There are two different versions of the Olympus Zuikos 3.5/21, a singel-coated and a multi-coated one. You recognize a multi-coated copy of the lens by the more or less distict green color of the coating. Older copys with written "G. Zuiko" on it are always singel-coated. On the first multi-coated copys of the Zuiko 3.5/21 you find the letters "MC". Later - serial number 120xxx or 121xxx onward - all Zuikos 3.5/21 were mutlicoated and the letters "MC" were not written on the lenses any more.
The Zuiko 3.5/21 I used for my tests is a multi-coated one.

The following pictures show how the different coatings affect ghosting, contrast and the sun star.
Specifications, first image: Canon EOS 3, Fuji Sensia 100, f/16
Specifications, second image: Canon EOS 10D, f/16, crop.

The smallest aperture of the Olympus Zuiko 3.5/21 is f/16. There is no f/22. If you compare this lens to other 20 mm lenses you unfortunately realize that f/16 of the Zuiko 3.5/21 is effectively only f/13 or f/12, depending on the copy of the lens. This is sad because you have to stop down as far as possible if you want to get the nicest sun star possible with a certain lens.

Depending on the position of the sun in the image, you can experience surprises with the Olympus Zuiko 3.5/21. If you place a strong light source in the red marked area in the middle you will get a more or less distinct red ring. If you place the light source in the red area at the edge region, you will get strong red ghosting in the opposite corner. If you place the light source in the green area, the lens is astonishingly resistant to ghosting.
Specifications: Canon EOS 3, Fuji Sensia 100, f/16

By the way, other Zuikos like the 2.8/24, the 2/21 or the 3.5/18 also can produce such a ring!

A big advantage of the Olympus Zuiko 3.5/21 is its very small filter thread. This makes it possible to use the Cokin-P-system, which gives you the possibility to use a Cokin or a Galen Rowell Singh Ray graduated ND filter and a polarizer at the same time without vignetting. Of cause other 20 mm lenses with small filter threads like the Pentax SMC M 4/20 or the two Nikon lenses 4/20 und 3.5/20 also offer this advantage.

On this picture you can recognize the green color of the multi-coating of the Olympus Zuiko 3.5/21 very well.

At the end a comparison of ghosting between Olympus Zuiko 3.5/21 and Contax Zeiss Distagon 2.8/21.