Lenses

Aperture and focal length 

The two terms to make sure you understand when choosing a photographic lens for 

an SLR are the maximum aperture and the focal length. The focal length indicates 

the angle of view (sometimes called the field of view), while the maximum aperture 

limits the fastest shutter speed usable, as well as the depth of field. 

Lenses are marked with their aperture and focal length. If you understand what these 

markings means you can choose the correct lens for the effect you want to achieve. 

Lets look at an example: 18-105mm f/3.5-5.6

What does this mean? 

18-105mm is the focal length of the lens. This tells us that it will ‘zoom’ from 

18mm, a wide angle shot, to 105mm a telephoto shot (a magnified shot). Look at 

the example shots below to see the effect on the picture. 

f/3.5-5/6 is the maximum aperture of the lens. Why are there two ‘f’ numbers? 

Well the aperture varies as you change the focal length. So if you are ‘zoomed’ 

out to 18mm this lens will give you an aperture of f/3.5, if you are zoomed into 

105mm it will only give you an aperture of f/5.6. This would mean it is classed as 

quite a slow lens (see further on in the notes for an explanation about fast and 

slow lenses) 

Lets look at another example: 50mm f/1.4 

What does this mean? 

50mm is the focal length but in this case it is fixed meaning you cannot zoom and 

therefore the field of view is always the same. This is often called a prime lens. 

f/1.4 is again the maximum aperture of the lens. There is only one number this 

time as the lens cannot ‘zoom’ so the maximum is always f/1.4. This would be 

classed as fast lens (see further on in the notes for an explanation about fast and slow lenses)

Examples of fields of view and how it’s affected by focal length:

Usually a specialist lens known as a wide angle. A wide angle lens lets you get more 

into the frame but distorts perspective – which can be a nice effect. (You can go even 

wider into what is called a ‘fish eye’ lens). Good for emphasising scale. 

Still classed as wide angle but often within the range of a ‘kit’ lens. I.e. a zoom lens 

often sold as part of a kit of parts with an SLR. So for example the Nikon D60 is sold 

with a kit lens of 18-55mm, the D90 with an 18-105mm so both could take this shot. 

Considered the same as the human eye – all ‘kit’ lenses will cover this range

Classed as telephoto, i.e. things are magnified 

Again telephoto but magnified more. Lenses that cover a range of focal lengths are 

classed as ‘zooms’. A typical zoom might be 100-300mm. There are lenses that will 

go to 1000mm and above but these are big and expensive. 1000mm+ lenses are the 

type of lens sports photographers and paparazzi might use.

‘Fast’ and ‘Slow’ lenses 

You will sometimes hear lenses described as ‘fast’ or ‘slow’ lenses. Here’s why… 

A fast lens lets you take a shot using a higher shutter speed as it will allow you a 

large aperture (thereby letting more light in). So for instance a 10mm f/1.4 lens would 

be considered very fast and allow you to take shots in low light. This large aperture 

would also give you very shallow depth of field. 

A slow lens on the other hand has quite a small maximum aperture letting less light 

in meaning a slower shutter speed for a given exposure. So for instance a 200mm 

f/5.6 lens would be considered slow but with considerable magnification. 

Lenses get more expensive the faster they are for a given focal length. So a budget 

100mm lens might give you a maximum aperture of f/5.6. A professional lens might 

give you f/3.4 on the same focal length. How’s this done? The lens quality is greater 

so more light gets through and the lens is a bigger diameter (again letting more light 

in). But this means the cost of the lens higher. 

Depth of field and lens choice 

To achieve a shallow depth of field with selective focus you need a large aperture as 

you know. So what lens should you choose? Well if you were working close to the 

subject (say for candid photography) you would generally use a prime lens with a 

maximum aperture in the region of f/1.4 or f/1.6. 

The catch with this sort of very large aperture is focusing. Getting the point you want 

in focus on a very shallow depth of field is hard. On the D40X and D60 you have to 

do this manually. On the D90 however you can use auto focus but your focus point 

needs to be very carefully targeted. 

Shot at f1.4 using Nikon D90 selectively focused on the hand (Targeted auto-focus)

Shot at f1.4 using Nikon D90 selectively focused on the face (Targeted auto-focus) 

Shot at f1.4 in close up using Nikon D60 (focused manually). Very shallow depth of 

field so hard to focus particularly when the dog keeps moving!