The lens you choose to attach to the front of your camera is arguably the most important factor when it comes to image quality, so let our guide ensure you choose the right lens for the job

Within serious photography circles it's generally accepted that the most critical hardware element of a DSLR or compact system camera is the lens you attach to the front of it. For this reason it pays to invest wisely. Lenses come in all shapes and sizes and many are designed for specific purposes, however the first thing to grasp is how the size of your camera's sensor will affect its stated focal length.

In the days before digital this wasn't an issue as 35mm film SLRs all used the same 24 x 36mm film stock. A standard 50mm prime therefore produced the same field of view regardless of the camera it was mounted to.

In the digital age, however, sensors come in a number of different sizes, which in turn affects the effective focal length of the lens. This is because sensors that are smaller than 35mm only capture a middle portion of the image generated by a lens.

Put simply, they effectively crop out the sides and magnify the middle. This has the effect of increasing the stated focal length of any given lens. The extent to which each type of sensor does this is usually referred to as its ‘crop factor'.

Because full-frame sensors are the same size as 35mm film they will capture images at the stated focal length of a 35mm lens. APS-C sensors are slightly smaller, however, which gives them a crop factor of 1.5x (Nikon, Pentax and Sony) and 1.6x (Canon). Cameras that use Micro Four Thirds sensors (Panasonic and Olympus) are smaller still, which gives them a 2x crop factor.

Thereby a 50mm prime will shoot at 50mm on a full-frame DSLR, but 75mm on an APS-C equipped camera, and 100mm on a Micro Four Thirds model.

Maximum aperture

Another important consideration when choosing a lens is its maximum aperture. This varies greatly, but as a general rule-of-thumb, lenses with faster maximum apertures, or apertures that remain constant throughout the focal range will be bigger, heavier and more expensive.

The main benefit of having a lens with a faster aperture - say a constant f/2.8 as opposed to f/3.5-5.6 - is that they will allow more light in, enabling you to use faster shutter speeds in low-light situations.

Another benefit of faster lenses is that the higher maximum aperture can be used to create a more limited depth-of-field effect, thereby giving you more artistic control over how much of the image you want to be in focus.

Stabilisation

Image Stabilisation is another useful technology that aims to reduce the image-blurring effects of naturally occurring handshake at slower shutter speeds and longer telephoto settings, where handshake is magnified.

Image Stabilisation technology is usually implemented either inside the lens via a group of optics towards the back of the lens that move to ‘correct' any detected handshake (Nikon and Canon), or via sensor-shift technology inside the camera body itself (Sony, Pentax and Olympus).

Each manufacturer has a different name for their take on the technology, but essentially they all share a common goal: to make an extra two to five stops of shutter speed available to help keep images sharper.

When choosing a lens you should also consider what you primarily intend to use it for. If you're looking for a lens that's great for portraiture then a 50mm or 100mm f/1.8 prime could be ideal, whereas if you're looking for a single jack-of-all-trades lens to take travelling then an 18-200mm f/3.5-5.6 superzoom may well be your best bet.

Last but not least, remember that prime lenses tend to offer greater levels of sharpness than zoom lenses as they have been precisely engineered to operate at a fixed focal length, whereas the flexibility offered by a zoom tends to come at the cost of some sharpness.

Telephoto Zooms

Telephoto lenses have a narrower field of view than wider optics, enabling you to crop in on details and magnify more distant subjects so that they appear closer to you than they actually are.

They're perfect for photographing things that you can't get physically close to (at least, not safely) such as elusive wildlife subjects and many sports.

They're also used for selective in-camera cropping, so you can fill the frame with a single tree in a landscape, or person in a crowd.

Telephoto lenses imbue your images with certain aesthetic traits, such as shallower depth of field, and the sense of compressing distance so that elements further away in your scene seem right on top of closer ones.

Although there's a wide choice of prime telephoto lenses, zooms offer the huge advantage of allowing you to zoom in or out to get the exact framing you want, which is especially useful given that in many of the situations in which you'd use one you may not be able to freely move around.

As a result, a telephoto zoom is a worthy addition to any photographers lens armoury.

To see our selection of Telephoto Zooms, click here

Superzooms / Travel Zoom

Many people are drawn to the idea of a single lens that covers every need from 18mm wideangle to 200mm telephoto and beyond. Such an optic would avoid the need to ever change the lens, so you'd never miss that once-in-a-lifetime shot because you had the wrong one fitted, and of course you'd never get dust on your sensor.

If you suspect there must be a catch then you're right; there are trade-offs with such lenses. In general, the longer the range, the more the optical quality tends to suffer, with lower contrast, poorer edge sharpness and greater distortion. Superzooms are jacks of all trades but masters of none, and will be outperformed by prime lenses, and many zooms with a shorter range.

The maximum apertures are pretty small too (as low as f/6.3 at the tele end) so you may have to raise the ISO more often to shoot handheld.

However, depending on what you photograph and the level of quality that you demand, you may find these to be sacrifices worth making.

The fact is that at smaller print sizes the average user is unlikely to spot many of these optical deficiencies, so they are fine for users who want reasonably good pictures that won't be printed too big or studied with a magnifying glass.

To have a look at our popular range of Travel Zoom, just click here