EXPOSURE

The next big decision, if you have chosen a DSLR camera, is which lenses to choose. There are loads to choose from, they literally come in all shapes and sizes, and indeed prices. So what do you need to know before you choose?
Prime Lens or Zoom?
A prime lens has a fixed focal length, and a zoom lens lets you . . . err . . . zoom in and out, in other words it has a variable focal length. This makes the zoom lens more useful as one lens may do the job of two or three prime lenses. There is always a trade off of course and, in this case, it is that the prime lenses are normally better quality. Although, having said that, modern zoom lenses are certainly good enough for most purposes and are the weapon of choice for most photographers.

Focal length determines the angle of view of the lens, a wide angle lens will include more of the scene giving the illusion that you are further away from the subject, whereas a telephoto lens will pick out a smaller area and make you seem closer to the subject than you really are, like a telescope. There are also ultra wide angle lenses (known as a Fish Eye Lens) on the market. The focal length that matches what we normally see is called the 'standard lens'. The exact number of the focal length of this lens varies according to the size of sensor you are using. On the old 35mm film cameras the number was easy - 50mm - but, as most amateur and 'prosumer' cameras use an APS-C size sensor, which is smaller, the standard lens has a focal length of 31mm. So, if you have an APS-C size sensor (and most DSLRs on the market, except the very expensive ones, do), then any lens with a focal length below 31mm is a wide angle lens and any lens longer than 31mm is a telephoto.
What focal lengths do you need?
Normally the camera is sold with an 18-55mm lens, which is called a mid range zoom. A mid range zoom is the most useful lens you'll ever buy for general photography. There are times, however, when you just can't get close enough to the action and you need a telephoto zoom to bring the action closer to you.


When you look at the specifications of a lens, after the focal length, you will see a number like f4 or f2.8. This is the maximum aperture of the lens and is very important. In the case of a zoom lens there may be a range of apertures, f4 - f5.6 for instance, this is because with some zooms, especially the cheaper ones, the maximum aperture is different at different focal lengths.
The larger the maximum aperture of the lens, which means the smaller the f-number, the more expensive the lens will be. This is because the lens will need a lot more precision ground glass and is more difficult to make.
The advantage to the photographer of a larger maximum aperture is that the lens will let in more light when the aperture is fully open, enabling you to shoot pictures in more demanding circumstances, in a low light situation or when using faster shutter speeds to capture action. Also, and just as important, the large maximum aperture allows you to limit the depth of field and throw the background of your photo out of focus. All of which you will of learnt about in this course. So when you look at a price list of lenses and see that a 70-200mm f2.8 is considerably more expensive than a 70-200mm f4 lens, there is a good reason for it and a good reason to buy the more expensive one if your budget allows.

One thing that can be a bit confusing is the issue of coverage of the sensor. In the old film days all SLR cameras were 35mm, that is the picture size was 35mm x 24mm, and all the lenses were made to cover this area. The large or 'full size' sensors on digital cameras are still this size but the APS-C cameras, which includes most of the cameras that us mere mortals can afford are, as discussed earlier, smaller. So the lenses that are specifically made for the smaller cameras can also be smaller and therefore cheaper. The problem is that all the top quality lenses are made to cover the full frame sensors, so the difference in price is even more than it might be otherwise. However, unlike camera bodies which need to be updated or replaced every now and then, a good lens will last a lifetime. So, if you have aspirations of owning a full frame camera one day, it's worth forking out for a lens that will cover the format.

The latest must have extra on lenses is an imager stablizer. Lots of lenses now offer this option for extra money.

So, buy the best quality lenses you can afford, get a large aperture if you can and try to get a zoom that has the same aperture all the way through the zoom range.

Try to steer clear of the all-in-one mega zooms that have focal lengths from 18mm to 300mm or so. There is something to be said for not having to keep changing lenses especially when you are out and about on a windy day, but the trade off for such convenience is usually a drop in quality and a smaller maximum aperture, you would be better off with two zooms to cover the same range. Having said that, others will tell you that they are very happy with such a lens.
A good lens should last you a lifetime if you treat it well, so they are a good investment. With so many millions of pixels on the sensors of modern cameras, that end of the camera is no longer a real issue as far as quality is concerned. What makes the difference between a so-so shot and a really crisp rendition of the subject, apart from your technique of course, is a really good piece of glass on the front of the camera.

If you can't run to the super lenses just yet, you will still get pretty good results from the 'kit lenses' that are included with the camera, so don't despair if you can't shell out thousands to start with, just get started with whatever you can afford. Don't forget that it's not how much the gear costs but who is standing behind it! A good eye for a shot is worth more...