This is part IV of a series of what glass you will need. Each part will discuss a certain lens type and its applications.
Medium primes are lenses in the range of 85-200mm (in FX terms) or 58-135mm in DX.
This segment is very interesting as you find here lenses with with the lowset f-number/focal-length (if we neglect long tele lenses). In other words: These lenses allow for extreme subject separation.
I am not talking about depth of field (DOF) here, I mean about the ability of a lens to blur a near background. This ability increasea the lower the ration above, I’ll write up the geometry behind this in another post (this assumes the subject appears with a constant magnification, therefore the separation from infinity background only depends on the aperture!)
An example, a 85mm 1.4 lens has a ratio of 0.0164, while a 2.8 lens reaches this only at 170mm, but this is sometimes a rather impractical long focal length. You can do better with a 300/2.8, but you will have to step back a lot.
The question want to use longer or shorter is a matter of taste and the subject. For filling the frame you have to get closer and have more perspective distortion. This can be desirable to bring out facial faces or something you want to avoid if “she” thinks her nose is too long.
85mm (FX) is long enough that facials feature won’t pop out exaggeratedly on a photograph, but still preserves enough to not flatten out everything - at 200mm not much is left. On DX 200mm is definitely too much, but if you want an effect like cut out of paper this works well for some purposes (wedding couple for the thank you photo).
The line up
The Nikkors (and third party) 85mm can be recommended throughout - the more expensive, the more you get. Special mention for the 85/1.8, which is compact and cheap, but decent quality for street photography and portraits when you take care of the background.
The 85mm is on DX on the longer side, so as well useful. For DX the shorter end is a bit weak, but 50mm lenses are close, Voigtländer’s 58/1.4 is perfect, the Tamron 60/2.0 a bit slower, but tack sharp.
The longer side is covered with 100, 135, 200 f/2.0 lenses (though the DC lenses are a bit outdated). The 200/2.0 is more the lens for portraits of sport stars.
Macro lenses shouldn’t be left out of consideration, they are not that fast, but frankly many times you won’t shoot wide open, especially with longer lenses you stop often down to f/2.8-4 to preserve context and achieve enough DOF