Light Metering.
Introduction:
Having decided upon your scene and its composition, you will now be faced with the challenge of determining how much light you have and how this light will be metered (measured) and interpreted by your camera to give you an accurate image exposure.
This month we will discuss how to measure the light and understand how your camera can assist (or vex) you with your photography.
How does your Camera Meter the light on your scene:
There are two ways of measuring the amount light on your scene. The metering built into your camera measures the amount of light ‘reflecting OFF’ your scene or subject and is called a ‘reflective’ light meter reading. Compare this with another type of light meter reading called an ‘incident’ light meter reading which measures the light ‘falling ON TO’ your scene’
‘Reflective’ metering: is the predominant type of metering found in modern-day cameras. This metering can be influenced by light (white) or dark (black) areas in your scene. The reflective meter is calibrated (standardized) to compare all light readings with an 18 percent grey colour. It is for this reason your ‘in camera’ meter may ‘see’ a black area as muddy grey or a white area as ‘dirty grey”. You may be familiar with snow images where it is very difficult to render them brilliant whites as you see them.
The ‘Incident light meter’ (hand-held meter): measures the amount of light falling onto your scene and is not influenced by background colour or bright or dark scene lighting.
They are/were mostly used by professional photographers to get very accurate lighting in bridal portraits in outside scenes, studio flash photography and in low lighting situations.
I used one over several decades and was most reluctant to move into the reflective metering on cameras with the advent of the digital age. That said, I find the high technology of today’s new generation reflective metering cameras extremely accurate - provided you understand how best to use them, and there-in lays a tale for the unwary! (and our discussion for next month)
How your camera uses the light reading from your metering system.
Once you have measured the amount of light ‘reflecting off’ your scene, your camera will interpret the light reading and set a correct exposure and /or shutter speed for your image.
Your camera has a number of different light metering modes to help you do this automatically. Most cameras have spot, center-weighted, and matrix/evaluative metering modes.
Matrix or Evaluative Metering: This is the default metering mode on most cameras. Matrix/Evaluative metering emphasizes different parts of the photo depending on how many sensors your camera has and how the light is distributed throughout the scene. It “looks” at different light level profiles, attempts to find one that matches your photo, and then picks an aperture and shutter speed accordingly.
Matrix/evaluative metering can be extremely sophisticated but you need to apply some common sense in scenes of high lighting variations. Matrix/evaluative metering tends to do a little better than center-weighted metering overall. However, occasionally it may interpret the scene lighting incorrectly and is in no way a replacement for solid photographic expertise. For these situations you may need to consider using one of the other metering modes.
Center Weighted Metering: The first automatic exposure cameras used center weighted metering. Center weighted metering gives the most influrence to the center of the photograph while still allowing other surrounding sensors to contribute in part.
As this image shows, the center of the sphere determines most of the camera’s “decision” when it comes to sensing the correct aperture and shutter speed combination. As you move further from the center, the other surrounding sensors play less of a role. Once you reach the black the region, the sensors will have almost no influence in the exposure calculation.
Many photographers like to use center weighted metering because it is very predictable and consistent. They know exactly what they are getting. If they focus on the center of an image, that’s the part of the image that will have the most accurate light levels.
Spot Metering: Much like center weighted metering, spot metering emphasizes the center of the frame. The only difference is the width of the sample area. Spot metering only emphasizes a small spot in the middle of the frame, discounting all other sections outside of it.
Spot metering is effective when you only wish to meter for a very small section of the photo. It is perfect for indoor objects that are illuminated by outdoor light. Focus on your subject, half depress the shutter (so your camera holds the exposure) and then frame the image before fully pressing the shutter button. (some early cameras may not have this function – check your manual)
This is an image where spot metering works well. The photographer set spot metering on the flower part of the image. While there is a bright light on the top right of the photo, the camera ignored the black background of the image and correctly exposed the flower subject. If matrix/evaluative or center weighted metering had been used, the camera would have tried to correctly expose the dark area and the remainder of the image would have been over-exposed.
Manual Metering: The final method of metering is to revert to setting your shutter and F stop as in the past. Use a hand held or set as ‘spot metering’ on your camera to read individual settings for high and low light areas of your scene and then use your experience to set the camera.
For those of us ‘who have been there’ we know that this usually more trouble than it is worth.
Of course, the hand-held ‘incident’ light meter will always perform well in most circumstances as you are reading the amount of light available on your scene wherever and however it presents itself.
Which Metering Mode Should You Choose?
Generally use ‘matrix/evaluative’ metering or the ‘center-weighted’ metering. (I have used the center-weighted metering most of my working life and find it performs well. In automatic mode the camera will usually default to matrix/evaluative mode.
With experience, you will learn all of the subtleties that come with the various modes. Always pick your metering mode based on the image you are trying to capture. Choose spot metering when there is a color or small section of the photo that you want to emphasize.
Next month we will discuss how to get the best from your camera’s metering system to ensure correct exposures.
