Copyright Michael Karbo, Denmark, Europe.
Chapter 17. Metering and Exposure
Metering the exposure is a brilliant little arrangement, which often isn’t noticed at all. It just works. Time and time again it ensures a successful exposure because a camera’s diaphragm and shutter speed can be adapted to the subject’s light.
At the same time there are usually two or three variants of exposure meters in the camera. Each of them has its particular advantage, which can be used in different situations – if you are aware of them!
Automatic light metering
The cheapest cameras only have automatic exposure programs, which manage both light measurement and setting the exposure parameters. But most other cameras have a handful of ”tools”, which can in different ways be used to set the camera. In most of the more expensive cameras, you can find:
All these functions help in different ways to ensure that an image is correctly exposed. In this way, the subjects’ “light metering” and “exposure control” becomes extensive when we start digging down into them.
Most photographers use a camera’s fully automatic programs, where you don’t have to worry about these conditions – the camera does everything by itself and the result is often very good indeed.
The various settings can, however, have an influence on the depth sharpness (diaphragm), shaken exposures (with long shutter speeds) and digital noise (with high ISO values).
A technically clever photographer understands the theory behind a diaphragm, shutter sensitivity, etc. and keeps an eye on the exposure values the automatics choose. There are lots of ways of correcting the individual exposures oneself.
But first and foremost you have to be familiar with the different types of light metering options the camera has. There are usually three systems of light metering, which can be used both in the manual and the automatic exposure programs.
Light metering through the lens
This is called metering or photometry. The image sensor functions as a light meter and, therefore, the light metering is done directly through the camera’s objective. This is called TTL, Through The Lens, and is a very effective way of measuring light.
Figur 65. Options for alternative light meters are seen here (from Fujifilm and Nikon)
The simplest way of metering light is by using the camera’s computer to measure the lighting in the whole of image sensor’s area. This is called an average metering – the computer finds an average of all the light, which hits the image sensor and then calculates the exposure from this.
Average metering was earlier used in all cameras and is still found in many models. It is, however, seldom sufficient to measure the average of light over the whole of the image surface. Very few subjects will be perfectly exposed by doing this. The main subject is usually only a small section of the total area of the image and it is the main subject, which ought to be properly exposed.
If the background is lighter than the main subject, then it will usually be underexposed. If the background is dark, the average metering will adapt itself to that, and the main subject will be overexposed.
In addition, you ought to be aware of the fact that a light meter usually can’t register colours; it only measures light in a grey colour scale, which ranges from black to white. This can in some situations thoroughly lead the light meter astray.
The three most used systems for light metering.
Light metering is in some ways a bit of a “matter of taste”. Professional photographers don’t, in fact, always want the same sort of lighting and exposure calculations, as happy shooters’ do.
So when engineers design programs for a camera’s automatic exposure program, they have to make compromises. With this knowledge, it is a good thing if you are familiar with your camera’s different light meters, to that you can compensate for their weak sides.
Figur 66. The background is well exposed but if the subject is the monument in the middle, then the lighting is completely wrong. The light meter has put too much importance on the background. This happens often with automatic light metering. It would have been better to use spot metering here.
Three methods of metering
If a light meter measures the whole of the subject and calculates an average lighting from this, the result will seldom be successful. This is why other methods of light metering have been invented, where the image is divided into various sections, which can be analysed individually. There are three to four ways of doing this, which are found in most cameras:
All these metering methods can be used in connection with both fully automatic and the more manual exposure programs.
A camera is manufactured to use one of the methods (preferably multi-segmented, which is the most advanced) so many new users only use this method. But there are alternative ways of metering.
There is a little button on some cameras so you can switch between the three types of light metering. In other cameras it is controlled via the menu system. What is important is that the choice exists and that the photographer knows how to activate it!
The light meter chosen is displayed on the LCD screen. It is usually a very small icon, which shows, which of the three types of metering is active.
Figur 67. The button used to select light metering can be difficult to find on the camera. Be aware of this! Here from the left you can see spot, center-weighted and multi-segmented metering.
A light meter can’t think by itself. It just measures the power of the reflected light in a particular area – as it has been told to. But we would like to have cameras, which could almost think by themselves so that light meters could at least come up with the information that could enable a camera to conjure up completely perfect exposures in all situations. This requires ”intelligence” in a camera and one of the ways of achieving this is called matrix- or multi-segmented metering.
The best and most overall form for light metering is when an image is divided into a number of segments, which are individually analyzed. The image area is divided into, for example, 8, 64 or 300 fields, where the luminosity is measured individually. Afterwards, all these meter results are collected together by the camera’s image computer, which then analyzes the information by comparing them with a library filled up with thousands of light meterings from well-known images.
This system is often called evaluative or matrix metering. Every manufacturer has his variant of the system, which has many different names.
Figur 68. The image is divided into a number of segments, which are metered individually.
There is a great deal of secretiveness in connection with these systems, so it’s not easy to get information just like that about how they work. Nikon is known to have a very efficient matrix light meter. The image’s area is divided into 256 fields and the light is measured individually in every field. The individual meterings are then compared with the information, which has been stored in a database with data from typical photographs.
Database of light meterings
The image computer is ”coded” with information about how photographs usually are exposed. Some models even include a complete database of light meterings from more than 50.000 exposures.
The image computer compares the light meterings from the segments with the corresponding meterings in the database in order to find an exposure, which matches. When a match is found, the exposure is defined according to this comparison.
Try to imagine, for example, that the topmost part of the image file is mostly one-coloured (blue) and light. Light blue colours are registered in all the upper segments. Then the camera’s software will guess that this is a sky from the information in the database. In the daytime a sky is always lighter than the rest of the subject, which is why a light meter will “play down” the sky when calculating an exposure.
Segment metering is an efficient system for automatic light metering. It is often successful but it is important to remember that it is automatic. The system will fail in many situations. The lighting can be very special – i.e. with large contrasting surfaces – so that the segments misinform the program and the computer completely miscalculates the exposure. This is why it is important to be familiar with the cameras other options.
Figur 69. Matrix or multi-segment metering gives the best results, if you want full automatic light metering.
Center-weighted average metering is a more manual method of metering without great ”intelligence”. It is a classical system, which is standard in those cameras, which don’t have the more advanced multi-segment meters. A center-weight measures the light in the whole of the image field and then takes an average of it. This is a development of normal average metering. As the light in the image’s center is seen as the most important, then it has much more emphasis on the calculations than the rest.
Center-weight average is a good system for light metering when the main subject is the middle of the image area. Typically portrait photography, where the light metering is concentrated on a person but where the details in the background should also be retained. It is often used in connection with the function exposure lock.
Figur 70. Center-weight light metering calculates an average of the light in the whole of the image’s area but with, for example, 80% emphasis on the area in the center.