The Yashica ML 3.5/21mm was Yashicas second superwide for the Contax/Yashica bayonet mount. The earlier ML 3.3/20mm (which is closely related to the Yashinon DS 3.3/20mm) is a pretty obscure lens, however. The ML 3.5/21mm, like the Nikkor 4/18mm, is a positive leading superwide (front lens with a positive element). As such construction it is somehow unusual, since most superwides are "negative leading" (i. e. with a negative lens in front). Positive leading superwides allow for easier correction of the distortion, but otherwise are more difficult to calculate. While the part in front of the aperture looks very similar to the Konica Hexanon 4/21mm (a lens more than ten years older), the rear part has similarities to the Minolta MC 2.8/21mm (1968). SInce the Hexanon 4/21mm clearly is based on the Carl Zeiss Jena Flektogon 4/20mm, also the Yashica ML is - to some extent - related to the much earlier Flektogon (1961 / 1964). 

YASHICA ML 21mm 1:3.5 (12 lenses / 8 elements)   (1976)

The Yashica ML 3.5/21mm is quite a bit smaller and lighter than the (much older and faster) Minolta MC 2.8/21mm (335g vs. 520g). It has about the same size / weight as the Canon nFD 2.8/20mm (1981), the Nikkor AiS 2.8/20mm (1984), and the Pentax-A 2.8/20mm, but it's bigger and heavier than the Nikkor Ai 3.5/20mm (230g, 1977), the Minolta MD 2.8/20mm (240g, 1977), the Konica AR 2.8/21mm (215g) and the Olympus 3.5/21mm (180g).

The lens barrel is completely made from metal, and very well machined indeed. It's  Focusing is smooth, and it goes in a 120° turn from infinity to 0.3m / 1ft. The aperture ring has full stops only, but it's much easier to turn than e. g. the Konica AR aperture rings. Personally I very much like the general haptics of the better Yashica primes - apart from the aperture and focusing rings (which are metal-only instead of being covered with a nice rubber), these lenses look and feel very much like the Zeiss CY lenses. There's one caveat though - surprisingly many of my Yashica ML lenses have grown some fungus over the years, and often they are difficult (=nearly impossible) to clean, due to their construction.

Unlike most contemporary superwides, the Yashica ML does not implement floating elements. As a result, focusing feels easier than with many other superwides, but its close range correction probably isn't as good. Infinity correction however is pretty good for a superwide of the late 1970s. Using 24 MP FF cameras, the image center as well as the borders (15-18mm from image center) are good even at f3.5. They don't improve much when stopping down. The extreme corners somehow lack resolution, and they gradually improve to good values (but not to "excellent") by stopping down to f11. This behaviour is quite different from e. g. the AF Nikkor AiS 2.8/20mm which has a really low corner contrast ("mushy" comes to my mind) at f2.8, equals the Yashica around f5.6, and becomes visibly better than the Yashica at f11.

The lateral CAs are clearly visible towards the corners, but they are benevolent and can be corrected easily by post-processing.

Distortion is quite visible, mainly towards the corners though (a mix of slight moustache and visible pincushion). That's pretty common for all superwides from the 1970/1980s.