Using Light Meters For Accurate Exposures

Using Light Meters For Accurate Exposures

by William Lulow

A light meter is a device for measuring the amount of light on a given subject. It is important to understand the various types of light meters in order to see how the metering system in today’s digital cameras work. It is also important if you are interested in obtaining good MANUAL exposures for maximum rendering of the Zone System’s scale in monochrome as well as good representation of colors. Auto mode settings take the decision out of the photographer’s hands and just provides an overall exposure. (The Zone System, by Ansel Adams & Fred Archer, is a method they devised for how to show all gray-scale tones in a black&white print. I have described it in a previous blog article.)

The REFLECTIVE light meter measures the amount of light that is reflected from the subject. It is usually pointed at the subject from the position of the camera. (In this case, it is contained in the camera itself and measures light as it is reflected from the subject through the camera’s lens). It works fairly well for most conditions, but can be “tricked” by shiny objects within the scene and other factors that affect exposures.

The INCIDENT light meter measures the amount of light that falls directly on the subject from the subject’s position. This is the most accurate method of determining a correct exposure for the subject as a whole. It can even be used for specific points within a scene if used properly.

The SPOT meter is a device which also measures reflective light, but does so for only about a one-degree coverage of the scene  It is excellent for measuring light reflected from a subject at moderate distances when the photographer knows what part of the scene he or she needs to have correctly exposed. It is not good for measuring general scene illumination.

Most good digital cameras these days, have REFLECTED light meters built in to them. (This one here is set to read a center spot, as you can see by the icon at the top right-hand box of the illuminated panel.)  They can measure an overall scene as well as various other types of readings. You can set your camera to measure light in several types of ways. The most common are:

1. OVERALL READING: This measures the light reflected by the entire scene in front of your camera.
2. CENTER WEIGHTED: This measures the light reflected by the entire scene in front of your camera but gives about 60% more importance to the center of the scene.
3. SPOT READING: This allows you to set a particular spot in the scene in front of your camera that you want all the importance placed in order to make a good exposure. It doesn’t take in shiny items that might be on the edges of the viewfinder.
4. AVERAGED READING: This is a method that uses the electronic chip in a digital camera to record two values within the field of view and make an average exposure between the them.

These are the main types I want to talk about here.

With today’s digital cameras, photographers have quite a lot of leeway when it comes to exposures, because they can also be adjusted in the post-processing step in the production of a good image. Software can very often make a bad exposure into a good one. In the days of film, the only way to adjust for a bad exposure was in the development process and then, you had to be sure your initial exposure was bad because once you over or under-developed the film, that was it! The only way you could change it is in the printing stage and that was often very difficult as well as time-consuming.

Given all of this, my routine for getting an optimal exposure is the following:

1. As noted above, I usually have my camera’s built-in meter set to SPOT METERING, because I decide what I want to show before I shoot and expose for that value, letting all the other parts of the image “fall where they may on an exposure scale.” I am always thinking that certain parts of an overall scene can be adjusted in post processing, but since I usually shoot portraits in a studio situation, I really don’t have to worry about my in-camera meter because my studio flash determines the exposure.
2. I mentally analyze the lighting. If it’s a bright, sunny day and I’m shooting with an ISO of 100, my usual exposure is f/11 @ 1/125th of a second. This just takes practice. If it’s overcast with no direct sunlight, I open my aperture to about f/5.6 @ the same shutter speed.
3. I then look at the LIGHT METER GRAPH in the viewfinder or in the window on top of the camera and adjust my APERTURE FIRST so that the arrow points to a spot 1/3 of an f/stop less than the center mark. This is underexposing the image by that much. I do this because I usually want to saturate the colors as much as I can in the original exposure, keeping in mind that it can be altered later.
4. I make an initial exposure and then look at the camera’s LCD, which I have set to “hold the image.” Then I can study it to see if I want to change anything. Again, if I’m in a hurry for some reason, I always know that I can change it when I download the file.

In the days of film, if I was doing an assignment with an art director present, I would do a Polaroid print just to make sure everything was exposed properly. Then I would show that print to her. I am treating the camera’s LCD as if it were a Polaroid print. I can view it quickly or enlarge certain parts of the shot to check for sharpness, exposure or both right on the spot.

When I was shooting film, most of my concert photographs were made with the use of a SPOT meter so that I could measure accurately the amount of light reflected from the musician’s face. An arena or another concert venue was always dark, but the stage lights themselves were very bright. I often was able to shoot with an aperture of f/5.6 or even f/8. In addition, these days, I’m using the spot feature of my camera’s meter to measure the exact place I want to expose for in the shot. I can then decide where I want the most detail to show in my image. Once I’ve done that, I can decide further how much detail in both highlights and shadows I want to show. This is a conscious decision on my part as the photographer. I’m not letting the camera decide the exposure. I’m only using it to obtain readings off the places in the image I choose! I then use the camera’s “exposure scale”, usually visible in the viewfinder, to fine-tune my exposure.

Here’s an example I like to use:

This is the famous, St. Francis de Asis adobe church in Taos, New Mexico that Ansel Adams actually photographed from the back in 1929. I used my camera’s meter set to SPOT mode. I physically went up to the front of the church and took a reading of the light reflected from the wall. I then went to the side facing the camera and took a reading from that wall. I then averaged the two. The initial exposure was ISO 800, f/13 @1/160th of a second. The camera was on a tripod. I was thinking about rendering the details on the adobe walls visible in both the shadows and the highlights. I wasn’t concerned about the bush or anything else in deep shadow, just the walls.  I wanted to emphasize the clouds, so I used a red #25A filter over my lens. That was the main reason for a slightly higher ISO setting. I knew I could get a very sharp image even with a setting of 800.  (From experimentation and a lot of printing, I can get really sharp images even with an ISO setting of 1250!) In the days of film, you really had to sacrifice some quality if you underexposed your film that much.

I wanted the statue in front to be a bit brighter so I used my  quick selection tool to select it and then increased the exposure digitally by one stop.

So, I’m actually slowing down my approach to exposure determination by THINKING ABOUT what I want to show and what will be important in the image. I also use my “back button focusing” technique to select the part of the image I want to be tack sharp and then re-composing the shot the way I want. This also helps to pinpoint my exposure to coincide with the part of the image I want to be most important.

This is the way I ensure that the image is sharp, but also exposed the way I want it to be. If needed, sometimes I use the post-production phase to further tweak any exposure and/or composition. Being able to toggle between Lightroom, Bridge and Photoshop really makes the post-production process and any retouching very easy to do.