Understanding Aperture Sizes

What is an Aperture? An aperture is the variable opening in a lens that allows light through. If you were to hold up a lens from an SLR camera and look through it from the rear, you would see the aperture situated midway between the front and rear lens elements. Iris diaphragm made up of a few overlapping metal blades which enable the size of the aperture to vary from nearly the full diameter of the lens to just a small hole. This feature will determines the how many amount of light that falls on the film or sensor. For SLR lenses, this aperture size can be changed via the aperture ring on the lens or via the dials on the camera, with the latter the method most commonly found on consumer-level digital cameras with fixed lenses.

Why do cameras have variable apertures? The key reasons for having a variable aperture are: (1) to vary the amount of light passing through the lens so as to achieve the desired exposure when using different shutter speeds to shoot photos, and (2) to affect the dept of field in the resulting image.

How are variable apertures expressed? Although, apertures are variable circles, they are not measured by a linear scale like the length of their diameters or radii. What is more important to a photographer is how a change in the size of the aperture will affect the amount of light being allowed to pass through the lens. However, this is not only depends on the size of the aperture, but also on the focal length of the lens. In the example mentioned later, you will see that a lens with a large aperture size and a long focal length allows the same amount of light to pass through as one with a small aperture size but short focal length.

In other words, aperture settings are expressed as the ratio of the aperture diameter to the focal length of the lens. These ratios are represented by relative values known as f-numbers that also indicate the amount of light being allowed to pass through the lens. The f-number system is a universally agreed standard, which means that a lens set at a particular aperture setting, say f/2.8, will let the same amount of light through and produce a matching brightness to another lens also at f/2.8, irrespective of their differences in focal length or manufacturer.

F-numbers are calculated by dividing a lens’ aperture diameter by its focal length. Simply put, an f-number expresses the diameter of the aperture in terms of the effective focal length of the lens. Lets use this example to demonstrate: A lens that has an aperture diameter of 20mm and a focal length of 160mm will have a matching brightness as another lens with an aperture diameter of 10mm and a focal length of 80mm. this is because in both cases, dividing the aperture diameter by the focal length will give you 0.125, or 1/8. However, expressing this ratio as the fraction rather than the decimal number is more convenient and causes less confusion. As f-numbers are expressed as fractions of focal lengths, the f-number for the aperture settings of the lenses in the example above can be written as f/8 or F8. Because f-numbers are relative fractions, the smaller the relative aperture, the higher the numerical value of the f-number.