When you buy a camera that can accept interchangeable lenses, you're not so much just buying a camera as buying a system.
Why? With little exception a Nikon body will only accept lenses made for it, and the same for Canon, for Pentax, for Sony, for Olympus and so on.
Some lens manufacturers such as Tamron and Sigma make lenses to suit a range of bodies, but those still have different mounts to the others. A Sigma lens made for a Pentax won't fit on a Canon, and vice versa. Sure, you can buy adapters that will allow some lenses to be used on other bodies, but they work with varying (read 'limited') success.
So back to my original premise - that buying a camera is effectively buying into a system.
The system I've bought into at the moment is Pentax. My father was a Pentax man so when it came time for me to move into the world of Digital Single Lens Reflex (DSLR) cameras, I opted to buy into the Pentax system. The reason? Well, I had fond memories of Pentax gear, and there are some high-quality Pentax lenses available to me that can be put to use in a Pentax-based system.
In late 2012 I purchased a Pentax K-30. My current kit includes:
The quality of the 18-55mm kit lens is quite poor. I look at photos taken with it next to those taken with the 50mm prime and they are poles apart in my view. (A prime lens has a fixed focal length - no zoom). That kit lens doesn't cut it!
After reading around the Pentax Forums and their review pages I decided it was time to replace the kit lens. I quite like the versatility of that range of zoom. The K-30 contains a APS-C sensor that is smaller than 35mm film so it applies a cropping factor - meaning a 50mm lens on my camera has the same field of view as a 75mm lens on a camera that accepts 35mm film (or sensor size). Basically you take the focal length of the lens as suited to 35mm film/sensor and multiply it by 1.5 times to find the equivalent focal length when it's on my camera. The 18-55mm zoom thus acts/feels like a 27-82mm lens on my K-30.
I pondered some prime lenses and some zoom lenses around that 20-50mm range. I wanted good quality, preferably one that could fit on a full frame camera (remember, you're buying into a system!), nice and sharp, a fairly wide aperture for lower-light situations that wouldn't cost an arm (or a hand or even a finger).
Those I considered were:
I quickly discarded the 24-50 from my thinking because it receives lower ratings tha the Sigma 17-50. I was quite attracted by the 28mm prime, but they are now nearly 30 years old. The 35mm prime seemed reasonable, but given its APS-C equivalent of 52mm I wanted a little more flexibility to go wider or narrower. That left me with the Sigma. Their RRP in the US is $669, but they were on special down to $369. This would work out at around $510 including postage in real (Aussie) dollars. A few sites in Australia had them for around $520 and that included a 2 year local warranty. The Sigma zoom also gets an excellent review at the Pentax Forum review of the Sigma 17-50mm F2.8 lens.
The Sigma has been ordered and will be a direct replacement for the 18-55mm kit lens. The kit one will be relegated to a bag somewhere. No point selling it - who'd really want it?