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WHAT IS… Focal Length?

Focal Length Defined

When parallel rays of light strike a lens focused at infinity, they converge to a point called the focal point. The focal length of the lens is then defined as the distance from the middle of the lens to its focal point.

[Note that since most camera lenses are made up of more than one lens elements, the definition "middle of the lens’ is adjusted slightly.]

The focal length of a lens is usually displayed on the lens barrel. Pictured below is a Nikon lens with a focal length of 50mm. The maximum aperture is f/1.8 (also often written as, F1.8).

Nikon 50mm f/1.8 AF Nikkor

Normal, Wide-Angle, Telephoto, Zoom
We usually categorize lenses as having a wide-angle, normal or telephoto focal length. A normal 35mm lens covers a 24x36mm film with a field of view that corresponds approximately to our normal vision; a lens with a focal length of 50mm (55mm is also popular) is considered as normal. Any lens with a focal length less than 50mm (or 55mm) can be considered as wide-angle; any lens with a focal length greater than 50mm (or 55mm) can be considered as telephoto. A zoom lens offers a range of focal lengths. The table below lists some of the more popular focal lengths.

Lens Popular Focal Lengths for 35mm cameras
Wide-angle 18mm, 20mm, 28mm, 35mm
Normal 50mm, 55mm
Telephoto 90mm, 135mm, 200mm, 300mm

35mm Film (135 Format)
Do not confuse ’35mm’ lens with a focal length of 35mm. The former refers to traditional 35mm film camera format (also known as 135 format). In other words, ’35mm film camera’ refers to the 135 film format — not the lens format. [35mm is the width of the film including the sprockets.]

35mm Equivalent

Nikon
28-105mm
f/3.5-4.5D AF-Zoom Nikkor
Nikon Coolpix 5400: 5.8-24mm
(28-116mm, 35mm equivalent)
f/2.8-4.6 Zoom Nikkor ED

In the example of the two Nikon lenses illustrated above, the lens on the left is a 35mm film camera zoom lens with a focal length of 28-105mm. It is a popular zoom lens because it provides a good wide-angle (28mm) as well as enough telephoto reach for a good portrait lens (with 105mm, you can fill the screen with a face without getting ‘in your face’ too close to your subject).

The lens on the right comes with the Nikon Coolpix 5400 and has zoom focal lengths of 5.8-24mm. Believe it or not, this has a 35mm equivalent of 28-116mm!

Because the image sensor size used in digital cameras are of different sizes, the same focal lengths may be expressed using different numeric values. Smaller image sensors can use smaller lenses; larger image sensors require larger lenses to ensure all the surface of the image sensor is covered. Other factors, such as amount of optical zoom provided, may further affect the distance between lens and image sensor.

By using the 35mm equivalent, we simply make it more convenient for everyone to use a handy reference point. Most, if not all, camera manufacturers will list the 35mm equivalent in the specifications of a digital camera. Interestingly, some digital cameras are now even engraved with the 35mm equivalent on the lens barrel to express the focal length of the lens. It makes more sense in a way, though it is important to bear in mind that 35mm equivalent does not mean 100% compatibility with the real 35mm lens. This is especially true in the area of depth of field.

Focal Length Multiplier
If you use digital SLRs (dSLRs) that use lenses made for 35mm film cameras, you need to be aware that the focal lengths expressed on the lenses must be multiplied by a factor. That factor, the Focal Length Multiplier (FLM), depends on the image sensor size used.

Of course, if the image sensor is full-frame, i.e. it is the same size as 35mm film, then the multiplier is 1, and the focal length of the lens is accurately represented.

However, only a few dSLRs use full-frame image sensor, with most using a smaller image sensor, usually APS size (or roughly half-frame). That is why you will read that a focal length multiplier of, say, 1.6 needs to be applied to the focal length of the lens to obtain the "true focal length."

So, for a dSLR using an APS-sized image sensor, the focal length of a 55mm lens becomes approx. (55 x 1.6 = ) 88mm lens. This is both good news and bad news. The good news is that you can now achieve super telephoto coverage without buying costly and unwieldly lenses. For example, a 100-300mm zoom lens, with a focal length multiplier of 1.6, now acts like it was approx. 160-480mm. The bad news is, of course, that your 18mm super wide-angle lens is now acting as though it were approx. 28mm, making it almost impossible to obtain the kind of super wide-angle coverage that most professionals desire.

When you use a 55mm lens on a FF DSLR, the field of view you get is 55mm.

When you use a 55mm lens on a APS DSLR, the field of view you actually get is not 55mm but (55 * FLM) = 88mm, if FLM = 1.6.

I.e., a 55m lens used on an APS DSLR gives the same field of view as an 88mm lens used on a FF DSLR.

To get the same field of view of a 55mm lens as used on a FF DSLR, you will need to use a (55 / FLM) = 34mm lens on a APS DSLR.

Some readers have objected to our use of the term "focal length" in our explanation above and have insisted that the technically correct term is "field of view." They are right, of course. So here’s the technically correct version: "The focal length of the lens used on a full-frame sensor or an APS-sized sensor is the same (i.e. achieves the same magnification) but since the smaller sensor only captures a smaller central portion of the image projected by the lens, the field of view obtained in the latter case gives the impression that the lens is acting as one with focal length times the FLM."

However for all practical purposes, you might find it just easier to simply multiply the focal length by the FLM, and think of that as your "true focal length." ;o)

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