• METERING

  • Just Do It...

    • Use spot metering mode to measure exposure

    • Center the exposure on a grayish target in the scene

    • Use the spot meter to evaluate the exposure across your composition

  • Why

    • Evaluative and Matrix metering modes let the camera guess at what the right exposure for the scene is — we are smarter…

    • Light meter in camera is calibrated so light reflected from a gray surface with 12.7% reflectance is “correct” — says who… ?

    • Spot meter lets us control what part of the scene to meter off of — great for backlit and high contrast scenes.

  • Gotchas...

    • If you meter off a bright white surface, and center the light meter,  it will be exposed to grey

    • If you meter off a dark black surface, and center the light meter,  it will be exposed to grey

       

    • Spot metering does not work in live-view mode — no problem, just use the live histogram

       

    • Gray cards you can buy reflect 18%, but the camera light meter is calibrated to 12.7%  — 0.5 EV difference!  (I rarely use a gray card in nature photography…)

       

“Angel of an Egret”

“Ute Mountain in the Shadow”

Key Points

  • The light meter in your camera measures the amount of reflected light from the scene.

  • The area of the scene that is used to measure this reflectance depends on the metering mode you are using:

    • Spot metering mode measures ~ 2% of the scene, usually in the center of the field of view.

    • Center-weighted metering mode measures the exposure over a larger, centered, area than spot metering.

    • Evaluative and matrix metering modes average over the entire scene and make estimations on the best exposure reading for the composition.

  • The light meter in your camera considers the correct exposure to be the light reflected from a gray target that has a 12.7% reflectance.

  • When you meter off bright white (snow, whitewater) the light meter will suggest a center exposure that results in the white being underexposed (2-3 stops) and gray.

  • When you meter off dark black (shadow, raven, bear) the light meter will suggest a center exposure that results in the black being overexposed (~2 stops) and gray.

  • A good approach is to spot meter off a grayish object and center your manual exposure based on that reading.  It does not have to be perfectly gray, especially when you shoot RAW files.

  • Pay attention to metering when:

  • The contrast in your scene is high (fox in the snow, waterfalls, full moonrise, bright sky – dark foreground)

  • Winter snow scenes (white) and very dark scenes (night)

Metering Modes

Most cameras provide different ways to meter the light reflected from the scene.  Once again there is a choice between letting the camera “decide” what the proper exposure is, or letting you decide which part of the scene you want to meter and what subjects in the scene you want to control the exposure.

I highly recommend using spot metering mode since you know the exact part of the scene the meter is measuring exposure and you can intentionally move the spot meter around your composition to survey the extremes.  For example, meter the sky and see that it is at +3 EV and the meter the shadows are at -4 EV.  Now you know there is an approximate 7 stop range of exposure and you can estimate how well your camera will record that dynamic range.

Note that when shooting in live view mode (looking at the scene on the LCD on the rear of your camera) spot metering does not work.   Instead the camera is metering the entire scene, or a larger fraction of it.  I use live view mode when shooting landscapes on a tripod so that I minimize vibration from mirror slap and to see the histogram in real time.  The histogram is telling me the whole story for the exposure of the entire composition.

Metering modes on a Canon 7D2

Spot Metering Mode

Adjust the exposure metering mode so that the light meter in your camera is only measuring a small spot in the field of view. This is known as “spot metering”.

In spot metering mode your camera measures exposure from a small spot in the scene  instead of taking a spatial average exposure measurement over the entire field of view.

Other exposure metering modes are known as “evaluative”, “matrix”, “average”, etc.  These other modes “help” us by adjusting the exposure to what the computer inside the camera thinks is best.  We can do much better by using spot-metering mode, manual exposure, and a small amount of thinking.  The camera does not realize that small brown patch in a sea of bright white is a beautiful red-fox in the snow.

Your Friend the Histogram

Being able to view the histogram of the image you just took is a great thing!  It is always good to take a quick look at the histogram on the LCD on the back of your camera after each shot, or at least when you are setting up on a new scene.  I set my cameras so that the histogram of the last image automatically appears for 2 seconds after I take a photo.  I have learned how to quickly glance down from the viewfinder at the histogram.

I look at the histogram to check whether either side of the histogram is bunched up along its limit (under-exposure is bunched up on left side; over-exposure is bunched on the right side).

It is a good habit to keep checking in with your histogram.

Most mirrorless cameras, and recent DSLR cameras, show you the exposure histogram in the viewfinder.  This is awesome!  I tells you what is going on across the grid of pixels quickly.

When photographing landscapes on a tripod I compose my image through the viewfinder, then switch to live view before taking the photo. In live view I can see the histogram (not all cameras do this…) in real time.  I normally use the two second shutter delay built into the camera to avoid camera and tripod shake.  Live view also helps here since the mirror is up and there are no vibrations from mirror slap.

Always check the histogram while shooting !

Viewfinder information display for a Canon 7D2.  The spot meter circle is shown in the center.  Higher end cameras often have the ability to link the spot metering circle to an autofocus point.  I do not worry about this, instead I periodically check my exposure using the spot meter circle on a grey target throughout the day, depending on how much the light is changing (i.e. sunny to cloudy, mid-day to sunset)

Gray Matter

The light meter in your camera measures reflected light from your subject (or scene).  Normally, the reflectance will vary from the different objects in you composition.  Snow can reflect 90% of incident light, whereas a black bird may only reflect 10% of incident light.  What level of reflected light does the camera’s light meter consider to be “correct” given the large variance of reflectance we see in the real world?

Light meters in cameras are designed so their “correct” exposure is the amount of reflected light from a matte gray subject that reflects 12.7% of the light incident on it.

Note, there is an unfortunate claim that the light meters assume a 18% reflectance gray subject, instead of 12.7%.  This is historical and a widespread error (sort of like people thinking that ISO adjust the sensitivity of the sensor…).  Unfortunately gray cards are created with a 18% reflectance.  Your camera light meter is using a 12.7% reflectance as “correct exposure” (i.e. centered on zero on your exposure scale).  The 18% gray reflectance comes from the printing world where the “mid-gray” was chosen to be half-way between bright white and solid black on a grayscale — this turns out to be the 18% gray reflectance level (i..e that gray reflects 18% of the light incident on it).

The light meter in your camera assumes that light reflected from a gray target with a 12.7% reflectance is the “correct” exposure.  Unfortunately gray cards, used to meter off of, are a 18% gray reflectance.  If you use your camera’s light meter on a 18% gray card, then overexpose by 0.5 EV to correctly set the exposure.  The blotches in this figure are where I have increased the exposure in Photoshop (using a brush mask) in the lower left by +0.5EV, and decreased it in the lower right by -0.5EV.  (note this reflects overexposing by 0.5EV and underexposing by -0.5EV respectively, with the camera in the field).

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