Tutorial: Measuring and Adjusting for Light Falloff

By Don Chick, M.Photog.Cr., CPP

Have you recently purchased a new light modifier for your main light? Maybe you have one that you’ve had for awhile, but you’re still not happy with the results you are getting. Your pictures just don’t look anywhere close to those dramatic images shown at the seminar where you ordered the product. The light falloff (decrease in the intensity of the light) across the scene just isn’t right! The part of the subject nearest to the main light is over exposed, while the other side of the subject is underexposed.

You’re frustrated. Now what do you do?

Do you send your new light modifier back for a refund? Do you put it in the closet with all of the other pieces of equipment that “don’t work”? Before you do anything else, go right to your camera bag where you will likely find a very simple solution to this common problem. Your light meter.

A light meter is a professional photographer’s best friend. If you’re not getting the results you expect, stop first to measure the amount of light output coming into your scene. Think of your light meter as being like a thermometer. A thermometer tells us if a temperature is too hot or too cold. Too high or too low means that you have a problem to take care of.

In similar fashion, your light meter is a gauge to show you whether there is too much or too little light falling into your scene. The good news is that you probably do not need to return your new light modifier or stash it away. This tutorial shows how to use your light meter to determine how a modifier affects the light. This one simple tool may just solve your lighting dilemma.

Figure 1 

To repeat the steps shown in the illustrations, position your main light as you normally would near the subject. Using your light meter, adjust the settings to Measure Flash with the shutter speed set to 1/125 second. Measure the quantity of light from one side of the subject to the other.

In Illustration 1, I used a small 22x32 F.J. Westcott soft box. I first measured the light at a distance of 30 inches (from the soft box to under the subject’s chin). I also measured the light at a distance of 18 inches (from the light to the subject’s shoulder). The shoulder reading was 1/2 stop brighter than the reading under the chin. I also measured the light at a distance of 42 inches (from the light to the subject's far shoulder). The shoulder reading was nearly 1/2 stop darker than the reading under the chin. This is a 9/10-stop light falloff reading across the subject, which means there was nearly one complete stop of light falloff across the 24 inches from shoulder to shoulder!

That is not a big distance, so why is there such a drastic change? In layman’s terms, the closer your subject is to the main light source, the more rapid the light falloff will be. The end result of this will be that you are at a much greater risk of underexposing or overexposing your image. Rapid light falloff conditions like this can cause you fits when you're trying to get consistent results from your lab.

One way to solve this problem is to increase Distance A (see Figure 2, below) by moving your main light further away from the subject (or the subject away from the main light). In order to maintain the same light pattern on the subject’s face, you will also need to increase Distance B (Figure 2) by bringing the light forward from the subject.

If you do not do these two steps in tandem, you will have a split lighting pattern as opposed to a Rembrandt or Loop lighting pattern. Angling the soft box slightly back toward the subject will help ensure an even wash of light across your subject area. When I made these adjustments, the falloff had decreased to only 2/10 stop.

 

Figure 2

Increasing the distance between the light and the subject will have a slight effect on your shadow edge. The larger a light source is, the softer the shadows that are produced. Even though the physical size of your light source does not change, when you move the soft box further away from the subject, you produce the same effect as having a smaller light source. You have increased the Light Source to Subject distance (Distance A), which effectively decreases the size of the source and increases the hardness of the shadows. Increasing the shadow hardness is a small price to pay, however, for proper exposure.

 

Figure 3

Personally I don’t use a small soft box for my main light modifier. I prefer the look of a large soft box, either a 3x4 or 4x6 Larson Soff Box. I conducted the same test shown in Figure 1 with both Larson soft boxes and achieved very pleasing results. Starting with the 3x4, I positioned the back edge of the soft box in line with the front plane of the subject area (Figure 3). I took meter readings at position 1 and position 2, and recorded only 4/10-stop falloff of light, less than 1/2 stop.  When I conducted this same test with the Larson 4x6, I recorded only 3/10-stop of light falloff—less than the smaller Larson 3x4.

Remember that a larger light source will have less falloff than a small light source will have for a given distance. You can easily remember these four points in a rhythmic sentence with four parts, each with five syllables:

Put a larger light source
in the same position
to lessen light falloff
and make softer shadows.


What is the practical application of this information? Thank you for asking! 

If you use a small light modifier (i.e. soft box) on your main light and your images aren't coming out the way you want them too, stop and evaluate your situation.

Are you getting a proper exposure? If not, move the light further away from the subject (increase Distance A). Greater distance provides a more gentle transition in your light falloff, and will help eliminate a blown out or overexposed side of the face in your studio portraits.

If you're trying to decide what kind of light modifier to purchase, consider a larger soft box. This will give you images with softer shadows and an all-around beautiful quality of light that your clients will love. You will also experience less difficulty creating a properly exposed image, and you will enjoy more peace of mind when opening the portrait orders from your lab. (This assumes that you have properly metered your lights and have followed the other pointers in this article.) Keep in mind that some specific portrait looks like “Glamour” or “Hollywood” are better achieved using other lighting techniques and equipment (or software).

The large soft box at a greater distance also works well for group portraits. Maintaining even exposure of larger portrait groups can be a problem. When all of the individuals are positioned for a family group portrait, your subject area can be six or more feet across. Your expertise in properly exposing the group will be a key element of consideration when your client sits down to make their decisions about buying. Why would your client want to buy copies of a portrait where Aunt Martha (who is closest to the light) is all washed out and looks like a ghost, and Uncle Henry (who is the furthest away from the light) looks dark and dim?

Become a master of your lighting tools (light modifiers and light meters) and you have a win-win situation. Your client wins because they have a better product in the end. You win because you should have better sales and increased opportunities to sell larger prints. If they love it, they’ve got to buy it!

 

Figure 4

Look at Figure 4. Here you can see that as the distance from the light source increases (shown left to right in this illustration), the amount of light falloff lessens. Notice that you have one complete stop of light falloff in the distance from 2 to 2.8 feet (less than 10 inches!), but at the distance from 11 to 16 feet it takes 5 feet for the light falloff to diminish one stop because the subject and the light source have moved farther apart.

Set aside some time soon to do these experiments on your own. They don’t need to take hours, and you don’t need to do them all at once. Experience is the best teacher, so take accurate measurements and meter readings, and keep a detailed logbook of your findings. Don’t let some intimidating name like The Inverse Square Law scare you off. Follow these basic steps and repeat them again and again until they become second nature for you. If you do this, you will conquer a giant that keeps many photographers from becoming masters of light.

Visit Don's Web site at www.donchick.com for additional resources.

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Comments (2)

VERY informative!!! Thanks for sharing this knowledge, you have no idea how long I've been battling with this little problem when everything I had read until now had been telling me to "just place the soft box as close as possible without it showing up in the frame" I KNEW I was missing something!!! Now I know and understand exactly what it was.

Thanks again!!!

Thank you for posting this article!
I, too, have struggled with this blown-out/dark image battle for a while, not truly understanding why. You have shed some light in a small dark corner of my mind!

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