The subject of PRIME Lenses was brought up and this really can be a real "mine field" to navigate, because there are various advantages to the different focal lengths and apertures.
The most dynamic aspect of a PRIME lens is Depth of Field (DOF). Having a
short focal length PRIME lens with an ultra wide aperture (f/1.4 - f/1.8) provides a lot of light, but also a
relatively deep DOF at close range. The same aperture and distance on a medium prime length PRIME tightens up that DOF considerably (shallow). On a telephoto PRIME lens, the DOF at wide aperture becomes very thin.
A couple of DOF examples:
f/1.8 @ distance of 10-ft
Focal length → DOF
20mm → 5.8-ft
24mm → 3.8-ft
35mm → 1.8-ft
50mm → 10-inches
85mm → 4-inches
135mm → 1.5-inches
Now, increasing the distance between you and your subject does have large effect, but the subject is reduced in size. Increasing to
20-ft, the subject is half as large. That can be a real problem with a wide angle lens, as the subject gets kind of 'lost' in all of the background:
Focal length → DOF
20mm → 30.6-ft
24mm → 17.5-ft
35mm → 7.3-ft
50mm → 3.5-ft
85mm → 14-inches
135mm → 6-inches
While this is hard to envision, without the accompanying images, you can kind of get the feel for the problem if you have a typical 17-50mm, 18-50mm, 18-55mm, 24-70mm or 18-70mm "kit" lens. Ignoring the DOF for a few moments, focus on your subject (10 feet away), then simply watch through the viewfinder as you zoom from
50mm down to
24mm. Now, step back 10 more feet (to a total distance of 20 feet) and do the same thing. Yeah, a lot more background to fight the "framing battle" with.
So, as you can probably tell ... picking the proper PRIME lens depends almost entirely on the situation. The low-light struggle with DOF has to be reconciled, before you spin wide open to f/1.4 or f/1.8, otherwise ... everyone may be bright, but they are going to look a little fuzzy around the edges.
So which PRIME lens to own? Well, generally speaking, the
50mm f/1.7 would be a place to start, because it is a relatively low-cost choice and splits the difference between wide and telephoto. The next one is a bit more difficult to choose, because the cost goes up considerably and kind of depends on whether you have a APS-C (
α100,
α200,
α300,
α350,
α700) or a Full Frame (
α900) sensor camera.
Note: The new
α230
, α330 &
α380 DSLRs may NOT have motor drive in them and using screw-driven AF lenses (currently ALL non-SSM/non-HSM lenses) may not be an option, because
autofocus (AF) will not be operational. This still remains to be seen, at this time. A new
50mm f/1.8 is being developed at this time, for this reason. Yeah, the ground is a little shaky on these new offerings. If you thought "lens selection" was bad before ...
Here is a "short list" of available prime lenses w/ filter diameters
* (non-MACRO despite description)
Non-fisheye Wide Angle PRIME
TAMRON
SP AF 14mm f/2.8 LD Aspherical Rectilinear (no filter ring)
SIGMA
20mm f/1.8 EX DG ASPHERICAL RF ø82mm
SONY
20mm f/2.8 ø72mm
SIGMA
24mm f/1.8 EX DG ASPHERICAL MACRO ø77mm
SIGMA
28mm f/1.8 EX DG MACRO ø77mm
SONY
28mm f/2.8 ø49mm
SIGMA
30mm f/1.4 EX DC HSM ø62mm
SONY
35mm f/1.4 G ø55mm
Normal PRIME
SIGMA
50mm f/1.4 EX DG HSM ø77mm
SONY
50mm f/1.4 ø55mm
SONY
DT 50mm f/1.8 ø55mm
Minolta
50mm f/1.4 ø49mm or ø55mm
Minolta
50mm f/1.7 ø49mm or ø55mm
Telephoto PRIME
SONY
85mm f/1.4 CZ ø77mm
SONY
135mm f/1.8 CZ ø77mm
SONY
135mm f/2.8 [4.5] STF ø72mm Manual Focus
SONY
300mm f/2.8 G SSM ø42mm (internal)
* Some folks pick and choose their lenses to have similar filter-ring diameters to reduce filter costs, so I included it.