Originally posted by y0chang Part of the reason is that modern optical designs have aspherical lens elements and other things that while improving the numbers, create sterile image rendering.
Very true, as far as I can see. The Zeiss OTUS 55/1.4 is a great example here. You can get older lenses that produce more beautiful images, but this $4000 wonder does an amazing job of balancing good measurements with beautiful images. The competing Sigma 50/1.4 Art is more biased towards the measurements (even though the OTUS beats it in measurements too). In contrast, a third new lens - the badly maligned Nikon 58/1.4G - is biased toward beautiful images. While it may be disappointing in some respects, it appears incapable of producing the busy backgrounds that can come out of the Sigma.
People decry the high price of the OTUS, apparently not realizing how impossible the goal is - a goal it has nearly reached! At the same time many praise the suddenly-affordable-looking Sigma. And the Nikon is often regarded as an absolute waste of money. I think this shows how much people's thinking has changed in the digital age.
BTW, some Pentaxians are really hoping the Sigma will be released soon in K-mount. Sometimes we forget that the Pentax DA*55/1.4 was already (but many estimations) the best 50ish lens around - until these new lenses came along. Like the Sigma, it can produce (a different kind of) busy background. But the overall images look nicer, I think.
For those who want the appearance of a "modern" 50, the DA*55 is still the best you can do for less than $4K.