The exposure triangle

Today we are going to get into some of the fundamental basics that every camera user should know and understand, but in reality most don’t – it’s really easy, just bear with me for two pages kids.

There are three major elements that influence the exposure, that is how bright or dark your picture will become: The aperture, shutter and ISO speed. Let’s break those down a bit and see what else they do.

The expsoure triangle

… is the locked relationship the three values have to each other. Let’s assume your picture is perfectly lit, not too dark, not too bright, but you want to change a value for one of the reasons explained below, say increase the aperture by one stop. To maintain the same exposure you will have to decrease either the shutter speed by one stop, or raise the ISO value by one stop, which in some cases might not be possible, technically probably yes, but it might ruin the picture, again for the reasons explained below.

The aperture

… is controlled by a ring in your lens consisting of an x amount of blades (not to confuse with knifes) that opens or closes more, letting extra or less light come through. This also controls the depth of field, meaning how much of your scene is in focus. When someone is talking about a wide open aperture he is referring to letting a lot of light in, with only a very small a.k.a. shallow area being in focus, while a stopped down or small aperture passes less light onto the camera, but also increases the amount of detail in focus and sharpness, especially in the corners of the photo.

Opposite to the other two settings this value is not as easy to calculate, while you can just double or cut the others in half to know the next stop, the so called f-stops are as following:

LIGHT = f1.0 <> f1.4 <> f2.0 <> f2.8 <> f4.0 <> f5.6 <> f8.0 <> f11 <> f16 <> f22 = FOCUS

Depending on your camera you have third or half stops in between the above numbers, increasing or decreasing the effect by, well, a third or half stop. It also has to be noted that the depth of field is tied to the sensor size, compacts have smaller sensors and thus the DOF is bigger, allowing less creativity, one of the reason to go for a DSLR. The maximum aperture can only be improved by buying a different lens.

Conclusion: The aperture represents the quantity of light gathered and is responsible for the DOF.

The shutter

… is a super fast curtain in the camera, that opens once you said “Cheeeeese” and punched the big button. In most scenarios it only stays open for a fraction of a second, but in special cases you want to keep it open longer – you don’t have to hold the button for this, it is determined by the shutter speed setting. The major effect the shutter has is controlling motion. If you choose a slower speed it will allow for more blur to show up, e.g. negatively: trees shaking in a landscape, people moving in a portrait etc. and positively for water flowing smoothly, or to create a sensation of speed on cars and so on. On the other hand you will want a fast shutter speed to freeze action, e.g. in sports or wildlife photography.

There is also a rule of thumb for handheld camera shake created by you in coherence with the shutter speed and focal length in use: Shutter speed > focal length. Another example, you use a 50mm lens on a 7D, thus your shutter speed should stay higher than 1/80 (50mm x 1.6 crop). This becomes more visible and problematic the longer your tele lens is. Keep in mind that an image stabilizer is only good for inanimate objects, since it only reduces your shake, not the movement of your subject, a common beginners mistake – a football player won’t run slower because of your lenses IS/VR ;).

Shutter stops are basically halves/doubles as you can see here (in seconds):

BLURRY = 30 <> 15 <> 8 <> 4 <> 2 <> 1 <> 1/2 <> 1/4 <> 1/8 <> 1/15 <> 1/30 <> 1/60 <> 1/125 <> 1/250 <> 1/500 <> 1/1000 <> 1/2000 <> 1/4000 <> 1/8000 = FROZEN

Again depending on your camera you have third or half stops in between the above numbers. Since the shutter is in the camera, an even faster shutter speed can only be achieved by replacing the camera. Most compacts go up to 1/2000, which is enough in merely any situation, generally the focus speed and shutter lag are the limiting factor in point and shoots.

Conclusion: The shutter represents the duration of light gathered and is responsible for motion blur.

The ISO

… defines the sensitivty of the camera’s sensor to light. The sensor is like the film in analog cameras, ultimately responsible for holding on to the picture. Low ISOs like 100 are used in bright situations, mostly outdoors with sun, whereas high speeds like 1600 or 3200 are used in dark situations, mainly indoors, when cloudy or in the evening. The important thing to understand is that the ISO quality is not the same across cameras, due to the size of the sensor and the amount of pixels (MP) cramped on it, but also its construction, like the pixel pitch, distance between pixels.

The bigger the sensor, the bigger the pixels obviously, dictating the the maximum sensitivity available, and quality plus mass of the so called noise, or grain. You will notice that at high ISO speeds a lot less of it will be visible when used in a well lit atmosphere unlike in dim surroundings.

BRIGHT = 100 <> 200 <> 400 <> 800 <> 1600 <> 3200 <> 6400 <> 12800 <> 25600 = DARK

Same as the other two settings your camera also has third or half stops in between these numbers. Because the sensor is in the camera, higher and better ISO performance can only be achieved by buying another camera. Most compacts start to struggle more or less heavily around 800, today’s DSLRs prop that up to about 3200.

Conclusion: The ISO represents the amount of light gathered and is responsible for picture quality.

Other sources

Cambridge in Colour: Understanding Camera Exposure: Aperture, ISO & Shutter Speed

Any questions or corrections, let me know in the comments!

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