Part II: Getting a lens. Buy a vintage prime.
To understand lenses you need to understand two basic numbers. The focal length and the aperture.
Focal length is the distance from the point where all the rays of light meet inside the lens to the sensor. This tells us the angle of view and the magnification. The lower the number the wider the angle of view will be and the lower the magnification will be. The higher the number the narrower the field of view will be and the higher the magnification will be. This is pretty simple. Low focal length means wide angle. High focal length means zoomed in.
Aperture is more complicated but also more important. Aperture is a measurement of how much light the lens allows onto the sensor. At the rear of the lens there is a hole and this hole can be made wider or narrower to change the amount of light that's let in. The ratio of the width of this hole and the focal length calculates the 'f-number', a numerical representation of the aperture. For instance, f/1.7 on a 50mm lens will mean that the hole will be roughly 28mm wide. Lower f numbers mean wider apertures and higher f numbers mean narrower apertures. Most lenses have a range of apertures e.g. f/1.7:22. This measurement has a huge impact on the final image. Wider apertures will mean more light let in and focus being concentrated on a very narrow plane. Narrower apertures will mean less light being let in but more things will be in focus. When choosing between two lenses with the same focal length the lens capable of a lower aperture is almost always better; f/1.4 is better than f/3.5. Often, lenses capable of lower apertures will be called 'fast'. Most lenses will be capable of high apertures but not all lenses will be capable of the lowest apertures. This is a very important factor when choosing a lens. Usually, the faster the lens is the more it will cost.
When you choose a lens you also need to remember that it has to be compatible with your camera body's lens mount, a Pentax K mount lens will not fit on a Canon EF mount body unless you buy an adaptor. Adaptors can be a great help but will often reduce your lens to purely manual operation unless you pay a lot of money for a 'smart adaptor' that allows signals to be sent between the electrical contacts on the lens and the camera. If your camera body has no built-in stabilisation you also need to consider whether or not the lens has image stabilisation built in. Likewise, if you have no in body autofocus you need to know whether or not the lens has autofocus built in or if you do have in body autofocus whether or not the lens can connect to it.
The last major thing to remember is the difference between prime lenses and zoom lenses. A prime lens is a lens that has only one focal length. Prime lenses do not zoom. Primes are often said to produce better images because the light has to go through fewer elements before it hits the sensor and because prime lenses are often capable of lower apertures. Prime lenses will have just one focal length such as 50mm or 35mm. Zoom lenses are lenses that can cover a range of focal lengths which means that they can zoom into, or out of, a scene. Often when looking at zoom lenses you will see a range of focal lengths such as 18-55mm or 70-300mm. The advantage of a zoom lens is the flexibility they offer.
If you already have a kit lens then for learning how to compose photographs a prime lens will stand you in much better stead and if you need the flexibility of a zoom then you have your kit lens as a backup. My advice for buying a second lens is to buy a vintage prime lens. Vintage lenses usually cost less while offering high image quality. However, they may not have autofocus or built-in stabilisation and you might have to set the aperture manually using the aperture ring on the lens instead of through the camera. This shouldn't discourage you because the experience of manual focus and using the aperture ring will bring you closer to understanding exposure and help you to become a better photographer. When buying vintage lenses be mindful that they were mostly made for full frame film cameras and therefore with an APS-C sensor there will be a crop factor applied. This means that you have to multiply your focal length by a factor of 1.5.
If you have a Canon body then your lens choice will be decided mostly by your budget. Canon changed their lens mount when they introduced autofocus and made their FD mount obsolete. There are EF mount lenses available but they aren't cheap and unless you're willing to shell out some serious cash they won't be stabilised. There is an online Canon lens database to help with finding a good lens. Buy vintage for best value and if crop factor is an issue then Canon do make a range of lenses designed for APS-C cameras which have the EF-S designation for their lens mount. If you have an EF-S mount lens then the focal length will be exactly what it says on the lens.
Nikon is a slightly different story. The Nikon F mount has been around for a very long time just like the Pentax K mount and there is a wide selection of lenses available. However, they are not cheap. You might be able to find some bargains at eBay auction or on your local classified ads website. Just like for Canon there is a Nikon lens database.
If you followed the advice of the previous post and bought a Pentax with a Kit Lens then you will have the easiest time buying a vintage prime. There are many available for the Pentax K mount and all of them are different. My favourite is the SMC Pentax A 50mm f/1.8. It cost me AU$50 and it shoots beautiful crisp images with vivid colour. It has auto aperture but no autofocus but at the price I paid, I didn't mind. When I got tired of manually focusing it I found out that Pentax made an adapter that allowed its manual focus lenses to have autofocus at the expense of some light and a slight increase in focal length. This adapter is rare and pricey but well worth the money for the ability to autofocus all your vintage lenses. If my favourite lens isn't your pick then there is a Pentax Lens Database online with reviews and test shots to help you choose.
Now that you know how to buy a lens you're all set to learn how to use your gear properly. In the next post we will be discussing the three main elements of photography and how they relate to exposure.