If you’d like to know more about the essential role that light plays in photography, then you’ll love our course, A Photographer’s Guide to Light. In this lesson, you’ll learn why brightness is the most important quality of a light source.
Noticing Brightness
If a light isn’t bright enough, photography isn’t possible. At least, not regular photography. Modern cameras can take photos in incredibly low light, but this doesn’t mean that you don’t need to be concerned with lighting, because low light restricts the creative possibilities.
If you’re shooting in very low light, you’ll be using wide apertures, long exposure times, and high ISOs just to get an exposure. A single lighting source can drastically change your options in a case like this, and usually the brighter the light, the more options you have because you can do more with a bright light. You can spread the light over a larger area or you can photograph subjects at a greater distance, so brighter, at the most basic level, is almost always better.
What Is Brightness?
You’ve observed varying levels of brightness throughout your life. You could easily say that the sun is definitely brighter than any constant lighting source you’ve ever had in your house, but could you tell the difference between the sun’s brightness on two separate days? That’s a little harder: you know it’s bright, but how bright?
Our perception of brightness is subjective, and your eyes and brain can deceive you. Light—like all other forms of electromagnetic radiation—is made up of photons. Photons at a certain wavelength have a certain level of energy. If you have a level of brightness in a given area and you want to increase the brightness, you need to get more photons in that part of the spectrum to hit that area. More photons equal more light.
To measure this increase in brightness, it makes sense that we need a device to measure the number of photons in the visible spectrum, and this is how light meters work!
Light Meters
A modern light meter uses a semiconductor like silicon or cadmium sulfide to measure light. When photons strike a silicon photodiode, it creates a very small current. The more photons that strike the photodiode, the more current you get.
When photons strike a photoresistor made with cadmium sulfide, the electrical resistance of the circuit decreases. The more light, the lower the resistance.
With a power supply and electronics, these changes can be translated into useful information to help you set the exposure.
Cameras have built-in light meters that measure the light and can give you some indication about brightness. They do this by measuring the reflected light often, in many areas. The information is then used to set some or all of the exposure settings for you, or it tells you that the settings that you’ve chosen will produce an image that’s too bright or too dark.
This meter reading on your camera is all relative to the metering mode that your camera is currently set in and your camera’s exposure settings. What in-camera light meters don’t do is give you an indication of the actual level of brightness in any one area. They also don’t work with manual flash lighting, so in order to use manual flash lighting and get readings at different points in your scene, you need a handheld light meter.
Advanced vs. Basic Light Meters
The simplest handheld light meters let you measure the light in different parts of your scene, which can be very helpful in determining what the light is actually doing. This is usually done by measuring the incident light. Unlike reflected light, an incident light meter measures the light falling onto the light sensor. This will often give you a much more accurate result because the reflectance of your subject doesn’t factor into the measurement.
Sometimes, taking a photo of a highly reflective object will cause a reflective meter to give a reading that’s way off, so the exposure would be far too dark. A handheld light meter can usually measure reflected light in addition to incident light. Most of the time, you would use incident light metering because it’s much more accurate.
The other advantage of a handheld light meter is that you can measure different parts of your scene, and you can measure the intensity of separate lighting sources. This is very useful for replicating lighting looks and making sure the illumination is even across backgrounds.
Light meters measure the light and display the measurement in different units. Advanced meters will display the measurement in lux, foot-candle, foot-lambert, and candela per square meter. These are all standards for measuring light.
Foot Candle
For example, foot-candle is the amount of illumination the inside surface of a one-foot-radius sphere would be receiving if there was a uniform point source of one candela in the exact centre of the sphere.
Basic Metering
In contrast to advanced meters, basic light meters will usually have an aperture value mode and an EV display mode. These modes calculate the light reading and then display useful information.
In aperture mode, you input the exposure time and the ISO and then take a reading of the light with the meter pointed at the lens of the camera. The meter then displays an F number. If you’re setting the exposure for your main light, you’d take this reading and set your camera to that F number. If there was a particular aperture you wanted to use, say f/4, you’d take the reading of the light and then make an adjustment to the light so that your meter was reading f/4.
EV Mode
In EV mode, the meter gives you a reading in EV—exposure value—which represents a level of light intensity for a camera set to a certain ISO. It has the same logarithmic exposure scale as everything else in photography. For example, EV4 is twice as bright as EV2, or in photo terms, one stop brighter.
This number can then be used with a table to figure out the exposure.
Let’s say we had a reading of EV11 at ISO 100. If we wanted to use a 1/250 of a second exposure time, we’d need to use an aperture of f/2.8. At this point, you could then work out any combination of exposures that would work.
You can also work out a conversion from EV to other luminance units.
Light at Distances
Another property of light that’s useful to understand is what happens to light at distances. To figure that out, we use something called the inverse square law. In physics, an inverse square law is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity.
Imagine a light bulb radiating in all directions. The total amount of energy coming off the light is constant. If a card is placed in the field of the light, the card will be illuminated with a portion of this light. You can see that on the left side of the image above. If the card is moved back twice as far, like on the right, it gets darker. The reason for this is that the light that was filling the card before is now spread out over an area four times as large. This means that at twice the distance, the card now has one-quarter of the light falling on it.
At double the distance of the first measurement, the new intensity is one-quarter. If it’s doubled again, we get one-sixteenth. And so on. What you see is that light falls off very quickly from the source and then tapers off at a much slower rate. We can use this to our advantage in a number of ways.
Example of the Inverse Square Law
In the left image above, the light is positioned very close to the subject, less than a few feet away, and he’s about 6ft away from the background which is nice and dark.
The metering is the same for both images, but in the one on the right, the light has moved to about 10ft away from the subject, and there’s a pretty big difference. The exposure of the background has really come up, and it’s a good 16 feet away from the light, so you can see how the inverse square law gives you a lot of options in the way things look.
Summary
Brightness is key to how you approach your photography. While sometimes you might be able to judge the light by eye, there are times when you might need a meter to help you get more accurate results and make the right lighting decisions. A basic light meter will definitely do the job in most cases, but again, if accuracy is key for you, then you might want to invest in a more advanced one.
If a lot of the in-depth stuff and the maths is a bit over your head, don’t worry—it is for most of us, and it’s certainly not going to prevent you from making great photographs. As with most things, though, the more you understand about why something works the way it does, the better and more confident your choices will be. Eventually, it will become part of the rhythm, and you won’t need to think about it quite so much.
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About the Authors
David Bode created the video course that includes this lesson. Dave is an expert on video and audio production, and he lives in the upstate NY area. He works as a camera operator, editor, inventor, motion graphics designer, recording engineer, and studio musician.
Marie Gardiner wrote the text version of this lesson, and it was edited and published by Jackson Couse. Jackson is a photographer and the editor of the Photo & Video section of Envato Tuts+.
