zarek rahman © 2011
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How to use your camera’s built in exposure meter

Your digital camera has an exposure meter built into it which is the reference for every photo it takes. You need to understand how that meter works so that you can use it effectively, either with exposure compensation or by reading and adjusting it accordingly in manual exposure mode. Today I’ll briefly explain how it works, how to use exposure compensation, and how and when to use manual exposure mode in conjunction with the exposure meter.

Background

The camera makes decisions based on what it thinks is the perfect exposure for any given scene based on the light meter built into it, the metering mode you have chosen, your ISO level, your chosen aperture, and your shutter speed.

The light meter takes a reading from the light reflected off your subject and back into the lens. If that reading is in the range of 13% to 18% grey (also referred to as medium or middle grey), you will have what your camera deems to be the ‘perfect’ exposure.

There are clearly a few problems with this approach and this is why your images often come out too bright (overexposed) or too dark (underexposed).

To deal with this, you need to be able to make use of the different metering modes available to you, as well as exposure compensation. If you like to shoot in manual mode (as you should be if you are using flash), the built in meter is invaluable in helping you choose the correct aperture and shutter speed for your photo.

Note: when referring to the amount of exposure compensation applied to an image, the general term is to use the ‘ev’ (exposure value) suffix, which you will see shortly.

What is 13% to 18% grey? Who cares?

I don’t really understand the technical reasons of why camera manufacturers and printers have chosen this particular colour as the basis for all digital camera light meters, but the point to note is that it doesn’t really matter so long as you appreciate that your camera sees things differently than you and I, and you adjust your settings accordingly.

If the scene you are trying to photograph is very bright (e.g. snow), your camera will see it as being brighter than middle grey, and underexpose the image automatically. Your photo will then come out too dark. You would fix this by overexposing the image, either with exposure compensation or by making the appropriate changes in manual exposure mode.

If you are finding that your photo is coming out too bright and you are losing detail in the highlights, it’s because your camera has metered the scene as being darker than middle grey, and made the photo brighter accordingly. You would fix this by underexposing the image, again using exposure compensation or making changes in manual exposure mode.

If the scene you are trying to photograph contains both dark and light elements (e.g. a sunny backdrop with a person in the foreground in the shade), your meter will get confused and it’s anyone’s guess what the final image will look like.

Understanding metering modes

The light meter will change its behaviour depending on the metering mode that you select.

Matrix (Nikon) or Evaluative (Canon) or Multi (other manufacturers) metering mode is the default setting for most cameras. In this mode, the camera will average out the exposure based on the image as a whole. This is a highly advanced mode and is what my camera stays in most of the time.

Spot metering takes the reading based on a single focus point, ignoring everything else. This is best if you are dealing with smaller objects, or backlit people. It is also invaluable at concerts, where the lights are constantly changing but your priority is the performer’s face. I use this fairly frequently, especially if I want to go for a purposefully overexposed photo.

Center weighted metering mode takes the reading based on the center of the image only, ignoring everything outside that central area. This can be useful if the image contains a good deal of information in the central area (the size of which you can usually adjust). I never use it.

Depending on the metering mode you select, the camera’s light meter will behave differently. If you are shooting in Program, Aperture or Shutter priority mode, your exposure meter will stay at 0 but you should be able to see the shutter speed and aperture that your camera is automatically selecting based on the metering mode. If you switch metering mode from Matrix to spot, you will get a totally different reading.

How to use exposure compensation

• Look at your scene through the viewfinder/LCD.
• Decide which metering mode is appropriate. In most cases, matrix/evaluative/multi will do.
• Take the photo.
• Look at the photo on the back *.
• If it looks too bright, move the exposure compensation to the negative side, take another picture, check, repeat until you are satisfied.
• If it looks too dark, move the exposure compensation to the positive side, take another picture, check, repeat until you are satisfied.
• That’s it.

You should note that the meter is just a meter which is calibrated to grey. You need to make the decision whether the photo that meter produces is what you want.

(* The LCD display on the back of your camera can be deceiving. For a more reliable preview, you should be looking at the photo with blinking highlights, or at the histogram. But that’s outside the scope of this guide.)

[box_grey]How (and when) to use manual exposure mode[/box_grey]

When you switch your camera over to M (manual exposure mode), you are in total control of the aperture, shutter speed and ISO.  You set your metering mode depending on the scene in front of you, and point your camera at what you want to photograph.

You’ll notice that the exposure meter is no longer static and resting at 0. Instead, it will jump wildly around from minus to plus depending on the brightness of the scene, and whether you are taking an average reading or one from a spot/the center. The readings for aperture and shutter speed will change constantly. It can be a little bit overwhelming and you’ll probably be wondering why you bothered when aperture or shutter priority does the job just as well.

There are limited circumstances in which I use manual exposure mode:

If I am using flash (TTL or manual) I always shoot in manual mode. I generally want consistency in my photos and so I need to be in control of the exposure for the ambient light (controlled by the shutter speed) and the foreground element (lit by the flash and controlled by the aperture). Shooting in aperture or shutter priority means that either one of these variables might change and I will miss a shot.

If I am shooting a concert, ceremony or other event with bright spotlights hitting my subject, or if the lights around them are constantly changing, I will be in spot metering mode. I will generally want to shoot wide open (f2.8 or greater), with a shutter speed fast enough to freeze the action (at least 1/125). Again, using shutter or aperture priority in such a situation might result in the camera choosing settings that don’t take the photos I want.

Once you get the hang of how the meter operates, this becomes relatively easy. All you need to be aware of is that the meter will jump to the plus side if it thinks the photo will be overexposed with the given settings, or it will jump to the minus side if it thinks the photo will be underexposed with the given settings.

It’s then up to you to alter your settings accordingly. So if for example you have set your ISO to 400, are already wide open at f2.8 and can’t afford to go lower than a shutter of 1/125 and your exposure meter is still at -2, your only option is to increase your ISO by 2 stops to 1600. Once you’ve set this, all of your photos will look the same, unless the lighting changes in the scene you are photographing.

Remember: being at 0 EV doesn’t mean your photo is perfect!

If you only remember one thing from reading this guide, remember this: just because your camera meter is resting at 0, it doesn’t mean that your job as the photographer is complete. You are the one making the decision about how the final image should look. If the meter is at 0 but the photo is clearly too bright, then you need to change the exposure accordingly. This is true not only for the semi-auto modes (program, aperture and shutter priority), but also for manual mode. When the meter is at 0, the camera sees grey.