I own both the 21/4.0 and 21/2.8 Konica Hexanon lenses. I have owned the 21/4.0 since 1970 and have generally been pleased with it's performance, although I admit to using it infrequently (the 24mm 2.8 is my full time wide angle). I purchased the 21/2.8 a year ago or so in order to compare it to the 21/4.0. I made the comparison a few months ago and decided to sell the worst performer on eBay. Life isn't always fair, and such was the case in this test of the two Hexanons. I used Kodachrome 25 slide film and a tripod mounted FT-1 body. I took shots of a busy commercial area with edge to edge and corner to corner detail from a hillside vista at wide open aperture and at F 8.0. I evaluated the finished slides with a 30X magnifier, and the results were unmistakable: the 21/2.8 Hexanon was noticeably sharper at the center, and the 21/4.0 was noticeably sharper at the edges! Either lens could be preferable for a given circumstance. What to do? How to choose? Keep both? Flip a coin? Evaluate needs? Build a decision matrix? Arrrrrgh! In the end, nostalgia ruled the day. I decided to keep my old 21/4.0. At 30X, the center still looks pretty good when you don't make direct comparisons, and like I said, I don't use it a lot anyway. The 21/2.8 will likely be up on eBay soon. It is perfectly mint in box with hood, caps, and case. It has been on a camera one time for two pictures. Thought those of you who worry about such things would like to know (as did I). Now you know, so go out and take some pictures!
My favourite lens is the 21mm. I got it very cheaply with its original lends hood and leather case in perfect condition. There's a little loss of sharpness towards the extreme corners but it's a cracking lens.
The Hexanon / Hexanon AR 21 mm / F4 is of very good optical quality and renders sharp images with high contrast. Vignetting is visible with this exteme wide-angle lens, but is not too disturbing. Distortions are corrected very well, although there are pincushion distortions detectable with straight lines running parallel to the frame – the extreme wide-angle effect is always clearly visible, however. The faster successor model is said to have better sharpness, but I have no own experiences with this lens.
Due to its huge, not recessed front element, the Hexanon / Hexanon AR 21 mm / F4 is a bit prone to flare. With its huge field of view, the sun is frequently lying in or just outside the image. Therefore you should use the original lens hood if possible, which was delivered with the lens. It gives much better protection than a collapsible lens hood, that gives almost no shading with the big diameter and the wide image angle. The original lens hood covers those parts of the front element that are outside the image area on the film, leaving open a rectangular opening . The precise aligning is ensured by a groove in the lens barrel with a corresponding feather in the hood.
The viewfinder image is a bit dim with the widest aperture of 1:4, in low lighting split-image or microprism focusing aids tend to black out – precise focusing is a bit tricky under these circumstances.
When new, the Hexanon / Hexanon AR 21 mm / F4 was quite an expensive lens, because of its extreme focal length it is quite rare and difficult to find. As a rare collector's item, it tends to fetch high prices nowadays. On the other hand, due to its extreme focal length and its very strong wide angle effect, possible use is somewhat limited. Whether the high price can be justified, is a matter of your personal needs. If you find yourself having much use for this focal length, the Hexanon / Hexanon AR 21 mm / F4 will serve you well.