Focal length, the simplest and most important number of them all!

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If there is one value that you must understand to gain true photographic enlightenment, it is the focal length (ƒ). This statement may seem somewhat counterintuitive given that for many, focal length is just a fancy way of saying "zoom". However, the truth is that ƒ is the single dimension upon which the entire convoluted system that we call photography is founded.

In a world where we are familiar with camera lenses that comprise numerous aspherical elements glazed with Super Spectra coatings, one can be forgiven for asking precisely what focal length is.

Unusually, by photography standards, the answer is decidedly simple. If during your youth, you tried to set objects alight with the aid of sunshine and a magnifying glass – a luckless ant, perhaps – then you are well versed in the very essence of focal length. Moving the magnifying glass back and forth trying to focus the dot of light for maximum destruction is, in effect, the same as finding its focal length. Measure the distance between the smouldering subject and the centre of the lens, and you will have discovered ƒ.

I admit, modern camera lenses are certainly more complicated than a single lens magnifying glass, but their impact on the world is still modelled in precisely the same way. A 50 mm lens, which may well be packed with more glass than a cathedral, is conceptually equivalent to a simple single lens magnifying glass designed to focus all its light on a point precisely 50 mm away from its centre.

To express just how deeply the simple measurement of focal length is embedded into photography is beyond the scope of this little blurb, but can you take my word for it, it's deep. The most obvious manifestation is, of course, so-called "zoom" but other notable occurrences include the aperture value, which is expressed in terms of focal length, as well as the associate depth of field.

We will dig deeper into the nuances of focal length and photography at a later date, so, for now, just make sure that you understand exactly what focal length is and if you're struggling with the idea, try playing with a magnifying glass.

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