Digital SLR: Buying a lens
What you can buy
Lenses for Digital SLRs (DSLRs) come in many different sizes and types.Two of the most common types are zoom lenses, and 'fixed focal length' lenses (lenses without a zoom).
Fixed focal length lenses can give you better image quality, but zoom lenses are far more practical, and the best ones are very good.
You can also buy macro lenses for better extreme close-up photos, for example butterfly-on-a-flower type shots.
Used lenses can sometimes be very good value - but check any used lens you are thinking of buying is compatible with your DSLR.
Fitting
There's some incompatibility between lenses and DSLRs. For example, Canon lenses only fit Canon DSLRs, Nikon lenses fit only Nikon DSLRs. Our table (see 'Lens Best Buys') gives information about compatibility.
When you buy a lens made by a third party manufacturer, like Tamron, you should specify the make of your DSLR to ensure the fitting is correct. Fitting and removing lenses is easy. It’s usually just a case of snapping the lens on and twisting to secure it, or pressing a button and twisting the lens off to remove it.
Take care when you're removing and fitting lenses though, as dust can enter and cause problems (especially on DSLRs without dust cleaning features). Remove and replace lenses quickly, with the DSLR switched off and facing downwards.
Focal lengths
Unless you own a top-end DSLR costing into the thousands, the stated zoom range on any lens is normally not the zoom range you get when you use it on your DSLR.
For example, the Canon 18-55mm lens has an optical zoom range of 18-55mm. This is the zoom range you would get if you fitted this lens to a film SLR.
However, when you fit this 18-55mm lens to the Canon EOS 450D DSLR, say, the zoom range becomes 29-88mm, due to this camera’s internal design.This optical zoom range change is sometimes called focal length multiplication. In the Canon EOS 450D's case, the focal length multiplier is 1.6. The actual, real-life zoom ranges of all lenses (for example 18-55, 100-300mm and so on) are subject to a multiplication of 1.6.
Fitting a lens on a DSLR nearly always changes its zoom range like in the Canon EOS 450D example. Usually, both numbers in the zoom range increase when you attach the lens to the DSLR.
The first number on any zoom range shows the angle of view - the lower the number the more you can zoom out. The second number shows how far the zoom extends – the bigger the number, the further you can zoom in.
Aperture and depth of field
All lenses have an aperture (a little hole), through which light passes to create the photograph. Changing the size of the aperture can give different amounts of ‘depth of field’. Depth of field is the distance in front of and behind the subject that is in focus.
More depth of field means that more of your picture is in focus. Little depth of field can mean a blurred background in a photo, and is a useful technique for pleasing portrait shots.
Consider an example of a photo of a person standing in front of a field of donkeys. With lots of depth of field, the person and the field of donkeys will both be in sharp focus. With little depth of field, just the person will be in sharp focus and the field of donkeys will be blurred.
The size of the aperture is represented by the f-value. The smaller the f-value, the bigger the aperture and the less depth of field you have. f2.8 and f3.5 are examples of small f-values.
You usually change the aperture size using the DSLR’s controls, although each lens has a maximum and a minimum aperture size (and corresponding minimum and maximum f-value).
More on f-values on lenses
The 'maximum' aperture of a lens, in other words how big the aperture can get, is usually stated on the lens. The bigger the aperture (in other words, the smaller the f-value), the more light can get onto the DSLR sensor.
This means shutter speeds can be shorter, reducing the chance of blur. This is in addition to less depth of field.
Lenses usually have their maximum apertures stated as a range, as the maximum aperture is different at different ends of the zoom. Recall the size of the aperture is represented by the f-value.
For example, the Canon EF-S 18-55mm f3.5-5.6 has a maximum aperture range of f3.5-5.6. f3.5 is the maximum aperture at the zoomed out end of the lens and f5.6 is the maximum aperture at the zoomed in end of the lens.
Autofocus (AF)/Manual focus (MF)
All our lenses are AF – this means they're compatible with your DSLR's AF system. All can work in MF mode too if you prefer – simply flick a switch on the lens body.
We recommend using AF on DSLRs , as finding correct focus using MF on DSLRs can be hard sometimes – and slow. MF is nevertheless useful to have in some circumstances.
Weight and length
Our lenses on test range from around 200 grams to over 500 grams. Add a heavy lens to your DSLR and you could have quite a heavy package to carry around with you. It’s always worth considering the weight, and dimensions, of a lens before buying.
Most of the lenses on test measure less than 10cm. Zoom lenses can of course replace the need for carrying around many fixed focal length lenses.
Filters
Filters can be added to your lens. There are lots you can buy.
For example, a polarising filter can make the blue of sky in your pictures more intense. A starburst (sometimes also called ‘cross star’) filter can create 'star' effects from light sources like candles or lightbulbs.
A UV/Skylight filter can absorb UV haze in sunny landscape pictures. Since this is the only effect it has, it is a good filter to leave on as it offers protection to the glass of your actual lens.
Many effects that filters can create can also be done using image editing software on the computer, but having filters can help the photographer visualise the image better, and can also save time in the long run.
Finally, make sure you buy the correct size filter for your lens.