where is the best place online to find a nikon used?
No "best" but they are common in all the usual places
1) Craigs list,
2) Amazon
3) eBay
4) KEH.com (this is one of the best on-line places)
5) B&H Photo these guys are good also, bought a 50mm there for $90.
6) Adorama only a small step down form B&H
7) almost EVERY local camera store has some Nikon used lens for sale. Yes EVERY one of them I've ever seen. most have dozens of them.
8) any of several camera forums. fredmiranda.com is a big one.
The reason so many Nikons are on the used market is that the F-mount has not changed in decades. Even 1960's vintage lenses still fit. Millions of lenses were made and they last nearly forever. There is some limitations but the supply of used Nikon gear is endless.
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I always read this, but the DOF does certainly not cover everything.
At 2.8 their is a noticeably shallow dof on my 14mm.
I would try to get the samyang used. they are a bargain!
The lens in question is f/3.5 not f/2.8 and also 8mm. for that lens the hyper focal distance is about 3 feet. In other words you only have to move the focus ring in the subject is very close.
You are right about them being a bargain. All of them are I think the same samyang lens sold under different brands. The only disadvantage is that some people may not understand manual exposure. With some bodies the lens would not meter at all. and with others it would work.