Focal Length written 4 years ago

The image below shows the focal length of a simple converging lens. It is the distance between the centre of the lens, and the point at which parallel incident rays of light are focused to one point on the image plane.

Camera lenses are usually made up of many more lenses than just one, so when measuring the focal length it is taken from something called the rear nodal point. Finding the rear nodal point of a lens is actually pretty tricky, and is discussed more here.

When calculating the focal length of a camera lens, it is always quoted at the “infinity position”. The infinity position is where the lens is set to focus on the furthest point away from the camera (where the light rays can be approximated as being parallel).

In practise the focal length of a lens is usually printed on the front of the lens itself, so working it out manually is not necessary:

The focal length of a lens is always specified in millimetres, and is sometimes given in the form f=xxx. The focal length of a lens on its own doesn’t really tell us much though, as the focal length of the same lens can be different, depending on the size of the sensor used.

The image above shows how the same lens can be classified as having a different focal length, depending on the sensor size. For the image to fill the same proportion of the sensor, the sensor must be moved back — increasing the focal length. This is why, on the images above the focal lengths vary from 4.8mm - 50mm. The 4.8mm lens must be coupled with a very small sensor, otherwise the photos it produced would have a very very wide field of view.

In order to provide a standard focal length, which can be used to compare different lenses, focal lengths are often quoted as being 35mm equivalent. This is the focal length of the lens, if it was being used on a standard 35mm film camera.

35mm Equivalent Focal Lengths

Lenses can be broadly categorised into three groups:

Wide Angle Lenses

These lenses have a focal length < 40mm.

As the name suggests they give a wide angle view of the scene, and can squeeze more into a typical shot. They are useful for up-close photography — where obtaining distance from a subject would be difficult.

The widest view lens has a focal length of 8mm, giving a 180? view from the front of the lens.

---

Normal Lenses

These lenses have a focal length of around 40-58mm. They are called “normal” lenses, as they approximate the view from a human eye.

The size 40-58mm is not chosen arbitrarily, it is roughly the diagonal across a 35mm film frame. This gives a field of view of around 90?.

Indeed, for sensors of varying size, a “normal” lens is one which matches the diagonal length of the sensor.

“Normal” lenses are good all-round lenses, and are useful in that resulting pictures will have a natural feeling perspective. Lenses supplied in kits bought with cameras will usually incorporate a 50mm focal length.

---

Telephoto Lenses

Telephoto lenses have a very narrow field of view and so focus on very specific parts of a scene. Lenses with a focal length > 70mm are usually regarded as being Telephoto. Although 1200mm lenses are available, the majority of consumer telephoto lenses top out around 300mm.

When using a long focal length lens, camera shake becomes a big problem, and the camera in general will be unusable without a tripod or sturdy resting point.

---

DSLR Focal length multipliers

Included with the documentation for any DSLR should be a multiplier value. The multiplier value converts 35mm equivalent focal lengths into the true focal length for that camera. So for example on a Canon 350D with a multiplier of 1.6, a 18-50mm (equiv) lens is actually a 28.8-88mm lens.

This has a few implications

• When looking for a telephoto lens, shorter focal lengths can be substituted, meaning that lighter lenses can be used, which will cut down on camera-shake.

• Wide angle lenses become not so wide. If a (super expensive) 10mm lens was fitted to a 350D, it would become a not so-wide 16mm lens.

• Usually with long lenses, it comes at the expense of having a large aperture. However, in the case of digital cameras, although the focal length gets multiplied, the maximum aperature is unaffected. This means that “fast” lenses stay fast when coupled with digital cameras. Another upside to this is it reduces the cost of lenses: a 300mm f/5.6 lens with a 1.6 multiplier becomes a 480mm f/5.6 lens — compare this with the price of a true ~480mm f/5.6 lens!

References

• Measuring Focal Length. How to accurately measure the focal length of a lens.

• Wikipedia: Normal lens. Wikipedia Entry for “Normal Lens”

• The “Multiplier” Factor. Discussion on what the multiplier for a DSLR means.

← previous entry | next entry → Wed 2nd Aug 2006 - 21:43 | no comments | tagged with