>>4124471firstly, this guy
>>4124494is chatting baloney.
"flat" rendering is when you have a flat focal plane, with no astigmatism or coma on the bokeh.
What does this mean in real life?
Most lenses don't have a flat focal plane, which means if you take a photo of a flat wall, the edges will not only be softer due to being on the edge of the optics, but also slightly out of focus, a flat focal plane will have the entire frame perfectly in focus.
And for the bokeh, look at something like the helios 44 with it's "swirly bokeh", this is the astigmatism and coma that causes oof highlights to not appear totally round. When this is well corrected, no matter what part of the frame bokeh occurs in, it will be even and consistent. And this isn't just applicable to colour soup bokeh, the effect is noticeable even with a little bit of DOF falloff, even if not as obvious as something that's not completely blown out with fairy lights in the background.
Why do some people claim to not like this "flat" rendering? because it's new, and expensive. Older lenses were filled with these kinds of faults but since we can use computers to accurately model optics now, instead of the old way of endless trial and error, we can ensure certain qualities within a lens.
However there's also still compromises to be made, typically one of the most important is prioritising sharpness across the entire frame or having less vignette. In the film days, film wasn't very high resolution and fixing vignette was arduous to say the least, so they opted for designs with a high centre sharpness and low vignette. Now though, vignette is fixed in one click, and we have the resolution to see image quality deteriation towards the edge of the lens, so our design choices for lenses has typically done a complete 180.
People are scared of new and change.
