AreBee: Side by side shots are ideal but rare. Few people have both lenses at the same time and few of those people will be so meticulous.
If you look at 100s of images from each lens, you can begin to notice the characteristics and reduce the influence of photographer-specific variables (post processing and lighting choices) and the weaknesses tend to reveal themselves. I need to go through large numbers of images before I feel confident, but a variety with backlit background items or brightly lit background items in the OOF area are the most useful.
The funny thing I have found is that as I go through enough images, I will notice that a few seem out of character and each time this has happened I have gone back to check the image and discovered that it was misfiled and taken with a different lens.
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When I say larger numbers of images are needed, I mean for better lenses. Poorer lenses tend to fail more quickly and noticeably, even under less challenging lighting.
Also, I should add that the 85mm f1.4 Planar lens acts differently wide open in two specific ways that could be a significant negative for many. It is a special lens, but it isn't for everyone.
1. There is focus shift from f1.4 when you stop down. This is not a problem if you are focusing at your taking aperture with live view or if you focus at f2.8, but it is something a prospective buyer should understand.
2. The lens is softer wide open. For portrait clients, especially women, this is often a big positive, but for those who live by sharpness, you would be better off with the Zeiss 100mm f2 or the Otus 85mm f1.4.
To be clear, the 85mm f1.4 Planar is NOT the best choice for everyone and those who are skeptical of the values I am expressing will not be as happy with this lens.
The reason I posted was to point out what I noticed. The Otus is superior in almost all ways in a highly competitive space (there are several great, fast 85mm and 90mm lenses from Canon, Nikon, Leica, Zeiss and Sigma). But it doesn't do as well in these two characteristics. Granted, these will not matter, at least not on a conscious level, to most people.