Understanding focal length

This lesson covers understanding focal length in practical terms. Read it with your camera or phone nearby so you can test the idea rather than trying to memorise a list of rules.

What you need to know first

Photography becomes much easier when you separate cause and effect. While learning this subject, change one thing at a time and keep everything else as similar as you reasonably can. That gives you a fair comparison and makes mistakes useful rather than frustrating.

Focal length and field of view

Focal length is written in millimetres. On the same camera, a shorter focal length normally gives a wider field of view and a longer focal length gives a narrower one. Sensor size affects the field of view a given focal length produces.

Zooming is not moving

Changing focal length changes how much of the scene fits in the frame. Moving the camera changes the relative apparent size and spacing of near and far objects. This is why two similarly framed photographs can have very different perspective.

Compatibility matters

For interchangeable-lens cameras, the mount must be compatible with the camera. Do not buy a lens because its focal length sounds right without checking the exact mount and any sensor-format limitations.

Find it on your own camera

Camera manufacturers do not all use the same button names or menu layout. Start with the mode dial, Quick or Q menu, Info/Display screen and main shooting menu. If the control described in this lesson is not obvious, use the index in the manual for your exact model. Do not copy a button sequence from another brand and assume your camera is faulty when it differs.

Make a controlled comparison

Choose a simple subject that will stay in the same place. Make the first photograph, change only the setting or technique taught on this page, then make the second from the same position. Compare the pair at the same size. Ask yourself what changed, what stayed the same and whether the change matches the explanation above.

Do not move on until you can explain it

• I can explain the idea in my own words without repeating a definition.
• I can find the relevant control, menu option or technique on my own equipment, where my equipment provides it.
• I can make two photographs that demonstrate the difference.
• I can recognise when the result has not worked and change one thing to test why.

If your result looks wrong

First check the simple things: battery, card or storage, shooting mode, focus point and whether the camera actually accepted the setting. Then check whether the light or subject changed between photographs. Do not reset five settings at once. Return to the last result that worked and change one thing.

Now prove it with the practical

The practical is part of the lesson, not optional homework. It turns the explanation into photographs you can compare. Work through it in order and repeat any step that does not give you a visible result.