Demystifying Photography: Exposure

In a previous post, I talked about Composition and Exposure being primary elements of the images you will take. I’ll start with the exposure side of things since its more ‘sciency’ and easier to stomach.

A little bit of terminology

When you take a picture, you control the amount of light entering the camera and focus it onto an image sensor. This light that you let in is measured in ‘stops’. A stop is not an absolute measure like say speed, in the sense that you can’t state: “That wall is 2 stops bright”. Instead, it is used as a relative term: “That wall is 2 stops brighter than the ground”.

Most modern digital cameras take the guess-work out for you. They look at the scene you’re trying to shoot and decide what the best exposure for it is (that is, to make it look like your eye sees it). They then give you a readout allowing you to make the image brighter or darker.

This is the meter on a 5D Mark II showing the exposure of an image. "0" is the base exposure of a scene. Selecting "+1" will cause the image to be twice as bright as "0" while "-1" means half as bright. "+2" and "-2" correspond to four times and 1/4 times bright as 0 respectively.

To get an idea of what one stop is, it’s simply the doubling of the amount of light. 2 stops means 2 doubles, so the wall I described above would be 4 times as bright. The images below explain this concept more clearly.

These three images of the Burj Al Arab were taken simultaneously at three different settings. The middle is the 'base', or what the camera thought would be 'proper' exposure. It decides this using a complex algorithm built in. The '-2' is two stops darker (hence 4 times less light) than the base, while the +2 image is 4 times brighter. Shooting the same scene at different exposures is called exposure bracketing. But more on this in another post!

If not all of that stuck, no problem! As a photographer, you only need a general understanding of what stops are and only a photographer Nazi would hate on you for not knowing exactly what they are.

Nazi cat is not amused you lack knowledge of the basics of photography.

Affecting Exposure

Let me just start off by saying that photography is a game of trade-offs. Any button or dial that you change on your camera affects your image in both positive and negative ways. Good photography is like finding good compromise. As you will see, by trying to improve the exposure of the image, you will inevitable change its composition and vice versa.

This post will concentrate on the exposure side of things, and you can expect the details of composition in a later post.

Shutter Speed

Onto methods of controlling exposure. We’ve already discussed how a camera uses a shutter to let light into the sensor. It’s a no brainer that the longer this shutter is open, the more light gets in and the brighter your picture is. The camera’s shutter controls the number of seconds the shutter will stay open.

If there is too much light, when the sun is out for example, modern SLRs can open their shutter for just 1/4000th of a second to make sure not too much light gets in. In a very dark room, the shutter might be kept open for minutes to capture all the light available.

Since the night sky is so dark, this image was taken with the shutter open for a full minute. By keeping a shutter open this long, the sensor even detects faint traces of gaseous nubulae that are invisible to the naked eye!

If you want an image to be brighter, keep your shutter open longer. How much longer? Remember your stops! If your current shutter speed is 1/80s and you want to double the brightness of your image (1 stop!), just double your shutter speed to 1/40s! Luckily, your camera probably allows shutter speeds in the middle as well so you can make more fine tune brightness adjustments. Cameras by default go through 1/3 stops, but can be set to 1/2 stop increments!

Now your trade-off, and some of you already guessed it, is blurry images. Our world is constantly in motion. From that kid you’re taking a picture of to your hand holding the camera, nothing ever sits still. The longer you have your shutter open, the more likely this movement is to affect the image.

For example, unless you’re a robot, you cant possibly hold a camera still for a minute to take an image like the one above. I used a tripod, but even that didn’t stop a second form of movement. If you look closely at the stars, you can tell that they’re not perfect points, but elongated smudges. It turns out, that since the earth is revolving, it causes stars not to sit still. Keep a shutter open long enough and it could ruin your image.

Aperture

The second way to affect the amount of light coming into your camera is aperture. If you recall, your aperture is the size of the ‘hole’ inside the lens. Logically, the bigger this hole, the more light that gets in. Therefore, an aperture of f/2.8 allows more light in than f/4. Exactly twice as much in fact. What is completely illogical however, is what all those numbers mean: 2.8 isn’t half of 4. Illogical only, of course, if you’re not willing to get into the math. Please read the following, only if you want details. Ignore it otherwise:

An f-number denotes the ratio of the focal length of your lens (the mm number of your lens) to the diameter of the opening for the aperture. So if you have a 50mm lens on, an f-number of f/2.8 means that the diameter of the hole is 50 / 2.8 = 18mm. Now, the amount of light reaching the lens depends on the AREA of the hole and not its diameter. The area of the lens above, if you remember your basic math, at f/1.8 would be pi * (18/2)² = 254mm². To halve the amount of light, we need to halve this area to 127mm². Reverse the previous formula and you get the area needed to be 2*sqrt(127 / pi) = 12.7mm². This corresponds, using the ratio formula, to 50/12.7 ~= f/4.

Thus proving that changing f numbers actually doubles or halves the AREA of the aperture. It also proves that I am a huge nerd. But more on that later.

If you’re an engineer, working through that last paragraph just made you smile. If, on the other hand, looking through that write-up makes you want to slap me, just know that f-numbers affect the width of a cameras hole, while stops are affected by the area. Also, please don’t slap me. I bruise easy…

What this means is that you need to increase the width of the hole with weird increments to get doubling/halving of the light going to sensor. Your camera takes care of the math anyways, listing the common apertures as f/1.0, 1.4, 2, 2.8, 4 and so on. Most cameras will also allow for 1/3 stop increments, with f-numbers in between.

Moving on to trade-offs, the aperture of your camera affects your image, albeit very subtly. The smaller your aperture number, the smaller something called ‘Depth of Field’ (or DOF) is. I will not go into details here, since this is a discussion for the ‘Composition’ section, but it suffices to say that sometimes, small depth of fields can cause parts of your image to blur out and other parts to be in perfect focus. This could be both wanted and unwanted, but as a photographer, you should know how your messing around with the exposure settings is changing your image.

A large aperture can be used for highly creative effects!

ISO

Probably the simplest concept to understand, the camera’s ISO is how sensitive the sensor is. Holding shutter and aperture constant, if you make your camera more sensitive, it will brighten an image. Making it less sensitive makes your image darker. ISO speeds in most cameras are listed as 50, 100, 200, 400, 800, and so on, each number increasing the brightness of the scene by 1 stop.

There is, however, a trade-off, again. Making a the sensor more sensitive to light, also makes it more susceptible to ‘noise’. You can think about it this way: lets say you’re talking to a friend over the internet, and there’s a low humming sound in her room right now. To hear her clearly, you increase the volume of your speakers. Not only is her voice louder now, but so is that annoying humming in her room. Similarly, because of imperfections in optics and electronics, there is a constant ‘hum’ of bad light hitting the camera sensor. By increasing the sensitivity of the sensor, you increase the effect this light has on your final picture (for the engineers: upping the ISO lowers your signal-to-noise ratio, hence the camera can less easily differentiate between the two). Hence noise!

In Conclusion

Admittedly, that preceding discussion was a lot to take in. But what I hope you take away from here is a basic understanding of the tools photographers have at their disposal to affect a scene’s exposure and knowing that there are trade-offs to each decision you make in shooting your scene!

Next up, the considerably less erudite: Composition!

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