[Note: This post is intended for novice, and not experienced, photographers.]

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In photography, the aperture is the adjustable opening of a lens that determines the amount of light entering through it and onto a camera’s sensor, be it digital or film.  If the aperture is large, a large amount of light will pass through the lens and enter the camera; if the aperture is small, a small amount of light will enter the camera.

Simple, right?  But, it gets a little confusing…

The problem with using the entrance aperture size to communicate a lens’ light transmitting ability is the physical reality that a longer lens will require a larger aperture to achieve the same level of light transmission as a shorter lens.  In other words, if “x” is the amount of light we wish to reach a camera’s sensor, a 75mm lens will require a bigger “hole” (aperture) at the entrance than a 50mm lens to achieve “x”.

To avoid confusion, and to standardize notation across all lens types, f-numbers — instead of actual aperture sizes — are used to communicate the light-transmitting ability of all lenses.   An f-number (also known as an f-stop, or focal ratio) is defined as the focal length of a lens divided by the aperture diameter.

You’ll usually see f-numbers labelled on lenses as a sequence of fractions:

f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, etc.

The important thing to remember is that because the f-number is a fraction, the smaller the denominator, the greater the light transmission.

(please click on the image to view LARGER)

Thus, when considering light transmission, f/1 > f/1.4 > f/2 > f/2.8 > f/4, etc.

In fact, for every step to the left in the sequence above, twice as much light is being transmitted as the preceding step.  Thus, f/1 allows for twice as much light to hit the sensor as f/1.4, and f/1.4 allows for twice as much light as f/2, etc.

Incidentally, lenses that are able to achieve maximum f-numbers of f/1, f/1.4, or f/2 (or even f/2.8) are often loosely referred to as “fast” lenses because they allow photographers, for a given amount of light, to shoot at faster shutter speeds than so-called “slower” lenses (exactly twice as fast for each step up in the f-number ladder).

It follows from the above, then, that large aperture/f-number lenses are useful in low-light situations:

↑Leica M9 and Konica Hexanon 60mm @ f/1.2.

The other main use for large aperture/f-number lenses is to achieve subject isolation.*

Thus a lens of a given focal length set to f/1.4 (for example) will create a greater background blur than when set to f/4.:

↑Leica M9 and Leica 75mm Summilux @ f/1.4

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↑Leica M9 and Leica 75mm Summilux @ f/4.

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I hope you found this useful,

—Peter.

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*The other way to achieve subject isolation is to shoot with a longer focal length.