Exposure with an Exposure Meter
Exposure meters come within many cameras but are also available as free standing accessories.
Most cameras today have a built in exposure meter, some several. Most Nikon cameras for example have 3 built in meters:-
Matrix or Evaluative metres
These look at a range of areas within the image and base the exposure on making the majority of the areas able to be seen without editing. Manufacturers sometimes say this is done by comparing the results obtained with a pattern made up from very many photographs. This is the form of metering used the majority of the time by most photographers today. The down side is that in order to make the general image viewable without editing, it often cannot cope with the exposure range required. Different manufacturers will have different solutions, Nikon images for example often allow skies to be blown (white'd out) loosing detail. You can use an exposure variation to make images darker and by this means keep in more sky detail, but then it requires editing to bring out the detail in the darker area.
Central Weighted Metering
This method takes an average exposure over the central area of the image, often a circle that's size can be adjusted in the camera menus. With some cameras you can also set it to average over the whole frame. Some photographers like to use centre weighted metering for portraiture suggesting that metering off the face is the solution.
The spot in most cases is the focusing point, so can be moved around the frame. With this method whatever you point the spot at becomes mid grey in tone, but not colour. Grass is often considered to be mid tone, and used to set exposures in landscapes.
It is the spot meter that is used in conjunction with Exposure Targets and some of these have, as well as a mid tone grey, tones representing one stop up and down.
Spot Meters used with a Tonal Range
You can also use spot meters to explore the range of tones in an image, then select an exposure to capture the range you want, often allowing you to capture the best detail in images that would have lost highlights or shadows with other methods. The problems encountered in doing this are in having something that is comparable, and this can be overcome by selecting a set of standard conditions, for example ISO and shutter speed and then looking at just what aperture change occurs. If this is looked at then as an EV value, the easiest way to do this being to use an EV table, then you can get to see and understand the tonal range of your image.
In past times a photographer, particularly with a medium format or larger format camera, would in a day take a very small number of photographs and strive to take each perfectly, while today the tendency is to take a very large number of images and rely more on the technology and editing , plus selecting the good from the volume. In this rush its often argued that there is no longer the time to consider and take individual photographs that require as much thought and work. Some point out that in a series of photos it is only the first that requires this level of thought.
In practice, although it does slow down a photographer, when the maximum effort is being made to get the perfect image, generally an understanding and use of this is not as demanding in terms of time or mental activity as many imagine. Those of us who use this and the applied form, known as Zone Systems Photography, because we don't need to apply it all the time, we can use it with difficult situations and use matrix metering for at least 80% of images. Some cameras such as the Nikon D200, D300, D3 etc have one or more function buttons that can be programmed to do things when pressed and one of these is to spot metre allowing matrix to be generally selected but the spot metering used to check any situation quickly. This is particularly easy to use and apply when exposure variation is added to the spare command dial via another menu setting.
The Zone Systems just extends this into a systematic approach with known tonal range values, allowing spot metering to be done on any tone in the image, and the correct exposure adjusted from this. Zone Systems Photography is thought of as an academic, difficult, and complex technique, and there are largish books written on the subject, but what is known as the modified or simplified zone system is used by many professionals as it is both easy to understand and apply.
Reflective and Incident Readings
Normally when looking through a camera we are taking reflective readings, measuring the light that is reflected from our subject. Incident readings are where the light is measured that is falling onto a subject rather than reflected from it.
Many accessory meters have a cone or other means to allow an incident reading to be taken. The reading you get from incident reading should be the same as a reflective reading, obtained using a spot metre and measuring the light from a mid grey card or target. So although cameras cannot usually measure incident readings as such they can get exactly the same affect by using an exposure target in conjunction with their spot meter.
The free standing exposure meter was very popular at one point, before inbuilt camera meters became as common and reliable. At one time few professional photographers would go out without a meter and those who could afford them had Weston meters. Today there are a range of electronic meters available. You can also now find second hand many of the older meters including quality Weston meters, on places like eBay you may today pick up a Weston for under £30. We have two Weston's, although they don't get used very often. I favour the older non electronic meters simply because they don't get used very often and these older meters don't have a battery that can go flat or corrode.
So why use a hand held meter at all, well I can get a direct reading of EV without needing to compare values on the EV table to deduce them, and there are occasions where I am photographing something that may be too fragile to lean a target on and where I want an incident reading, for example some types of mushrooms. In addition I also need a meter to teach some people advanced exposure techniques, allowing them to see the difference in tone at different points, for example when using reflectors, or comparing what is in the light and shadow or contrast when using a diffuser.
The modern electronic meters can be quite clever bits of electronics, undertaking calculations, having sighting devices and more. However many who buy them are looking for an electronic crystal ball, hoping it will solve their exposure concerns without them needing to understand what they perceive to be a complex area. As it is often more difficult to learn to use the complex functions of the electronic meter than it would be to get to use the facilities in their camera and understand the basics of exposure, this makes little sense to me.
Some electronic meters also function as a flash meter, I have a separate flash meter I use with studio flash. What the flash meter does is to look out for a bright pulse after you tell it to look out, then records that peak of brightness. You use it with manual flash, allowing you to even up or adjust the lights and exposure. It's not necessary when using the Nikon Creative lighting system as the camera tells the flash units to send out test pulses that it analyses and is by this means able to set individual groups of lights and the overall exposure.
for details on Exposure Article Route see the Exposure page