Using Your Camera’s Meter to Set Exposure

In the last post we talked about shooting in Manual mode and the key to being successful in Manual mode is a full understanding of exposure. That means understanding the relationship between changing aperture and shutter speed. We discussed how cameras meter to a middle gray and how that can be thrown off. In this post, we are going to continue to understand how the meter works so you can better detect situations where the camera will get it wrong and how when shooting in manual to determine if the exposure is right.

First up we have to understand how metering works. Look at this diagram:

lightexposure

The light source, in this case the sun, is falling on the subject. The light then bounces off the subject toward the photographer where the camera measures the amount of light. In this scene, the light also bounces off of the tree, the sky and the ground and it reflects towards the camera as well.

In days of old when our camera’s did not have meters, we used hand held meters and read the light at the subject. Its called “Incident Metering”. This meter is the most accurate since it measures the exact amount of light at the subject and does not take the background into consideration.

When our cameras got internal meters, they were able to now measure the light reflected off the subject. These are called “reflectance meters”. They do a great job in most cases where the scene’s different tones all average out to a middle value and can be fooled.

Most cameras have multiple ways of measuring the light. Modern cameras will typically have three different options:

  • Matrix or Evaluative
  • Center Weighted
  • Spot or Partial Spot

Matrix or Evaluative metering is probably the meter mode you use the most. It is the default meter mode on most modern cameras. In this mode, the camera reads data from multiple locations across the frame. Some might be as few as 5 locations to over 1000. These values are then compared to a list of scenes stored in the camera’s database. The engineers evaluated over 30,000 different shooting combinations and they compare the information coming from the different cells in the matrix of sensors looking for a match in the database and it determines the best exposure. In theory, this is smart enough to detect snowy or beach scenes or troublesome backlit scenes and adjust the camera accordingly. This should cause more photos to be exposed properly.

Center Weighted reads the center of the frame (typically the inside area bounded by all the autofocus points) and measures the light there. It also measures the light outside this area. Then it averages the two giving a bias to the center area, letting it factor into say 2/3 of the exposure and the outer area 1/3 of the exposure. This is based on the fact that most people who shoot photos are people taking snap-shots and those people tend to center their subjects. This is the method that most old film cameras use.

The Spot meter measures a very small spot of the view finder. This is a very precise measurement. For many camera’s that spot is about the size of the center autofocus point and is located in the center. More advanced cameras can read its “spot” from any of the autofocus points.

If your shooting in automatic modes, Matrix meters will generally give you the best measurement. Many cameras recommend that if you shoot manual mode to not use Matrix metering but it still gives the best measurement for an in camera meter.

Manually adjusting Exposure

meter

The "250" in the display is the shutter speed, in this case 1/250th of a second. The 9.5 is the aperture, which is 1/3 stop above F8

Your camera has a gauge in its viewfinder. Older cameras might have a needle that slides up and down and when its level the exposure is “correct”, meaning the camera now thinks the camera thinks the camera’s settings will produce a middle gray exposure. More modern cameras will have a digital gauge in viewfinder that looks like +|–0–|- with a pointer underneath the gauge. Each pip represents 1/3 of a stop of exposure either over (towards the +) or under (towards the -) exposure. (some cameras will be in half stops and may show up to two stops over/under exposure). The exposure is “correct” when its at 0 .

While pointing your camera at your subject, you adjust either the shutter speed dial up or down or you adjust the aperture up or down (and with digital you can even adjust the ISO up or down) until the pointer reaches the 0 point. Of course there will be times where you don’t want a zero setting, such as shooting sunsets, or the white background high key portraits and you can adjust the camera to over or under expose for the effect your going for.

When you recognize that the matrix meter is not going to get you the right exposure, its time to switch over to the spot meter and take advantage of it.

In situations where you are in control of the setting, say family portraits, you can have your subject hold a “Gray Card” which can be picked up at most camera stores. These gray cards are middle gray. You then aim your spot meter at the gray card and adjust the meter is centered. This should give you a very accurate measurment for the exposure.

Sometimes you can’t use a gray card in that case you try to find something thats lit with the same light as your subject. Outdoors you can use grass as long as its not washed out from the sun reflecting at you or you can use a light asphalt or darker concrete to get close. I’ve even used the referee’s stripes to meter off of geting a black and white stripe in the area of the spot.

So put it all together. You have light. You read it using the meter in the camera. You adjust the shutter speed or aperture up or down until the meter reads the right amount of light. Take a perfectly exposed photo. Sit back and enjoy the marvel and wonder.

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2 Comments

  1. Mar 11th 5:43pm

    thanks…well said and easy to understand. so i was just playing around with the metering options on my 50D for bright backgrounds and off center subjects, which is mostly what i shoot. it’s been fun.

  2. Mar 28th 7:10pm

    Good thorough guide, takes the mystery out of it. Wish I had something like this when I was getting started!

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