San Francisco is a nice-looking font. It has the same invisible feel that Helvetica has with touches of DIN to aid in legibility and add a bit of style. I’m really excited to see Apple embracing optical size but I can’t help but think they could have pushed this even further (time permitting).
I’m writing this right now on a late 2012 iMac — non-Retina. SF UI is sharper than Helvetica Neue, but on non-Retina displays it’s not that much better in terms of legibility. SF UI still has blurry apertures and counters when set at most of the commonly used sizes. The grey tones are still spotty across long passages of text. It’s an over simplification, but it still looks like Helvetica with most of it’s shortcomings on 1x displays.
On Retina displays it’s a whole different story. SF UI feels more airy than Helvetica. The apertures render with clear definition and feel comfortably open. The letter are easily distinguished in a long string. It performs pretty well, even without my glasses on, especially in the text sizes. The spacing is a little tight in some areas, but not by much. I love SF UI on retina Macs and iOS devices—I deal with it on my iMac.
Some of the methodology here is a little suspect; for example, though I agree that Helvetica Neue is not a particularly good body typeface, I disagree with the author’s use of an upscaled rasterized string of characters to speak to its legibility. I also disagree that San Francisco isn’t much more readable on non-Retina displays — I find it far more legible, with much clearer structure in smaller type.
But, as an analysis, this is fantastic. It’s a really deep dive into the nuances of Apple’s new one true typeface. They’re using it in hardware and software, and even in product packaging. With that in mind, it’s no wonder there’s so much attention paid to the vast differentiation in sizes, weights, and cuts. I anticipate subtle changes will be made over time, too.