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Exposure

In this chapter we'll see how the amount of light captured on the image is controlled, as a function of three fundamental parameters: aperture, shutter speed, and ISO sensibility.

Aperture

The lens has an iris that can be opened or closed to allow more or less light in. The size of that opening is measured on a scale (f/) which you don't need to know exactly what it is, but that works as follows:
* the smaller the value, the more light gets in
* the relation is quatratic
For example, f/4 allows twice as much light in as f/5.6: (5.6/4)^2 = 1.4^2 = 2.

An increase of aperture that allows twice the light in is called a stop. Most cameras allow you to control aperture in thirds of a stop (with three clicks you allow half as much light in: 4.0 - 4.5 - 5.0 - 5.6) or half stops (two clicks: 4.0 - 4.8 - 5.6).

Defining quantity of light = 1 with f/5.6 in a completely arbitrary manner, the following table shows how much light is allowed in with some common aperture options:

f/ 1.41.61.822.22.52.83.33.544.55 5.66.37.1891011
light16.012.710.18.06.35.04.03.22.52.01.61.3 1.8.6.5.4.3.25


The problem with allowing more light in by increasing lens aperture is that:
* there is a limit: every lens has a maximum aperture, and you can't open it beyond that (that's why it is important to have fast lenses, i.e., ones with wide aperture, for example many primes, and, to a lesser extent, some zooms)
* in general when we approach the maximum aperture of the lens we lose sharpness in the image, specially in the corners; this depends on each specific lens design, so you should test and investigate your kit

Here are my sharpness tests with all my lenses, where you can see how at wider apertures the corners tend to become softer. Another phenomenon that you can't see in those tests, is diffraction: when aperture is very narrow (generally beyond f/11 or f/16) you also lose sharpness, in this case in the whole image, not just in the corners.





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