Technical

Understanding light meters

Light meters are colorblind. They only see gray. In fact, they can only see one very particular shade of gray. It’s called 18% gray.

18% Gray

Why this precise shade of gray? The theory is this: If you took all of the colors and all of the shades of brightness in the world and mixed them together to form the average "color/brightness" – it would be 18% gray. The theory is solid. An average scene is indeed 18% gray. However, there aren't very many perfectly average scenes that you want to photograph. You want photos of the unusual… the gorgeous sunset, the close-up of a rose, and such. If you learn to think like your light meter, you can predict when it will make a mistake. Here’s how:

Imagine the average scene with a little of everything in it. Some trees, some grass, some people, some buildings, some sky. Think in black and white, your meter does. The trees and grass aren't green – they are gray. Everything is a shade of gray.

An average scene

Now mix all the shades together. Some shades are dark, some medium, some light. You get a nice medium gray. You will also get a nice exposure from your light meter since it tries to make every scene average (18% gray).

What the meter sees

OK. Now let's imagine a different kind of scene – a raven in the snow. The raven is a small black spot in an otherwise white photo.

A raven in the snow

What happens when you mix the shades in this scene? Think of a bucket of white paint with a drop of black on the surface. Mix it up. Do you get a nice medium gray? Of course not. It is still almost completely white.

Your light meter, though, is still going to try to make every scene 18% gray. What is going to happen? You have an image that is almost entirely white, and the meter is going to make it gray. This means that the whole image will be underexposed. The snow will come out a nice medium gray, and the raven will be totally black.

Not very much like what you had in mind, is it?

Let's look at another scene. This time you're at a rock concert, and you want a photo of the band. You are in an dark auditorium with the band brightly lit far below you. This time imagine a bucket of black paint with a spot of white on the surface.

The band in an auditorium

When you mix it up, it doesn't turn medium gray either. It stays almost completely black. Your light meter is still going to try to make every scene 18% gray. What is going to happen? You have an image that is almost entirely black, and the meter is going to make it gray. This means that the whole image will be overexposed. The auditorium will come out a nice medium gray, and the band will look like they are on fire on the stage; they'll be totally white.

Not very much like what you had in mind, is it?

Even though it seems like the wrong thing to do, for darker scenes you need to give less exposure, and for lighter scenes, you need to give more exposure. This will make darker scenes (which would have turned out too light) darker. And it will make lighter scenes (which would have been too dark) lighter.

As a rule of thumb, always think in terms of the subject of your photo. If your subject is darker than the background (the raven scenario), you need to add light to the subject to make it match the background (give it more exposure). If your subject is lighter than the background (the band scenario), you need to subtract light from the subject to make it match the background (give it less exposure). It will take a little practice to get it just right, but here are a few guidelines:

• Adjust, plus or minus, .5 to 1 f/stop for scenes that vary only slightly from average.

• If you have a camera with "matrix," "honeycomb," or some other version of multisegmented metering, cut down the adjustments to about 1/3 of the suggestions above.

• Adjust, plus or minus, 2 or 3 f/stops for extreme scenes (the raven and the band are both extreme).  

• Adjust, plus or minus, 1 to 2 f/stops for medium scenes (a photo of someone indoors but with a window in the background). 

Red eye

At one time or another, red eye is a problem for almost every photographer. Most problems arise because red eye is not understood. The basic causes lie in the somewhat technical laws of physics.

The eye is a spherical device that has a double-sided convex lens (both sides of the lens curve outward away from the center) for a front element. A double convex lens has the property of projecting any point in space onto only one corresponding point on the other side of the lens. This is the reason that our pupils appear black to each other even though the retina, which lies directly behind the opening, is bright, blood red. In order for you to see into your friend’s eye, the light source illuminating their retina would have to be where your eye is. A doctor looks into your eye with a special tool that allows him to place his eye directly behind the light source, something not normally possible.

Red eye happens when the light source is close enough to the cameras lens and bright enough to illuminate the retina.

Click here for an illustration of the solution that physics offers for red eye (get a taller flash)

Red eye is very hard to predict. Sometimes you get it, sometimes you don't. Children get it more often than adults do. People with blue eyes are more prone than people with brown eyes. One can often predict when they will get red eye, but predicting when you won’t get it is an entirely different problem.

Keep in mind, as we discuss some solutions, that NONE of them are an absolute cure. There is no cure. Even so, we can reduce the number of times you have to groan when looking through your prints.The best way not to get red eye is to use a taller accessory flash on your camera. This will make more difference than anything else you can do. The further apart the flash and the lens of your camera are, the better. Many professionals go as far as putting their flashes on long cords or infrared remotes.If your camera will not accept an accessory flash, it may have a "red eye reduction" mode. This usually involves having the built-in flash blink, or strobe, just before the picture is taken. The idea is to cause your subject's pupil to shrink in size because of the brightness, and thus reduce occurrences of red eye. This is a hit 'n' miss procedure that is more effective with young people than old because younger pupils will close faster. It is not a cure but is often well worth trying.

Another way to shrink the pupil is to turn on as many room lights as you can. Even though you are using a flash, and the room lights will hardly affect your final picture, they will help you to reduce red eye. Having your subject look slightly away from the camera can also help reduce red eye.

The last ditch effort: use a red eye pen. You'll know when to use the pen because you will have used one of the techniques above (or all of them combined), and when you get your prints back you still have a couple of shots with red eye. The red eye pen is a felt-tip pen with transparent cyan dye for ink. It is just the right color to counteract the red from an illuminated retina. It is also transparent so that it allows details, such as highlights on the eye, to show through. It looks very natural, so you can salvage your red-eyed prints.