Made in 1981, this thirty-something-year-old lens remains 

 one of the best wide-angle lenses for the F mount system.

Lighter, more compact, and less expensive than modern Nikkor lenses of similar focal length,

 the 20mm ƒ/3.5 Ai is a lens for which it’s easy to justify ownership. 

 When it’s understood that the optical characteristics of this lens are equal to (those same) modern lenses,

 ownership should be a foregone conclusion.

https://www.casualphotophile.com/2014/10/26/noteworthy-lenses-nikon-nikkor-20mm-ƒ3-5/

This tiny lens is extremely well corrected for flare and ghosting; 

accordingly it is the inside tip for nature photographers fond of shooting into the sun. 

 Used in combination with the elusive, ultra-thin K1 extension ring it can give stunning close-ups 

with a very steep and dramatically emphasised perspective. 

Although there is no CRC feature, it yields excellent sharpness used up close this way.

 For landscape work, however, its pronounced field curvature can be a disadvantage.

 The corner sharpness isn't as good as the centre either.

 I tend to bring it with me for those dramatic shots into the sun and set the aperture to f/22 to obtain a nice star-shaped sun.

 Otherwise, f/8 gives the sharpest results.

http://www.naturfotograf.com/lens_wide.html

 NB. I concur Bjorn Rorslett's comments above, for full frame digital cameras,

 which is why I think this lens is best for Apsc format, where it's central sharpness shines, 

and the extremes are never seen) 

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Please refer to photos

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