CAMERA METERING & EXPOSURE

Lesson objectives Knowing how your digital camera meters light is critical for achieving consistent and accurate exposures. Metering is the brains behind how your camera determines the shutter speed and aperture, based on lighting conditions and ISO speed. Some metering options often include matrix, center-weighted partial and spot metering

BACKGROUND: INCIDENT vs. REFLECTED LIGHT All in-camera light meters have a fundamental flaw: they can only measure reflected light. This means the best they can do is guess how much light is actually hitting the subject.

Let’s discuss how a light-meter - any meter, built- in or hand-held - determines the "right" exposure. For example, you want to determine the right exposure for your friend's face. You point your lens or a hand- held meter toward the face, and your meter tells you the "correct" exposure. Correct for what? Well, correct to produce a pleasing skin tone. But how does the meter know what a "pleasing skin tone" is? It doesn't - and that is the problem. The meter doesn't know if you're pointing to a human face, a field of snow, or a lump of coal. All the meter knows is how to tell you what shutter speed and aperture combination to use to produce an 18% gray tone. Why 18% gray? Why not 25% gray...or 12% gray...or 99% gray? Because it has been determined that if the light in an average scene is averaged out, it will produce an 18% gray tone. So meters are calibrated to produce “proper exposure” when it is exposed to produce an 18% gray tone.

How does the light meter work? Immediately, some questions come to mind. What is an "average scene"? Is a ski slope average? A beach? A neon sign? And what is an average skin tone? Is it the skin of a Swedish blonde? Or a Nigerian black? Or a tanned California sun-worshiper? The meter doesn't know or care. It will tell you how to produce an 18% gray tone no matter what you point it at! Knowing this, we can now proceed. If the light meter is set to read the best exposure when it is exposed to 18% gray light, why not read the light itself? Then, no matter what color your subject is - black, white, red, green, or anything else - if you read the light falling on it, you should come up with a "correct" exposure. And this is exactly what a gray-card reading or an incident-meter reading does. While metering, just make sure the card is tilted so that the light that falls on it is similar to the light that is falling on your subject. You already know that your meter is calibrated to give you a reading that will produce an 18%-gray tone. This means the tone of the gray card would be duplicated perfectly. It also means that all other tones, lighter or darker, will be faithfully reproduced too. So whether your subject is blonde or brunette, snow or coal, red or green, it will be faithfully reproduced and properly exposed on your film.

“Free” gray cards can found all around you…

Matrix (or evaluative) metering This is your cameras standard metering mode which most people use for most situations. It is well suited for most subjects including those that are backlit. It is basically just what your eyes do when looking at anything. The camera measures the light intensity in several points in the scene, and then combines the results to find the settings for the best exposure

Center weighted Uses the whole of the scene to meter but, as the name suggests, places a greater weighting to the light coming from the central area It is a kind of cross between matrix and partial metering where around percent of the scene towards the central point is metered for. Preferred by those wanting to photograph a particular thing, rather than the scene as a whole, such as portraits Works well for photographing people or objects in a scene, where the subject is the focus but you also want to show its surroundings

Partial (not on Nikons) This setting is useful when your subject is strongly or overly backlit. The metering is weighted towards the centre of the viewfinder covering approximately 13.5% of the area. This will inevitably lead to your subject being correctly exposed, with an overexposed background.

Spot Spot metering gives the photographer far more control over the exposure than any of the other settings With spot metering, the camera will only measure a very small area of the scene (between 1-5% of the viewfinder area). Spot and partial metering are also quite useful for performing creative exposures, and when the ambient lighting is unusual. Spot metering is usually used for pictures with tricky lighting, like very high contrast scenes (for example, if the subject is backlit). It gives the subject a proper exposure while overexposing the rest of picture.

In the examples to the left and right below, one could spot meter off of the diffusely lit foreground tiles, or off of the directly lit stone below the opening to the sky.

Spot metering (cont.) Great for taking pictures of the moon. Due to the very dark nature of the scene, other metering methods tend to overexpose the moon. Spot metering will allow for more detail to be brought out in the moon while underexposing the rest of the scene

Dave Wares

Hands on Find the different meters on your camera NIKONCanon

Hands on break! With your camera set to “M”, practice using the gray card and the different metering controls on the camera. Practice using my hand held (I only have one so you will have to share)