SLR Lenses Explained

What do all those numbers mean?

The two main features and specifications of any lens are the focal length and the maximum aperture. Most lens choices would be based upon these two factors (and price of course!). There are however many other features to take into consideration when buying a lens so this guide should help you to understand what you get for your money and what lenses should suit your need.

• Focal length.

You will notice that the name of a lens is written as a measurement. This can be a single figure (prime lens) or a range (zoom lens), e.g. 50mm or 24-70mm. These figures are the focal lengths and determine the view you will get when you look through the viewfinder. The lower the number, the wider your angle of view will be. Conversely the higher the number, the narrower your angle of view will be, and the more the subject will be magnified.

So for example, if shooting with a digital SLR camera with an APSc sized sensor (most digital SLRs have this) a 15mm lens would give you a very wide field of view, useful for sweeping landscapes and shots of groups of people. Using the same camera, you could attach a 300mm lens and get a great magnification of your subject, bringing distant objects much closer just like using a telescope. These lenses are popular with wildlife and sports photographers.

A lens designed for a digital or film SLR with a focal length anywhere between about 35mm and 60mm would be considered a normal or standard focal length. This is because the image it produces roughly matches what our eyes naturally see. Anything wider that this is considered ‘wide-angle' and anything longer is called ‘tele-photo'.

It is much harder for a lens manufacturer to design and produce a lens of a certain quality at wider angles or with great magnification so these lenses are normally much more expensive than those considered ‘normal' focal lengths.

• Aperture.

The other part of a lenses name will be the ‘maximum aperture', again expressed as a number. It will either be a fixed maximum aperture: f/2.8 or f/4, or a variable maximum aperture: f/3.5-5.6 for example.

The maximum aperture of a lens determines how much light it can capture. A ‘fast' or ‘bright' lens will absorb a lot more light than a ‘slow' lens. A fast lens has a wider maximum aperture than a slow lens. The speed of the lens is measured in ‘f-stops' which are the numbers that follow the focal length in the lens' description. The lower the f number, the faster the lens is. For example, an f/2.8 lens captures more light (twice the amount) then an f/5.6 lens and is therefore faster. With a zoom lens, usually the cheaper ones, as you zoom in, the maximum aperture becomes narrower. You will see this on lenses written 18-70mm f/3.5-4.5. This means that at the wider angle the maximum aperture is f/3.5 but towards the longer end of the zoom range the maximum aperture available is f/4.5 so the lens is ‘slower' at the long end.

The common markings of speed are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and so on. These are full ‘stops'. Each stop lets in exactly half the amount of light as the previous one.

Having a wider maximum aperture means there is more light entering the lens per second compared to a slower lens so in order to get the same exposure you can use a faster shutter speed. For example, if you have a lens set at f/4 and to obtain a correct exposure you need a shutter speed of 1/50 th of a second you could set the aperture to f/2.8 and get a shutter speed of only 1/100 th of a second and you would achieve the same overall exposure. For any sort of action photography, where you require a fast shutter speed to freeze the action, a wide maximum aperture is very useful.

The aperture of a lens doesn't just affect the shutter speed. It also has an effect on the ‘depth of field'. This is how much of the scene in terms of depth is in focus. A wide aperture creates a shallow depth of field, where you have a narrow plane in focus and a very blurred background. This is useful for portraits where you want to make your subject stand out and allows you to eliminate distracting elements in the background.

A narrow aperture results in much more of the scene being in focus. This allows you to have a scene that is sharp and in focus from near to distant points. You would use a narrow aperture for landscapes in particular. Having a narrow aperture such as f/16 results in a slow shutter speed in all but the brightest of conditions and this is why a landscape photographer would almost always use a tripod.

• AF-S, USM, SSM, HSM etc…..

Auto focus lenses have the internal elements moved by the camera itself, as opposed to you, in order to focus the lens. This is done by a motor in the camera body driving the lens.

In recent years various lens manufactures have employed new motors, built into the lens itself which use sonic waves to focus the lens elements. These motors can focus the lens much quicker than the in-camera motors and also operate much more quietly.

Nikon lenses which incorporate this system are called AF-S lenses with the ‘S' referring to ‘silent wave motor'.

Canon calls their system USM – ‘ultra sonic motor', Sigma: HSM ‘hyper sonic motor' and Sony/Minolta: SSM ‘super sonic motor'.

Whilst it is more expensive to produce a lens with one of these motors, they have become essential to sports and wildlife photographers requiring extremely fast focusing lenses to capture fast moving subjects.

• Anti Shake.

Canon, Nikon and Sigma have all developed a system for some of their lenses to counter the effect of camera shake. The 3 systems all work on the principle of having gyros detect movement in the lens, and then moving one of the lens elements in such a way that the image is slightly shifted, keeping the projection onto the sensor or film from moving. This results in a comparatively sharper image at the same shutter speed.

Canon refers to their system as ‘IS' – image stabilisation, Nikon: ‘VR'-vibration reduction, and Sigma: ‘OS'-optical stabiliser.

This technology is most useful in low light situations with a static subject, when you may not have enough light to achieve a high enough shutter speed to avoid blurring. The anti shake systems would help to keep the image sharp by minimising the movement of the light through the lens over the period of exposure.

As an alternative to this, some manufacturers have started building anti-shake systems onto the sensors in the camera bodies themselves

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