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2.2 Brightness

In this lesson you will learn why brightness is the most important quality of a light source.

2.2 Brightness

In this lesson, you will learn why brightness is the most important quality of a light source. If a light isn't bright enough, photography isn't possible, at least what I would consider to be, quote, unquote, normal photography. The latest generation of super sensitive cameras can take photos in incredibly low light. This doesn't mean that you don't need to be concerned with lighting. Because low light restricts the creative possibilities. For example, if you are shooting in super low light, you are going to be using big, 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. This is because you can do more with a bright light. You can spread the light over a larger area or you can shoot subjects at a greater distance. So brighter, at the most basic level, is almost always better. But what is brightness? You have observed varying levels of brightness throughout your life. You could easily say that the sun is definitely brighter than any constant lighting source you have ever had in your house. But could you tell the difference between the sun's brightness on two separate days? Now it gets a little bit harder. You know it's bright, but how bright. Our perception of brightness is subjective, and your eyes and brain can deceive you. Thinking about what you have learned in the previous lesson, you know that light and all other forms of electromagnetic radiation is made up of photons. Photons at a certain wavelength have a fixed level of energy. If you have a certain 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 equals more light. To measure this increase in brightness, it makes sense that we need a device to measure the amount of photons in the visible spectrum. And this is how light meters work. 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 will 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 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. It then uses this information to either set some or all of the exposure settings for you, or it tells you that the settings that you have selected are going to produce an image that's too bright or too dark. Now, 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 do not work with manual flash lighting. In order to use manual flash lighting and get readings at different points in your scene, you need a handheld light meter. The simplest handheld light meters let you measure light at different parts of your scene. And this 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 that is 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 is way off, and the exposure would be far too dark. A handheld light meter can usually measure reflected light in addition to incident light. But 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. Lux, foot-candle, foot-lambert, and candela per square meter are all standards for measuring light. For example, foot-candle is the amount of illumination the inside surface of a one-foot-radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere. In contrast to advance 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 are setting the exposure for your main light, you would take this reading and set your camera to that F number. Or if there were a particular aperture you wanted to use, say F4, you would take the reading of the light and then make an adjustment to the light so that your meter was reading F4. In EV mode, the meter gives you a reading in EV. EV stands for exposure value and 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, EV 4 is twice as bright as EV 2, or in photo terms, one stop brighter. This number can then be used with a graph to figure out exposure. Lets say I get a reading of EV 11 at ISO 100. If I wanted to use a 1/250th of a second exposure time, I need to use an aperture of F 2.8. At this point I can then work out any combination of exposures that will work. You can also work out a conversion from EV to other luminance units. EV 11 at ISO 100 is 476 foot-candles, 5,120 lux, 74.7 foot-lamberts, and 256 candela per square meter. Like I mentioned before, there are meters that will give you readings in lux, foot-candle, foot-lambert, and candela per square meter, but more basic meters don't have these options. Another property of light that is useful to understand is what happens to light at distances. To figure that out, we use something called the inverse square law. According to the Wiki, 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 that is radiating in all directions. The total amount of energy coming off the light is constant. If I place a card in the field of this light, the card is illuminated with a portion of this light. If I move the card back twice as far, what happens? The card gets darker. But why? The reason for this is because 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. It works exactly the same with the math. At double the distance of the first measurement, the new intensity is one-quarter. If I double it again, I will get one-sixteenth, and so on and so on. What you see is that light falls off very quickly from the source and then sort of tapers off at a much slower rate. And we can use this to our advantage in a number of ways. So I wanted to show you a quick example of the inverse square law in action here. I set up a little demo with my son Lincoln here, and I wanted to show you the dramatic effect the inverse square law can have on your subject and background elements. So, to start this little example off, I have my light here positioned very very close to my son Lincoln. He's about six feet away from the background. And then I took three more shots, and each time I moved the light further back. So this is what the first shot looks like. The metering on all of these is exactly the same. It all reads about F 8. So in this first photo here, you can see that the background falls nice and dark. If you look at the second photo here where I pulled the light back just a few feet, you can see that the background comes up a pretty good amount. You can see the exposure is pretty much the same on Lincoln's face. Now, the lighting looks different because the light is in a different position, and so the shadows are falling in a different place. But if you look at the parts of his face that are actually lit, they're very very similar in exposure to the previous photo. You can see in this third photo here, the light is now about seven feet away. It's behind the camera. And the exposure on the background has come up again. If we look at the fourth photo, we have even more of the background coming up. And there's a pretty big difference by the time we got to the last photo here. The light is about ten feet in front of Lincoln, which is about 16 feet away from the background. So it's a pretty big difference in distance, but with the inverse square law, it gives you a lot of options in the way things look. Coming up in the next lesson, you're gonna learn all about color.

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