What’s Your Preference?

AHutch · #1 (**permalink**) 12-02-2011, 03:31 AM

For the beginner to digital SLR photography it’s important to understand what the light meter is and what it does before discussing how to use the camera’s built-in meter to set and manipulate exposure. Like many aspects of DSLR photography, metering can be a somewhat overwhelming or intimidating subject. But it doesn’t have to be. In this introduction to your DSLR’s light meter, I will keep things simple by giving you enough information to grasp the basics so you can move to the next step. Once you progress in your overall understanding of digital SLR photography and advance in your photo taking, you will then have a concrete base on which to move to the more advanced areas of light meters and metering.

The first thing is to define a light meter in its most simplest terms:

A light meter is an instrument that measures the amount (or intensity) of light!

This sounds simple enough but we need to expand on this definition by defining the type of light that is being measured and how this measurement is used. Again, let’s keep this simple.

Basically there are two types of light when it comes to digital SLR photography:

1. Ambient Light – This is the light around us that illuminates objects.
2. Reflected Light – This is the light that is reflected off of objects.

This is an important distinction. The more advanced hand-held light meters generally measure ambient light. But I wish to stay focused on discussing the built-in light meter found in your digital SLR camera. This meter generally measures reflected light (although one could argue that light from the sky is ambient light, but let’s keep things simple for now).

Now let’s expand the light meter definition:

The built-in light meter found in your digital SLR camera measures the amount of light being reflected off objects through the lens and into the camera.

This measurement is then used by the camera’s computer to determine what it believes to be the best exposure for the scene, with that exposure being the proper combination of shutter speed and aperture (for a given ISO). In other words, based on the amount/intensity of light being reflected off the objects and into the camera, the camera will decided how much of this light will be permitted to enter the camera once you release the shutter button by setting the shutter speed and the aperture. Bear in mind that since we are talking about a digital SLR camera, you as the photographer ultimately have control over the final exposure setting.

Light Meters are Color Blind

The important thing to keep in mind is that the light meter is color blind. The best way to think about a light meter is by imagining that it only “sees” light in terms of shades of black & white. The light meter will take the light being reflected by the object and decide if there is so much light that the object will appear all white, or if the light being reflected by the object it is not enough to the point that the object will appear black, or if the light being reflected will render the object somewhere in between.

Now here is the important part. In measuring the light, the meter will take that measurement and decide on the best exposure. In deciding on the “best” exposure, it is attempting to limit the amount of light entering the camera so that it will be neither too bright nor too dark but EXACTLY in the middle. That “middle” is what is commonly known in photography as “18% grey” (It’s actually 12% grey, but we won’t argue this point). This “18% grey” is typically the middle of being too bright (white) and too dark (black).

Aiming For That Middle Ground

Let’s look at an example. Let’s suppose you are taking a photo of a “ruby-red” car. In taking the photo you want the final photo to show the car color as exactly ruby-red. Now remember the light meter doesn’t determine colors, it will only measure the amount of light being reflected off that ruby-red car. Assume you have no control over the light that is striking the car and you have the camera set to an auto-exposure mode.

So you aim your camera at the car, focus, wait for the “beep” and click the shutter. Whether there is too much or too little light striking the car, just prior to the “beep”, the camera’s meter will limit the amount of light coming into the camera so that the amount is exactly in the middle – it’s aiming for that 18% grey. After taking the photo you examine the LCD screen to see how it came out. This is when you discover that meters are not perfect! Instead of seeing that perfectly ruby-red car you may either see a car whose color is slightly too bright or slightly too dark. Why? Remember that the meter is color blind – it can only “see” the amount of light and attempt to adjust the amount of light entering the camera and in so doing it is attempting to render the color perfectly but since it cannot “see” the color it can only guess.

Now this is why digital SLR cameras are so great! While it may be annoying that it doesn’t always give you a “perfect” exposure due to the limitations of the light meter, it does allow you to do something that you can’t do with a typical digital point & shoot camera. It allows you to make adjustments to the exposure until you have the photo just how you like it. And that can mean making an adjustment so as to have the colors render “perfectly” or making creative adjustments to under or over expose the subject to your liking.

Now let me give you the full definition of a light meter:

The built-in light meter found in your digital SLR camera measures the amount of light being reflected off objects through the lens and into the camera and attempts, with the help of the camera’s computer, to limit the amount of light by setting the appropriate shutter speed and aperture so that the amount of light entering the camera is neither too much nor too little but exactly in the middle.

Is it really that easy?

Trying to get the color correct by limiting the amount of light to that “18% grey” zone is not the only problem the light meter has to deal with. As you can image, there are usually multiple objects in a scene that are reflecting light into the camera. You may be taking a picture of a building in daytime and the meter then has to deal with the building that is not reflecting enough light because the sun is behind the building, and it has to deal with the bright light from the sky along with the light from the green trees surrounding the building. How does the meter deal with all these sources of reflected light and decide where the middle “18% grey” is? Well, this is where the meter needs a little help from the photographer whereby you have to tell the camera which objects you would like it to take a reading from. This is a lot easier than it sounds and the details of these different “metering modes” will be covered in another post about Metering Modes. But basically, by choosing a particular metering mode, you will be giving the camera one of three options. Either you tell the camera to choose a particular spot in the scene to measure the reflected light, or choose a particular limited area of the scene to measure, or take the entire scene and come up with an average measurement. Neither of these methods is perfect and you may still find you need to make adjustments.

I hope this has given you a better understanding of your digital SLR’s built-in light meter. With this understanding you will be better able to understand the concepts of exposure leading to more creative photographic experiences. This knowledge is also important to understanding why a picture has come out like it has and what adjustments you can make to improve it.