It probably shouldn't be surprising that every one of these lessons so far has talked about light to some extent. These are photography lessons, and photography means "light writing." Photography is about using the energy from light to create something permanent. There are different ways to do this, and each person can decide what they think is the best way (or ways) to interpret a given situation: what to emphasize and how to emphasize it. Lots of elements go into photography: composition, perspective, exposure, and focus to name a few. Some I've discussed (focus and exposure, for example). Some I don't feel qualified to talk much about (specifically composition). For almost every element of photography, though, a little knowledge about light goes a long way to help improve control. This lesson will be about the basics of light and color (one of its characteristics).
(Visible) Light and Color
So what exactly is light? Ask a physicist and they'll tell you it's an electromagnetic wave and that it comes in small packets of energy called photons. They would also tell you a photon of light will have more or less energy depending on what the frequency of the wave is (higher frequency = higher energy). But this general description isn't terribly helpful for photography, so we should get one thing straight early on: the light we're almost always talking about in photography is light you can see—what's called the visible spectrum.
If you've ever seen a rainbow, you've seen the visible spectrum. The lowest-energy photons that we can see are red, and the energy gets higher and higher as you go through the rainbow into yellow, green, blue, and then all the way to violet, which has the highest energy photons our eyes can see. The molecules in our eyes that are designed to respond to light don't respond to photons with more energy than a violet photon or with less energy than a red photon. So why would we only talk about light in the visible spectrum? Because not many people want to make photographs of things they can't see, but there are some exceptions.
To name a couple, infrared photography uses light that has a bit less energy than those low-energy red photons (infra means below), and astronomers look at things in space using the entire light spectrum from very low-energy radio frequency light to very high energy xrays, gamma rays, and cosmic rays.
Talking about the color of light, then, is actually the very same thing as talking about the energy of light. Change the energy, and you get a different color. But you may be thinking we've left out some important colors if all we talk about is the colors of the rainbow. What about things like black, white, and gray? It turns out you can get any of these by using a combination of the light from the colors of the rainbow. White is all 13 the colors combined, black is none of the colors, and gray is white in disguise (it's all the colors with less intensity than something you interpret as white). How do you take light of all different colors (energies) coming from different parts of your image and record that as something recognizable? You just record how much of each color comes to each point of your sensor, then tell the display medium (photo paper or a monitor, for example) how much of each color to display at each point on the image. It sounds easy enough, but as with many other things, nature throws us a curve.
Sources of Light and White balance
For better or worse, not all light is created equally. The band of colors that something emits is called its spectrum, and the spectra for different light sources can be very different indeed. Take as one extreme example light from a laser, which is only at one very specific color. You'll see only red if you use a red laser to light your subject. For the other extreme, let's take the light from the sun. It's a complex spectrum, with most of the energy it emits in the green energy range, and smaller parts in both the red (lower energy) and blue (higher energy). The main point is that it emits all visible colors, unlike the laser which emits at just one.
Sunlight is one example of a whole class of light sources called black body emitters. The idea is that if you get something hot enough, even if it normally absorbs visible light (hence, black body) it will start to emit light by radiation to get rid of some of its extra energy. You've probably seen this on an electrical stove's heating elements.
If you crank up the heat as high as it will go with nothing on the stove, as it heats up you'll see the element glow a dim red, then brighter red, and finally almost orange. Not surprisingly, the hotter something is, the more high-energy photons it gives off. Take the other classic example of a black-body emitter: an incandescent light bulb.
When you flip the light switch on to send current through the light bulb, the filament gets so hot that it actually looks white because it's hot enough to emit the higher-energy green and (hardly any) blue. If you hear someone talking about the color "temperature" of a light source, they're talking about the temperature in Kelvin that you would have to heat something to in order to get it to give off light with that color balance (Kelvin are almost the same as degrees celsius in this case). For example, direct sunlight has a color temperature of about 5500K.
The visible surface of the sun, aptly called the photosphere, is at a temperature of 5800K (5500 degrees C or 10000 degrees F). For comparison, light bulbs that use tungsten filaments can't get hotter than 3700K since that's where tungsten melts, so they give off more red and less green and blue compared with the sun.
I mention these examples to show the fact that you can't always control the balance of colors in the light that you're recording in a photograph. It's the most literal application of wysiwyg (what you see is what you get). There are times when you can help things out by adding light of colors that are missing, but other times the best you can do is to tell your camera (or computer) which colors to boost to get more realistic colors. This is what's called white balance.
You tip the scales toward one color or another until whites look white. In the days of film, you had to do this as you shot the images by putting a colored filter in front of your lens. For example, if you were shooting in incandescent light (again, this is mostly red), to lower reds while keeping blues and greens mostly constant you would put on some kind of aqua-colored filter that lets through blues and greens, giving them a boost relative to reds.
Now, you just increase one color or another by telling your camera or the software you're using to process your pictures what kind of light you're using, and it boosts the channels that need help to get a good balance. It's good to remember what you're doing here, though. If you have a picture with almost no blue light to begin with, boosting the blues is going to make noise in the blue channel more visible.
White balance control is one more advantage to shooting raw when you can. If you shoot raw, you don't need to worry too much about white balance because you'll do that on your computer before you convert the pictures to jpegs. Shooting jpegs, you run the risk of forgetting to set the on-camera white balance for the situation you're in, and it's sometimes so bad you can't fix it.
Confusingly, people talk about photos with more red and yellow tints being "warm" and photos with blue tints being "cool," but just remember that's backward from the black body temperature of the light source. Higher temperatures have more high-energy blue photons in the mix. What makes it even more confusing is that setting a higher white balance temperature on whatever is interpreting the sensor's data actually boosts reds, but that's because a higher temperature source would have extra blue, so it will boost reds to correct for it. That's enough of that!
Something else you sometimes have control over is the direction of light, and it can vary from directional (coming from a single point) to diffuse (coming from many angles). Directional light gives hard shadows and diffuse light will fill in cracks and wrinkles, so diffuse light is used more often in portrait photography, but you can use directional to get different effects. For a light source of a certain size, the closer the subject is to the source the more diffuse the light will be. This might make more sense when you think about how the light source will look bigger to your subject if it's closer, and the light will come in from a bigger range of angles.
One more useful light source in photography is flash. Flashes are based on gas discharge. What you do is shoot electrons through a gas (xenon in this case) with enough speed that they can rip electrons off of the gas atoms. When the knocked-off electrons find their way back to the atoms, they give up the extra energy by emitting light. This all happens very fast—faster than a millisecond (1/1000 s), so if flash is your main light source and you have a longer shutter speed like 1/60, your flash duration is effectively your shutter speed. This can really help for stopping motion.
There are a couple of disadvantages to oncamera flash. For one thing, we're not used to seeing faces and things lit by a light that's coming from right next to our eyes, so everything looks flat since the only shadows you see are behind what you're looking at. The other issue is called red eye, and that's 15 from red light that gets reflected by the retina at the back of people's eyes.
Annoying preflashes to make people's pupils smaller can help, but a better plan is to take the light as far from the lens as possible. If you've seen someone with a flash that mounts to the top of the camera, maybe you've seen them point it up at the ceiling (assuming the ceiling is a fairly neutral color so that you don't get some bizarre color cast). That's called bounce flash, and since the light from the flash reflects from the ceiling it gives a nice, diffuse, offcamera patch of light that is your light source instead of the point-like, on-axis light from the on-camera flash.
Another use flash is fill flash. Flash is white balanced to daylight so that you can fill in shadows in normal daylight. A good starting point is to get about a 3:1 ratio of the light on the bright side of someone's face to light in the shadows (true or not, someone told me this is the ratio Rembrandt tried to use in his paintings). To use the flash for this the output is set to 1 stop less than the incident light (so the flash gives half as much light as the sun where the subject is). That means you get 3/2 incident light on the bright side and 1/2 incident light in the shadows, for a ratio of 3:1. With flash output at the same level as the incident light, you get a 2:1 ratio.
You can experiment to see what looks good to you. How can you know how much incident light you have? You can buy a light/flash meter, or some camera systems have a flash you can buy that will let you do the metering through the lens.
The camera meters the ambient light and fires a metering flash to check the flash level, then fires the fill flash at the level you ask it to. For fill flash, don't bounce the light. Point it right at the subject because you may need all the power you can get from the flash.
Now I'll list some of the most common light sources with a few of their properties.
Direct sunlight
Color content: ~5500K black body spectrum. The color temperature depends on how much of the earth's atmosphere the light has to go through to get to the subject. The more it goes through, the more blues get scattered out, making the light look yellower (this scattering is why the sky is blue). Noonish, it just goes through the thickness of the atmosphere, so it's only slightly shifted— from 5800K to 5500K.
In the morning and evening, the light goes through the atmosphere at an angle so more blue gets scattered out and depending on how many particles are in the air (dust, smoke, etc.) the light can be anywhere from yellow to red. Since sunlight is directional (coming from only one direction), shadows can be harsh. It's a good idea to fill the shadows with some flash or a reflector if you have one.
Light from clouds
Color content: usually about like direct sunlight (it depends on the amount of cloud cover), but now the blue from the sky is more 16 important if the sun is covered by clouds, so you may have to kill some blues. Clouds act like diffusers, bouncing the light in from many angles. This fills in shadows more than direct sunlight, but even overcast can be pretty directional. Watch for this and use fill flash when you need to (bright clouds can provide the fill light if you're lucky).
Shade and window light.
Color content: often has lots of blue, since it's usually coming from the sky. Close to the shade/sunlight boundary, shade is very diffuse since light comes in from almost everywhere. Porch light can be very flattering for portraits if you get the white balance right. If you're using trees for shade, watch out for spots of direct light filtering down through the leaves.
Your eye might not notice them very much, but your camera will (they might be a few stops brighter than the rest). Window light is like shade, but it only comes from a small area (the window). For window lighting, you need to remember that the light falls off really quickly (if you double the subject-window distance, you get about 1/4 the light you had before). Shade is the same way, but not quite as extreme. It's surprisingly dark inside a forest or even deep in the shade of a building, so look for somewhere on the edge of shadows if possible. Be aware that light reflecting off of big, colored objects can change your white balance—a big yellow building will add yellow, etc.
Incandescent lighting.
Color content: ~3500K black body spectrum (very red with some green, very little blue, so it ends up looking yellow) With the right white balance settings, incandescent light by itself can actually render color very well, though you'll have extra noise in the blue channel. The main issue is that if you have mixed incandescent/ window lighting, you'll know which is which really easily because of the difference in color content.
Fluorescent lighting.
Color content: complicated Fluorescent lights are based on mercury gas discharge, which emits mostly in the ultraviolet range. They're coated with phosphorescent materials that convert the UV light into visible light, but only at certain colors—usually three different colors or sometimes four. What the camera sees depends on the mix of phosphors painted on the tube and on the red, green, and blue filters painted on the sensor, so it's harder to set white balance consistently. Another thing to remember is that fluorescent lights aren't continuous—they flash 120 times per second, so remember that when you choose your shutter speed.
Solid State Lighting (SSL).
Color content: like fluorescents These aren't so common now, but you've probably seen white LED flashlights and headlamps, and soon LEDs will be used more in light fixtures in homes, etc. White LEDs use a blue LED coated in phosphors that convert some of the blue light to red and yellow to get a natural white balance, but again it's only at a few distinct colors, so white balancing is tricky.
Rules of thumb
Now that I've gone over most of the important technical aspects of photography, I'll include a list of some practical statements based on what we've discussed in these lessons.
Exposure
• To double the exposure to light while changing the aperture, divide the f/number by a factor of 1.4 (the square root of 2).
• Smaller apertures (bigger f/number) make a bigger range of distances in focus.
• To double the exposure while changing shutter speed, make it twice as long.
• For hand held photos, a conservative shutter speed to stop camera motion is 1/ (2*focal length) for cameras with APS sensors (almost all digital SLRs) or 1/ (focal length) for cameras with full frame sensors.
• Optical image stabilization can help stop camera motion, but doesn't do anything about subject motion.
Sensors and File Formats
• Just because your eyes say two things have almost equal brightness doesn't mean they do.
• To double the camera's sensitivity to light, double the ISO sensitivity.
• Use the lowest ISO possible when you can (meaning the camera's base ISO), since raising the sensitivity amplifies the noise in the picture.
• Shooting JPEG is less forgiving than shooting raw, but you don't have to do any processing later.
• When shooting JPEG, make sure not to overexpose highlight detail that you want to keep. You'll never see it again.
• When shooting raw, expose to the right (err on the side of having bright pictures by about 1 stop, maybe a bit less), and then bring the exposure down in post processing. This helps with shadow noise.
Focus and Metering
• For smaller apertures, chromatic abberation and image sharpness (and I don't mean depth of field) are better since you're using the central sweet spot of the lens.
• To have the camera's metering system adjust the exposure, change the exposure compensation. Setting +1EV of exposure compensation will tell the camera to let in twice as much light as at 0EV.
• Use the histogram unless it scares you. Even if you don’t want to expose to the right, make sure your highlights are highlights or you’ll regret it when you see the noise on boosting the exposure later on.
• Try to avoid the programmed auto shooting mode (P), since the camera makes silly decisions sometimes. If you want the camera to make most of the decisions, it's better to choose aperture priority auto (A) or shutter priority auto (S or T) and have the camera just set one.
Light and Color
• Correcting for the different mix of colors from different light sources is called white balance.
• Shooting raw, don't worry too much about white balance since you can always 18 change it later.
• If you're using flash as your primary light source, try to bounce it off of the ceiling so that it's more diffuse and natural looking.
• You can use flash to fill in the shadows when the light on your subject is very directional.
• To make light more diffuse, get the subject closer to it if possible.
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