Copyright Michael Karbo, Denmark, Europe.
Chapter 15. Controlling the light
A camera’s most essential task is producing a well-lit exposure. But what does this mean? It means that a camera has to gather light through an objective. The light is reflected from the subject; it comes, for example, from the sun, which shines on the earth and the sea. From there the light is reflected into the camera’s lens.
There is a lot of light when we take a photograph of a landscape in the middle of a summer day. There is less light if it is cloudy or if the sun is setting. A camera has to adapt itself to these and many other lighting conditions.
The human eye can adapt to varying amounts of light. It happens quite automatically with the help of the pupils, which open and close themselves dependent on whether the light is weak or strong.
The trick is to expose the images properly in any situation. It can easily be done if a photographer is familiar with the diaphragm, shutter speeds, alternative exposure meters, etc. A camera has three mechanisms, which adapt an exposure to the light:
These three parameters can be varied individually. They have to be harmonized so that the image sensor receives the right amount of light. It only takes a little too much to make the exposures either overexposed or underexposed.
An aperture is a hole in an objective, which can be increased or decreased in size. This is done quite automatically with the help of some small metal plates inside the objective, which can be regulated so that the opening changes size.
Aperture size is described by a diaphragm step, which is a numerical value from a special sort of scale, which looks like this: 1 – 1,4 – 2 – 2,8 – 4 – 5,6 – 8 – 11 – 16 – 32. The secret is that for every diaphragm step the amount of light is halved. When the diaphragm number is increased, the hole in the object becomes smaller.
Technically speaking the diaphragm number describes the relationship between the focal length and the diameter of the diaphragm.
Focal length (popularly speaking) is the distance between the objective’s backmost lens and the image sensor. It could, for example, be 8,8 mm. If the hole in the lens is 1,1 mm in diameter, then we have diaphragm 8:
When calculating amounts of light, it is done in steps, where the amount either is halved or doubled. This is also the case with a diaphragm scale. If you, for example, change from diaphragm 8 to diaphragm 4, then four times as much light will come through the objective, because there are two whole diaphragm steps from diaphragm 8 to diaphragm 4. With every diaphragm step the amount of light is doubled, when we increase the sized of the aperture. Diaphragm steps, by the way, are described with a little f or a capital F in front of the numerical value. That’s how you know the number describes a diaphragm step.
Figur 52. A diaphragm scale has a fixed number of steps, each of which halve/double the amount of light in proportion to the next diaphragm step. The changes take place quite automatically when the hole in the lens is made bigger or smaller. The larger the aperture, the more luminosity the lens has.
The lens' luminosity
An lens has a fixed luminosity, one says. If the objective’s largest aperture is 2,0, then the lens is more luminous than if the largest aperture is diaphragm 2,8.
We talk about fast objectives, when the aperture is large. In this context, fast means that the image, due to high light sensitivity, can be taken with a low shutter speed. This is why it is an advantage if your camera has a large aperture. The lower the shutter speed, the faster an exposure is taken.
You can always read the diaphragm step on the camera’s LCD screen or in the little display, if there is one.
Figur 53. The biggest diaphragm is displayed in text on the front of the objective. Here it is between 2,8 and 3,1 – dependent on how far the camera has zoomed into the subject.
Figur 54. The camera’s LCD display shows the aperture.
Don’t start thinking that all objectives have the same sort of diaphragms. A diaphragm scale has, in fact, 12 steps from 0,7 to 32, but all the steps are definitely not to be found in every objective. In fact, there are relatively few diaphragm steps in a digital camera.
An lens can, for example, work from diaphragm 2 or maybe to diaphragm 8. This is normal for the objectives in small ordinary digital cameras. The luminosity cannot be varied very much with the help of the diaphragm. It is expensive to produce luminous and fast objectives; a big aperture requires large and expensive lenses. If the focal length is, for example, 21 millimeters, then the aperture has to be able to be opened with a diameter of 10,5 mm, if the camera’s largest diaphragm is 2,0 (then 21 : 2 = 10,5).
Figur 55. A simple camera only has a small part of the full diaphragm scale at its disposal.
If you instead choose 2,8 as the largest diaphragm, the biggest aperture only needs to be 7,5 mm in diameter (21 : 2,8 = 7,5). This makes the lens a lot smaller in diameter and cheaper to produce. The cheapest cameras, therefore, never have particularly luminous objectives.
Figur 56. The bigger the hole (diaphragm diameter), the lens has, the more luminous it is and the shorter shutter speeds necessary for taking images. The focal lengths displayed here are the shortest a camera can work with (wide angle image with 1X zoom).