Updated: Feb 19, 2020
Understanding How Aperture, Shutter Speed and ISO work together
When learning how to shoot in manual mode we need to know and understand the exposure triangle. Before we go ahead and master photography techniques understanding the three basic and key elements of the exposure triangle in photography namely the Aperture, Shutter speed and ISO is the primary step toward improving our photography skills. The use of Aperture, Shutter speed and ISO in conjunction to each other would help us to nail the correct exposure.
Now let me elaborate each one of them in detail and see how it affects the photos you click. These functions not only effect the exposure, but they also determines the global appearance of any image; thus, it is really importing to know about them both for technique and composition. For the purpose of this discussion I will assume that you have a camera that will allow you to shoot in manual mode and control these three elements independently.
The first important element of the exposure triangle is the Aperture. Aperture is basically the variable opening through which light enters in any camera. Aperture is referring to the lens of your camera and the actual opening in the center of the lens which can go wider or narrower depending on the effect you want to create.
Aperture is basically a measuring unit which determines how open or closed the lens’ iris is. A wider aperture (or lower f-number - f/1.4 or f/1.8) means more light will be let in by the lens, simply because the opening is larger. A narrower aperture (or higher f-number - f/16 or f/22) allows less light to reach the sensor.
You might wonder why we would ever want less light to reach the sensor. The answer the majority of the time is that we want a larger depth of field. Depth of field is a byproduct of aperture. Small apertures (higher f-numbers) give a greater depth of field, which allows more of a scene to be in focus (think landscapes which have everything in detail). Wide apertures (lower f-numbers) create a narrow depth of field, which helps to isolate a subject and is one of the greatest compositional tools at your disposal (think portraiture which have blur backgrounds).
You should also note that most lenses are their sharpest around f/5.6 or f/8. However, many photographers are willing to trade some sharpness for the subject-isolating effects of a wider aperture. Now let me explain this with two photographs shot with different f-stop numbers.
The first photograph is shot with a low aperture of f/2. Notice the fences at the back which are quite blurred and only the Monkey in foreground is sharp and in focus. The very same shot is made with a low aperture of f/22. Now both the fences and the monkey are sharp and in focus.
2. Shutter Speed
Shutter speed is probably the easiest to perceive. It is actually a measurement of time the shutter is open to let in light to the sensor when the shutter actuator is depressed. In a base DSLR the shutter speed can range between 30 seconds to 1/4000th. of a second. A shutter speed of 1/1000 means that when the shutter actuator is depressed the shutter only opens for a time of 1/1000th. of a second to let in light.
Faster shutter speeds give the sensor less time to collect light and thus, result in a lower exposure. Whereas, Slower shutter speeds allow more light to enter through the shutter and result in a higher exposure.
Shutter speed is also used to freeze action. When you photograph sports or a fast moving object or a flying bird you should be looking to freeze the pane for a perfect crisp shot with a fast shutter speed. A shutter speed of 1/2000th.of a second all the way up to even 1/10000th of a second depending on your lens capability and the type of motion you want to freeze. Here again you need to adjust your aperture and ISO accordingly to counter the increased shutter speed and get your correct exposure. There is no thumb rule for it and the kind of shutter speed you want to use will entirely depend on the kind of effect you want in your photographs.
Below are images of a mini fan with different shutter speeds. Check the effect of slower and faster shutter speed on the wings of the fan.
3. ISO - Sensitivity to the Light
The final element in the exposure triangle is the ISO. Now in the old days when film cameras were used different film speeds were measured by a term called the ASA. Obviously film cameras are obsolete nowadays so we would be discussing on ISO. ISO is basically the measurement of the sensitivity of your camera sensor to light. Lower the ISO less sensitive your sensor is to light. Higher the number more sensitive your camera sensor is to light.
However the trade-off here is, lower the ISO, darker the image with lesser grain or noise and higher picture quality. Higher the ISO brighter the image but also more grain or noise compromising on the quality of the shot. So I would always prefer to keep the ISO as low and possible unless I am compromising a great deal on my aperture and shutter speed. In recent times professional Dslr bodies are capable of handling very high ISO with minimal grains which can be removed through post processing in lightroom. When you turn your camera on in all probability the ISO would be set to a 100 or 200 which is the standard mark. The ISO increases in a multiple of two and the sensitivity of the sensor to light doubles itself with every increase.
Altogether: EVs and Stops
We call a specific combination of aperture, shutter speed, and ISO an exposure value (EV) and often refer to a change that either doubles of halves the amount of light reaching the sensor (or doubles of halves the sensitivity) as a stop. This is where the math can get a bit annoying. Shutter speeds and ISO respond numerically how you would expect: a change from ISO 200 to 400 is an increase of one stop; a change from a shutter speed of 1/30s to 1/120s (most cameras will make this 1/125s) is a decrease of two stops. However, f-stops, which correspond to aperture, are arranged in a geometric series that roughly approximates powers of the square root of two (I promise the reasons for this make sense, but it certainly doesn’t lend itself to an intuitive grasp).
In other words, in the following sequence, each new f-stop represents a decrease of one stop: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. So, how do you intuitively work with “doubling” or “halving” an f-stop? You don’t. Just memorize the sequence. It’ll become second nature very quickly and you’ll have no problem jumping between stops and understanding exactly how many stops you’ve added or subtracted to your exposure. For example, moving from f/2.8 to f/8 represents a decrease of 3 stops, so we're now using 1/2 * 1/2 * 1/2 = 1/8 as much light.
Notice how every time I reduced the aperture by one stop, I also decreased the shutter speed by one stop; thus, the overall exposure is the same in each shot. Nonetheless, the differences are quite noticeable. In the f/1.4 example, the flower in the center of the frame is quite prominent and isolated; the busy deck railing melts away rather nicely. By f/8, my depth of field is much bigger; the deck railing is much more prominent and it isn't clear what the focus of the image is.
When we take a shot even in full Auto the camera ideally combines all the three elements of the exposure triangle – The Aperture, Shutter Speed and the ISO. In manual mode we have the option of controlling these elements separately and hence have more control on our photography and the effect we are trying to achieve. No matter how much you memorize on the numbers and concept, the real learning would always come when you spend time on the field or in your backyard with your DSLR taking hundreds of shots trying to manipulate the aperture, shutter speed and ISO yourself.