I keep seeing posts that recommended the TC-14 instead of the TC-17. The image magnification of the Nikon TC-14 almost does not seem worth the purchase. Why would anyone prefer the TC-14 over the TC-17? Does the TC-17 really degrade the IQ that much? I have a friend that bought the TC-17 for use with his D300 and he loves it. He says he cannot see IQ degradation with this teleconverter.