Main types of camera metering systems

Exposure meters contain a light sensitive cell. Within the camera this measures the amount of light that is reflected back from an object. This enables the correct combination of aperture and shutter speed to be set which allows the appropriate amount of light to reach the CCD/CMOS sensor in order to achieve an accurate exposure. The following metering modes can be found on modern digital cameras:

Spot metering: This is used for metering a specific part of the subject or scene. The metering is weighted at the centre, covering a very small area of the frame (approximately 2 - 4% of the viewfinder).

Partial metering: Again used to meter a specific part of the subject or scene. The metering is weighted at the centre, covering approximately 15% of the viewfinder.

Centre-weighted/Average metering: This works by splitting the image into two parts – the centre and the surrounding area. Light is measured from all over the viewfinder but the central 40 – 60% influences the exposure slightly more (as this is where the main subject tends to be positioned).

Evaluative/Pattern metering: Also referred to as Matrix or Honeycomb metering, it is designed to prevent exposure error caused by tricky lighting conditions. The image is split into several segments with light readings taken from each. The information obtained is relayed to a central microchip computer where an average exposure is calculated based on different “model” lighting situations stored within the computers memory.

When metering a scene a few things have to be taken into consideration. What metering mode are you using? If you are using Evaluative metering - how much sky is in the scene and how bright is it? If you are using Spot or Partial metering – how bright/dark is the aircraft you are metering from? All of these factors will influence your readings.

A camera’s meter measures the amount of light that is reflected back from the subject. A bright aircraft reflects more light back than a dark aircraft. Metering systems however assume that everything is mid-tone grey and if it is not the meter will produce an exposure setting that will make it a mid-tone grey!! With a little knowledge about how to deal with tricky lighting conditions you will be able to achieve accurate results with just your trusty camera.

 

Typical tricky lighting situations the aviation photographer encounters

(1) Dark aircraft against a bright sky

Evaluative metering

The end result is a nicely exposed sky and an underexposed aircraft. But why? What has happened here is that the bright sky has fooled the camera’s meter into thinking that there is lots of light available. The camera has then closed down the aperture/increased the shutter speed (depending on which shooting mode has been set) to allow less light in.

The solution to this problem is to overexpose by 1-2 stops (set exposure compensation to between +1 and +2) depending on how dark the aircraft is.

Spot or Partial metering

The end result this time is an overexposed aircraft. The meter has been placed over a dark object and has therefore been fooled into thinking that there is not a lot of light available. The camera subsequently opens up the aperture/lengthens the shutter speed to allow more light in.

The solution to this problem is to underexpose by 1-2 stops (exposure compensation set to between -1 and -2) again depending on how dark the aircraft is.

Centre-weighted metering

This metering mode more often than not will produce accurate results. Both the dark aircraft and the sky influence the reading. The underexposure caused by the bright sky is balanced out by the overexposure caused by the dark aircraft. However, if you have composed your shot so that the dark aircraft is for example 1/4 face on and completely fills the frame then you will get an overexposed image and should compensate accordingly (see Spot or Partial metering above).

Of course the only way to determine which method gives you the best results is to try them out in order to determine the characteristics of your camera’s meter.

(2) Bright/white aircraft

White aircraft, especially against a bright sky or if using Spot or Partial metering, will tend to come out underexposed. As explained above, the camera is fooled into thinking that the scene is very bright. Accordingly it closes down the aperture/increased the shutter speed to allow less light in. The end result is that your white aircraft looks grey.

The solution is to overexpose by 1 - 2 stops.

 

Alternative methods for metering

(1) The sunny 16 rule

According to this rule the correct exposure, on a clear sunny day, is obtained by setting the shutter speed to the speed nearest to the ISO setting in use and the aperture to f/16.

For example, if using ISO 100 the exposure settings would be 1/125^th second at f/16. This is of course equivalent to 1/250^th at f/11, 1/500^th at f/8 or 1/1000^th at f/5.6.

(2) Grey card

As a camera’s meter is set to accurately meter mid-tone grey it is possible to take a reading from an 18% grey card (available from various photographic supply outlets) and then input the settings manually. When taking the reading make sure that the grey card fills the frame. This method is more appropriate for film rather than slide or digital.

(3) Hand held light meter

Unlike a camera’s meter, which measures only the light reflected from an object, hand held meters can be used to take reflected readings and incident readings (a measure of the light falling onto the aircraft).

For incident readings a small white dome, called an invercone, is placed over the light sensitive cell to gather up available light. For static shots at an airshow you simply move as close to the aircraft as you can and point the invercone back towards where you will be standing to take your picture. For take-off/landing shots you hold the light meter so that the invercone is in the path of the light that is falling onto the aircraft from your position. Switch the camera into Manual mode and input the exposure settings indicated by the light meter. Incident readings are more appropriate for slide or digital.

One final thing about exposure. The dynamic range of a digital camera it is approximately 5 stops (we will talk a little more about this in the histograms section). If you plan to utilise manipulation software such as Photoshop it is always better to have a slightly underexposed digital image than an overexposed one. Therefore on bright sunny days if you are unsure, it is advisable to meter for the lightest area of your image where you still expect to see detail, referred to as the highlights, as this will produce a slightly underexposed image. Metering with a handheld light meter with the invercone in place in order to take an incident reading does exactly this. In the case of digital photography an underexposed image still contains lots of detail which will become visible with post-processing. With an overexposed image on the other hand the highlight detail will be completely blown out and cannot be retrieved with post-processing.