What old lens can I use for my NIkon D3100?
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Camera-Specific Properties: Image-Specific Properties:
The D3100 does not have an in-house autofocus motor. You have two available scenarios of use:
1) Get Nikon F-mount lenses with integrated autofocus motors. https://en.wikipedia.org/wiki/List_of_Nikon_F-mount_lenses_with_integrated_autofocus_motor
2) Get any other Nikon F-mount lens and exclusively manual focus with it. The D3100 poses one problem if you do this: the viewfinder is pretty small. If you use S or A modes (but not M) you can compensate by enabling the rangefinder option under Setup. Alternatively, you can get a DG-2 eyepiece magnifier that will magnify the central portion of the image by 200% but cut off the view of the corners as it's round.
Once you have decided whether you're willing to use the limited selection of lenses with internal autofocus motors or use others with autofocus, you'll want to head for the reviews.
I'm personally a poor-as-fuck D3200 user, so I can't help you with that. But the Angry Photographer is a trusty guy, in my opinion. Here's his review of an out-of-production 135mm lens now that you mentioned old ones (it's a manual lens, too): https://www.youtube.com/watch?v=Fn4v4I40Fvo