Guide To Using A Hand-Held Light Meter

Metering

The main issue with metering, no matter how sophisticated it is, is that it's computed to ensure the subject comes out as an average of the scene it's measuring. In simplified terms, the scene's tones are scrambled up inside the camera's metering cell to calculate a single brightness value, which is then used as the basis for the shutter speed and aperture selected. The camera will ensure this single brightness level is equivalent to a mid grey, which is fine when the subject has a wide variety of tones, or is predominately of a brightness similar to mid grey. But things go wrong if the subject is all white, such as snow or a white car, or all dark, such as a black suited person or black car. In these situations the camera will adjust the exposure to compensate and ensure the snow comes out grey and the black suit comes out grey. If you use a hand-held exposure meter and point its meter sensor at the subject the same thing happens. This type of meter reading is known as a reflected reading, as it measures the light reflected from the subject, but a hand-held meter can be used differently and that's why they are still very popular, despite the availability of advanced camera metering systems.

By turning the meter around you can use the sensor to measure the light falling onto the subject, so it doesn't become fooled by the subject's reflective qualities. This type of metering is called an incident reading and can produce very accurate exposures.

How it works

To use it you have to slide a dome over the metering cell, which is usually a 180 degree translucent plastic diffuser. The meter then reads the light falling on the subject from all angles. It still scrambles the reading and adjusts the exposure to produce a mid grey, but this reading hasn't been affected by reflective or absorbent subject matter and will ensure that the dark subject stays dark and a light one stays light. If you've ever seen wedding photographers walking up to the bride and holding a gadget up to her face, now you know what they were doing.

This illustrates how the two types of reading would be taken when photographing the garden pagoda. An incident measurement is being taken on the left and a reflected reading on right.

Even very basic meters have this type of measuring, but the more you pay the more modes you get. Basic meters have a very simple full stop exposure adjustment, while more advanced ones can measure to an accuracy of 1/10th of a stop. Some meters can measure flash too, making them versatile in the home studio and some have a spot attachment that narrows the measuring angle down to between 5 and 1 degree.

Using a meter in ambient light

The first thing to do is set the ISO on the meter. On basic models this is a dial that you rotate, placing the ISO you are using against a marker. On the more expensive LED and LCD models you key in the number using up and down buttons, like you would on an electronic camera. You then press a button on the meter to activate it. Holding the button in on some locks the exposure on others it allows a continuous reading so you can move around checking the exposure levels around the subject. More advanced models allow multiple selection, where you can take a reading from, say, the highlight and one from the shadow and the meter then computes the average.

Taking advanced reflected readings

As reflected readings are more prone to errors one way to help you meter correctly is if you start to visualise pictures in black & white. By splitting the subject into grades from 0 (black) to 10 (white), with mid grey at 5, it's easier to evaluate the exposure required to ensure the subject appears the correct tone. This method of metering was developed by legendary landscape photographer Ansel Adams and is known as the Zone System where each brightness value is given a zone rating. If you struggle to see colours as greyscale, a filter called the Mono View is available to help you see in black & white.

You would then take a reading knowing that it would create a mid grey, whatever your subject matter, then you would adjust the reading to over or under expose, making the subject go whiter or blacker.

The easier approach

An incident reading saves you trying to understand how the Zone System works and is the quick and accurate approach to photograph difficult lighting situations.

When using a meter in incident mode on subjects that are front lit, just point the meter, with the diffuser cone in place over the sensor, towards the camera, making sure that the meter is positioned in the same light as that falling onto the subject. The reading will not be correct if you hold the meter in an area of shade, while the subject is basking in sunlight.

If you are photographing a distant subject using a telephoto lens and can't get the meter near, the reading will be the same, providing the same light is falling on the subject as the metering cell. For instance, you may be in an office that's illuminated with fluorescent lighting and want a long shot of an employee. A person sat at their desk nearby can be used as the metering reference point.

Similarly, a clear-skied landscape will have the same reading at the taking position as the distant tree that you're framing up.

A white subject with strong directional sunlight from above causes a very contrasty result, especially as it's offset from the dark background. This is an easy job for an incident meter.

Spot metering

DSLR cameras and a some exposure meters have a spot meter that narrows the angle of the metering cell so you can point it at specific areas of a scene. As this is a reflected reading you must, once again, use the reading only as a guide, being aware that its value will produce a mid grey. This type of meter is often used to take several readings from a scene and then using your head as a calculator work out what the best exposure should be to ensure everything comes out as expected.

Spot meters should really only be used by those who have grasped exposure and are able to apply compensation to ensure the results are correct. Just pointing and relying on the indicated exposure is a recipe for disaster.

Using filters

Whenever you attach a filter in front of the lens the amount of light reaching the sensor is reduced and the exposure needs to be increased to compensate. It's easy to use filters on a DSLR when you're using the camera's built-in meter because it sees what's in front of the lens and adjusts accordingly. This obviously can't happen when you use a separate meter, but to help, filter manufacturers print the filter's exposure details on the box, around the filter's rim or in the instructions. You simply take a reading and then knock off the number of stops suggested for the filter manufacturer. A red filter, for example, reduces the exposure by three stops. So if the hand-held meter suggested 1/250sec at f/8 you could open up the aperture to f/2.8 or increase the shutter speed to 1/30sec.

The problem comes when a filter increases the exposure by a third of a stop, and that's when the more advanced meter, with its 1/10th stop accuracy is again worth its weight in gold.

Shoot anything white using reflected metering and it will come out dull with either a neutral or colour cast. This Mediterranean wall has picked up a blue cast from the strong midday sun and is underexposed an incident reading produces a more natural exposure.

What about EV numbers?

EV is an abbreviation of exposure value and is something that's used mostly by professional photographers. It's a combination of the shutter speed and aperture and some meters give the reading as an EV number which is then set on a dial to give the shutter speed and aperture combinations. If you know the EV number you can select any aperture and the shutter speed is adjusted accordingly. It's almost like having program exposure on a meter.

Using a flash meter

A flash meter measures light in much the same way as an ambient meter so can often be used as incident or reflected with the same issues being raised. Most basic flash-only meters will only work in incident light mode because the dome is fixed in place. More expensive combination meters offer both options.

Taking a reading depends on the flash meter you use. More advanced ones allow the flash sync cable to be plugged in and you press a button on the meter to trigger the flash. This gives you the benefit of being able to move close to the subject. With a basic meter you don't have this option and would need someone at the flashgun to trigger it while you hold the meter in front of the subject. This could be the model you are photographing, but on still life sets you may have a problem.

Using a Grey Card

If you can't justify the cost of a meter and want better pictures using your camera's exposure meter, buy a grey card. This is an 18% grey card, which is the same as the tone that the camera's meter scrambles to. Taking a meter reading from a grey card placed in front of the subject will give a similar exposure to an incident meter reading.