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I'm currently looking for a general purpose lens, and I'm more inclined to opt for a prime lens. There are several reasons for that; I want a fast lens and prime lenses tend to be faster at lower prices. Also I think it will be a good way to compose my shots more consciously, as I have to actually move for certain compositions and can't just zoom in and out.

However, I'm not too sure about which focal length I want. I have a Pentax 50mm 1:1.7 which I like, but on my APS-C sensor camera the FOV is not large enough for me in some situations. At the moment, I have the option of buying a used Sigma 30mm f1.4 DC HSM for 270€, which I think is a good offer. But before I buy it, I wanted to ask if anyone has some experience with 30mm lenses (on APS-C sensors) and if they make for good general purpose lenses. I don't need to take epic panoramas, and I don't to zoom in on stuff that's kilometres away. With the 1,6 crop it has a full format equivalent focal length of 48mm, so quite close to 50mm equivalent which as far as I know many people tend to regard as a good general purpose lens.

Is that a good choice? Is there anything I should be aware of (i.e. certain limitations of that focal length)? And is 30mm on an APS-C sensor a good choice for portraits (i.e. can I take frame-filling portraits without too much distortion)?

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  • @MichaelClark That's an entirely different question, not sure why you tagged it as a duplicate – MoritzLost Sep 6 '16 at 16:23
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    I'm afraid this question is either a dupe (it's basically the same as asking if 50mm full frame is a good GP lens), or opinion-based. Every question and answer about 50(ish) mm full frame applies equally to this question. – scottbb Sep 6 '16 at 22:46
  • Related, effectively equivalent issues/suggestions: Do I really need a fast 50mm lens? – scottbb Sep 6 '16 at 22:48
  • @MoritzLost Because ultimately the general consensus is that a single prime used as a general purpose lens needs to be a normal lens: That is one which has a focal length roughly equivalent to the diagonal of the imaging medium. You even acknowledge such in your question: "With the 1,6 crop it has a full format equivalent focal length of 48mm, so quite close to 50mm equivalent which as far as I know many people tend to regard as a good general purpose lens." – Michael C Sep 7 '16 at 1:37
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IMHO, 30mm would be ok for general purpose photography. I have a sigma 18-35 and I often find myself using the long end of this lens (from 28 and onwards). Distortion depends a lot on the specific lens, but in my opinion 30mm should not be a problem on APS-C sized sensors. If you intend to fill the frame with that focal length, just check that the minimum focusing distance is not too big.

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A “normal” lens is one that is not telephoto and not wide-angle. This works out be a lens with a focal length about equal to the diagonal measure of the format frame. The Advanced Photo System (APS) frame size is approximately 16mm height by 24mm length. The diagonal measure of this rectangle is about 30mm. Such a lash-up delivers the following angles of view: 30⁰ vertical, 43⁰ horizontal and 51.5⁰ diagonal. It’s the diagonal angle of view that is most often published. Wide-angle is about 70% of “normal” and shorter = 20mm. Telephoto is about 200% of “normal” = 60mm. In art, there is no law cast in stone. Portraiture is likely best if the lens is 2X thru 2.5X or “normal” = 60mm thru 75mm. Primes are nice but zooms are more flexible, especially if you are restricted to a single lens

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I have a Pentax DA 35 f2.4 which is my general purpose prime in conjunction with my K5ii.

If you want a fast prime which fills the role of a 50mm from the days of film, somewhere around 28-35mm is where you want to be. Now which one as always is a matter of personal choice.

My DA is a plastic fantastic. The lens mount is plastic, there is very little damping in the focus ring, no distance scale and no full time manual override on the focus (quick shift in Pentaxese). On the other hand, it's so small and light that I have no reason to ever leave it at home, it cost very little (£120 brand new) and its optical performance matches lenses costing 3 times as much. If you are using a Pentax, I'd give the little DA35 some thought.

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