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An update to my previous review.
I forgot that this lens focuses VERY CLOSE with excellent IQ even wide open. In fact, it actually does blow away kit lenses in this respect... so I've adjusted the scores appropriately.
Retrofocus ultra-wide lenses are difficult optical design problems. They tend to have a lot of elements, so contrast suffers unless the coatings are very good. Relatively speaking, this Vivitar is actually quite good -- it certainly has much more contrast than my Spiratone 18mm. Sharpness wide open is quite acceptable and distortion is fairly low. The transition to out-of-focus is even fairly good. Unfortunately, my copy has the aperture stuck wide open, so I can't comment on how much IQ improves upon stopping down.
Build quality is very good. It's a pretty hefty hunk of metal and glass, mounted using an FL-style breech-lock, and is absolutely smooth in operation. The huge front element will impress anyone who sees it.
There are just three problems:
1. It's f/3.8 -- not very fast by modern standards.
2. Pricing on all old ultra-wides, this included, is often way too high (e.g., over $200).
3. Most folks who can mount an FD lens are using APS-C or micro 4/3 -- not full frame.
In summary, on a crop like APS-C, this 20mm isn't much faster or better IQ than a kit zoom lens designed for the cropped sensor... and the kit lenses are cheaper and can autofocus.