.