Coming from an automatized world where we are used to take fast pictures in automatic mode with our phones or compact cameras, the first time we try to approach to manual mode can result in an overwhelming experience: we change from using just one button to take the picture to a full set of options that we barely know how to manage. However, explaining each parameter one by one, without interference of the others, can lead to a very easy understanding of how each of them affects the overall image. To start with the basics of exposure we are going to talk about aperture, as the first parameter every photographer should be able to understand in order to get better and more creative images.
The aperture of a lens is the central space, filled with the glass pieces that form it, that allows the light to pass to the sensor. It is one of the most important parameters in artistic photography and one of the three parameters that determine the exposition of the final picture (being the other two the shutter speed and the sensitivity of the sensor). In the “real world” we tend to refer to the aperture as the area of the circle that allows the light to pass, but “photographically” we will see a more interesting way of representing it.
The maximum aperture in a lens is the one that allows the light to pass without obstruction to the sensor. It’s a characteristic value of the lens that represents how fast the lens is (the maximum aperture of a lens is usually printed in the front of it, next to the focal length): the greater the maximum aperture is, the more light the sensor receives and higher shutter speeds we will be able to reach with a correct exposure. It is obvious that in a low light environment we will want the lens that most light can capture, although many times other factors, as price or weight, can be the real limiters, as the faster a lens is, the more expensive and heavy it gets.
The maximum aperture of the lens is determined by its construction and cannot be changed, but what we can do is to reduce the aperture of the lens. The piece that makes this possible is the diaphragm, which is a device composed of 5-18 blades that can fit together to form a circle of lower area than the maximum aperture. If we reduce the area to the half, we will allow half the light to reach the sensor. The diaphragm can close to get lower and lower apertures until a point where it occludes all the available space.
Diaphragm on a Korinar 35 mm lens closed to f/16.
Measuring the area of the circle is tedious and little practical, so in photography we use the f number to refer to the aperture. The f number represents a relationship between the diameter of the current aperture (D, in millimeters) and the focal length of the lens (F, also in millimeters; the focal length is what many people call “the zoom”). In mathematical terms, it is expressed as:
f = F/D
This means that in a 100 mm lens an f value of f/4 gives an aperture of 25 mm in diameter, and an f number of f/2 gives an aperture of 50 mm in diameter.
Because the relationship between the diameter and the area of the circle is quadratic (the diameter is squared), doubling the f number doesn’t reduce the light to the half, but to one fourth of it (two squared). So if we have an aperture of f/2 and we want to half the light we will have to multiply the number by the square root of 2, which results in f/2.8 (2.8 = 2 · √2), and not by two. This gives us the following values as a guide, assuming we start counting on 1 (if necessary, you can calculate the f numbers on the left of the 1 by dividing successively by √2), each one of them representing half the illumination of the one on its left:
Image by Cbuckley (Wikipedia)
Each one of these values is called a stop. If we advance one stop to the right we are letting half of the light to pass. Advancing two stops means that we cut the light to one quarter. The advantage of this method is that the aperture is standardized, as it is given as a relationship with the focal length. This means that an f/8 aperture gives exactly the same amount of light on any lens in any focal length. It doesn’t matter if you are using a brand new 200 mm or a 35 mm from your grandparents’ first camera, an f/8 means always the same amount of light.
In order to fine tune the aperture, usually the cameras allow us to set halves or thirds of stop, being the thirds the more common and precise. The numbers in this case can be a little bit odd but working with them is easy in practice. If we have the camera configured in thirds mode, every time you turn the dial one “click” to set the aperture you are moving a third of stop, so every three clicks you are doubling or halving the aperture. The same applies to a camera configured in halves, but with only two clicks instead of three.
A simple example: We have achieved the right exposition for a scene with a 50 mm lens using a shutter speed of 1/100 s and f/4. If we wanted to take the same picture with a 100 mm lens and a slower shutter speed of 1/25 s, what would be the correct aperture to set? First of all, as we are using f values, the focal length of our lens doesn’t matter at all and we can forget about it. According to our exposition time, we are passing from 1/100 to 1/25 of second. If we divide 1/25 between 1/100 yields a result of 4, which means that we have doubled our exposition time twice. Another way to see it is that in a thirds configured camera we have to take 6 “clicks” to get from 1/100 to 1/25 (3 clicks to double from 1/100 to 1/50 and another 3 clicks to double from 1/50 to 1/25). As we have doubled our shutter speed twice (we have multiplied the amount of entering light by 4) we have to correct the aperture in the same amount to reduce the excess of light, going from f/4 to f/5.6 (one stop) and from f/5.6 to f/8 (two stops); or in clicks three clicks to get to f/5.6 and another three to get to f/8. As we see, knowing if the camera works in full increments, halves o thirds is usually enough to calculate the best aperture knowing the correct exposition for the picture we have in front of us.
Now that we know how the aperture affects the exposition and how to use it the question that arises is: Why would I want to reduce the aperture of my lens and let less light to reach the sensor? The answer will be the topic of the next post, in which we will see the effects of changing aperture maintaining the correct exposition.