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Other photography tutors refer to the "exposure triangle" which has three nodal components - aperture shutter, speed and ISO. Although this introduces the variables of exposure, it does little to illustrate their working relationship.
My Exposure Seesaw, on the other hand, illustrates how the most functional components, aperture and shutter speed, readily swap and balance values. Also in better context, ISO sensitivity and exposure compensation are represented on the diagram by moving the seesaw left or right, and up or down.
Each seesaw colour illustrates a combination of aperture and shutter speed values that balance to give the same exposure - that is, they let the same amount of light into the camera.
Aperture is the diameter of the variable diaphragm whole that lets light pass through the lens into the camera.
The f numbers work the opposite way from what you'd think because they are a ratio expressed as a fraction of the focal length of the lens, but you don't need to understand that - just work with the numbers.
On the diagram, as with shutter speed, each f number value lets in double of half the light of its neighbour: f8 lets in twice as much light as f11 and only half as much as f5.6.
Shutter speed - The shutter is like a door that opens to let light into the camera; shutter speed is how long the door is open.
In photography, light measurement units are logarithmic. So, on the diagram, each shutter speed value is double or half that of its neighbour: 1/50s lets in twice as much light as 1/100s and only half as much as 1/25s.
Besides their effect on exposure, aperture and shutter speed have other properties that need to be considered for the reqirements of each image:
Aperture affects depth of field (what’s in focus from near to far).
By using smaller apertures, such as f8 or f11, and focusing at the optimal distance, you can maximise your image's depth of field (DOF) so everything is sharp from foreground to infinity; this technique is used mainly for landscape and other genres where you want everything in focus.
Wider apertures, such as f2.8, allow selective focusing where the main subject is sharp and the rest of the scene is less in focus; this works well for portraiture, street photography, and sometimes abstract and still life photos.
Shutter speed, on most occasions, needs only be fast enough to take a blur-free shot when hand holding the camera. In addition to this, it can be set fast to freeze a moving object or slow to capture the flow of its movement.
For aperture, shutter speed and ISO to exchange values, they need a common currency. This currency is EV (Exposure Value), and its units allow shutter speed values to be traded for aperture and / or ISO.
In aperture priority mode, you select the right aperture for your shot and your camera sets the corresponding shutter speed to maintain the correct exposure. In the diagram, this is like having hold of the aperture end of the seesaw and moving it up or down while the shutter speed values change correspondingly at the other end of the seesaw.
In shutter priority, you'd have a hold of the shutter speed end of the seesaw.
The easy way to remember this is, when aperture f numbers rise, shutter speed numbers fall and vice-versa. As one goes up, the other goes down - easy
ISO is simply the bank of EV currency.
When light levels and shutter speeds are low, you can borrow EVs at the expense of image quality. There's a small trade-off of dynamic range too. On the diagram, this is represented by moving the seesaw to the right - a dirtier, but more light-sensitive, place; each EV you borrow can be spent increasing shutter speed or stopping down aperture, or shared between the two. Auto ISO is like having an overdraft arrangement; you get to spend EVs, but it's wise to keep a check on what you're borrowing.
When there's plenty of light, you don't need the extra sensitivity, so it's best to keep ISO levels low for the best image quality. On the diagram, this is represented by moving the seesaw left to a cleaner, but less light-sensitive, place.
You're taking a landscape shot, so you set aperture to f11 to get sharp focus from near to far. In the diagram, f11 is on the orange seesaw and corresponds to a shutter speed of 1/50s. You take the shot at those settings.
Next you decide to take a photo with your partner in the same scene. This time, you wish to focus selectively on your partner and have the rest of the scene less sharp, so you choose the aperture with the shallowest depth of field, f2.8. In the diagram, f2.8 is on the turquoise seesaw and has a corresponding shutter speed of 1/800s. (All other settings remain the same.)
In doing this, you've scrolled your camera's aperture +4EV, from f11 through f8, f5.6, and f4 to f2.8. To compensate, your camera adjusts the shutter speed -4EV, from 1/50s through 1/100s, 1/200s, 1/400s to 1/800s.
Aperture has traded with shutter speed in their common currency and the overall exposure remains the same.
Exposure compensation is where you choose to set a different exposure value (EV) than your camera's metering system has chosen. For more on exposure compensation including when and how to use it, read the tutorial, Exposure Compensation. And if you shoot RAW, you can learn how to deal with high contrast scenes, combat noise and obtain the highest quality post processed images by reading Exposure Optimisation.
In aperture or shutter priority mode, you use the EV button and command wheel to increase or decrease exposure; this is represented by moving the seesaw up or down in the diagram.
In aperture priority, the aperture numbers are fixed to the seesaw and the shutter speed changes; in shutter priority, the shutter speed numbers are fixed to the seesaw and the aperture value changes.
In fully manual, you set the shutter and aperture values and your EV display tells you how many EVs this may differ from the camera suggested setting (it's best to look at the histogram; see me further exposure tutorials, Exposure Compensation and Exposure Optimisation).
This would be like hinging the seesaw at its pivot and moving each half independently. If either end bends downwards, you're increasing the exposure, and if they bend upwards, you're decreasing it.
That's it. Shutter speed trading with aperture, borrowing EVs where necessary from the bank of ISO, and exposure compensation illustrated in aperture priority and fully manual.
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