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For many who are new to photography, the relationship between aperture, shutter speed, and ISO can be confusing. The terms “stop” and “f-stop” are common and are used interchangeably, but they refer to different things. In this article, the misery will be taken out by talking about the exposure triangle and how it is useful. We have simplified all the concepts for easy understanding, especially on ISO.
The exposure triangle is the basic way of associating the three variables that determine the exposure of photograph, shutter speed, aperture, and ISO. One must balance the three to achieve the best image. Adjusting one requires that you change one or both aspects for the best shot.
The exposure triangle
ISO, aperture, and shutter speed make up the three sides of the exposure triangle. They work together to ensure the photo is exposed correctly. Adjustment of one variable necessity the adjustment of one or both variables to maintain the correct exposure.
What is a stop of light? A stop refers to the doubling or halving the amount of light that makes up the exposure. Every photo taken needs a specific quantity of light to expose it appropriately. Adding a stop of light by doubling the exposure brightens the unexposed image, whereas halving the amount of light darkens the unexposed image.
How do you add or take away a stop of light? To achieve this, you need to change aperture, shutter speed, and the ISO. Let us look at each individually.
Aperture is a measure of how open or closed the lens’ iris is. A wider aperture (or lower f-number) means more light will pass to the lens. A narrower aperture (higher f-number) allows less light to reach the sensor.
Narrower apertures give greater depth of field, thus more area of capture. Wider apertures (lower f-numbers) create a narrow depth of field, which helps isolate a subject for the greatest composition. (think portraiture).
Most lenses are sharpest around f/5.6 or f/8. However, many photographers are willing to trade sharpness for the subject-isolating effects of a wider aperture.
Aperture is the size of the circular hole in a lens that allows light in. The bigger the hole, the more light that reaches the sensor. Doubling the size of the hole doubles the amount of light by or increase one stop. When you reduce the hole by half, half the amount of light reaches the sensor, thus decreasing the exposure by one stop.
F-stop is the ratio that relates to the size of the opening. Mathematically, it is the focal length of the lens divided by the diameter of the lens. Isn’t it confusing how small values correspond to larger openings and vice versa?
Here is the math:
F-stop = focal length/ diameter
And rearrange it to make diameter the subject. You get:
Diameter = focal length/f-stop
Given any focal length, we can calculate the diameter of the aperture by dividing the focal length by the f-stop value. When the f-stop value is large, you get a smaller diameter; hence the opening is small. Conversely, if you divide the focal length by a small f-stop figure, you get a large diameter. Bigger diameter means more light will pass to the sensor.
To double the area of the opening, the f-stop needs to be divided by the square root of two (1.414). To half the area, the f-stop needs to be multiplied by the square root of the two.
A little geometry helps prove this.
Remember the area of a circle:
Area = (π/4) diameter2
Use the formula to calculate the area of the aperture for a lens with a 50 mm focal length using varying f-stop values. What are your observations? As you move up the f-stop scale, the areas doubles.
Shutter speed measures the time the shutter remains open and how long the sensors are exposed to light. Faster shutter speeds ensure sensors get less time to collect light, thus lowering exposure. When the shutter speed is slow, sensors have more time to collect light resulting in higher exposure.
High shutter speed is very efficient when capturing moving objects because it helps maintain sharpness. When the shutter is open, the camera records elements in the frame. If the element moves, it results in blurriness.
Many photographers argue that a fast shutter gives the sharpest images, which is technically true.
To double the amount of light, you need to double the length of exposure. For example, moving from a shutter speed of 1/60 s to 1/30 s doubles a stop of light since the shutter remains open twice as long. Decreasing the shutter of light from 1 s to 1/8 s reduces exposure by three stops. From 1s to 1/12s is one-stop, 1/2s is another stop, then 1/4s to 1/8 is the third stop.
When the film was new, there was not the kind of flexibility we have now. The exposure triangle was two-sided. One could control the amount of light of the film, but once the roll was in the camera, it could not be changed. Today, we control the sensitivity of digital sensors on the fly. Technically, it is controlling the post-image gain applied to the signal.
Increasing the ISO makes it possible to work with less light. Increased ISOs result in increased noise and less detail. The noise comes from the random fluctuations in the electrical signal. The image signal is close to the noise which enters the image. At lower ISOs, the magnitude of the image is larger than the nose (signal to noise ratio), which means the noise is unobstructed.
Here is a simplified illustration: think of the image signal and noise as a pit of balls. If the image signal is 1,000, you will not notice when 4 or 5 balls are added, but if the image signal is like 10 balls, adding 5 balls is very noticeable. When the signal is amplified by a high ISO, the relatively high noise will be amplified accordingly. When high Iso makes the 10 balls into 1,000 balls again, the noise becomes 500 balls.
Higher values of ISO mean sensors can make a correct exposure even with low light and vice versa.
Altogether: EVs and Stops
EV (Exposure Value) is the specific combination of aperture, shutter speed, and ISO.
Stop, on the other hand, is the change that either doubles or halves the amount of light (sensitivity) reaching the sensor.
This is where the math gets complicated. Iso and shutter speed respond numerically how you would expect. i.e., a change from the shutter speed of 1/30s to 1/120s (most cameras make it 1/125s) is a decrease of two stops. However, f-stops, correspond to aperture are arranged in a geometric series that roughly approximates power of the square root of two. In simple terms, in the following series, each of the f-stops 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 work with doubling or halving of f-stop? You do not. Just memorize the sequence.
The exposure triangle is very different from exposure square. Image stabilization helps you get away with slower shutter speeds, which allow narrower apertures for more depth of field or lower ISOs for more detail. Do not confuse camera shake with subject motion. Image stabilization is perfect for static subjects. When the shutter speed is slower, the images will be blurry, and image stabilization cannot save you.
The bucket analogy
In any photograph, there is only one mathematically correct exposure. There are hundreds of apertures, shutter speed, and ISO that can e used to create that exposure. The combination chosen is the best for the artistic vision of the image.
In any exposure, when one of the three variables is changed, one or both must be adjusted in the opposite direction. If you decrease your shutter speed by two stops, you will have to increase the ISO or aperture by two stops or both. If ISO is increased by four stops, you will decrease aperture or shutter speed with the same effect or a combination of the two.
Here is a simple analogy of rainwater filling buckets. The exposure is the total amount of water collected. Let us say exposure is one gallon.
In our analogy, the shutter speed is the time the bucket is left outside in the rain to fill up. How hard the downpour is will be our aperture? Downpour is a wide-open aperture (large opening with small f-stop number), and a light sprinkle would equate to a tiny aperture (small opening, large f-stop value).
The width of the bucket represents the ISO. All buckets must measure one gallon to collect the correct exposure. A very shallow, wide bucket fills up faster (think very sensitive, high ISO) than a very tall, thin bucket (Low ISO).
There are many combinations that will help collect one-gallon ‘exposure. We can fix one variable and let the other two change. Let us start by fixing ISO. If we use two buckets with the same shapes, we can put one out in the downpour for a short duration to collect one gallon. We can put the other for a long time during a light sprinkle until it collects one gallon.
If we put the gallon for a long time in the downpour, the image is overexposed. Water will fill and spill to the ground. Conversely, putting the thin bucket in the light sprinkle will not fill the bucket because it is underexposed.
If there is steady rain and both buckets are left out, the wide bucket collects one gallon faster than the tall, thin bucket.
Lastly, let us fix the time. For us to collect one gallon, you need to put the tall thin bucket in a downpour while the wide bucket is put in a light sprinkle. Both will collect exactly one gallon at the same time.
Note: You can see lots of combinations of aperture, ISO, and shutter speed that yield one gallon of exposure. The choice of these combinations is entirely dependent on the type of photograph you want. The most useful point to note is that if you increase one variable in the exposure triangle by the number of stops, then you must make up by decreasing one or both or a combination of both by an equal number of stops.
Combining the settings
Below is an example of the typical range of aperture patterns:
f/1.4, f/2.8, f/4, f/5.6, f/8, f/16, and f/22. The numbers almost double every time.
For those that do not have (f/4 and f/22), they are the sum of the two previous numbers.
For ISO, numbers double each time. 100 goes to 200 then 400, 800, 1,600 and 3,200. The shutter speed follows suit with 1/125, 1/250, 1/500, 1/1000.
Every number is one stop. They either add or subtract one stop’s worth of light from your image. They all work together in the triangle. That is, the correct light scene is given by ISO 100, shutter speed 1/125, and aperture of f/16.
If the scene gets two stops darker, you need to add two more stops of light in your setting for correct exposure.
ISO can be changed from 100 to 400 (100 -> 200 -> 400). The image quality is compromised since higher ISO brings digital noise and grain.
Shutter speed changes by two stops from 1/125 to 1/30 (1/125 -> 1/60 -> 1/30). You get a high-level camera shake in your image.
The image aperture changes from f/16 to f/8 (f/16 -> f/11 -> f/8).
Key tips to help you
- Use the lowest ISO possible: it helps you achieve the cleanest images with the best color depth and dynamic range. Outdoor range for ISO 100-400, outdoor ISO 400-3,200, Dark Receptions ISO 1,600+. These values may vary depending on the ambient light present.
- For sharp photos and no motion blur, the shutter speed must be at minimum 1/ (double your focal length) for full-frame sensor cameras and 1/ (1.5 x focal length) for crop sensor cameras. The rule is used for stationary objects. For motion pictures, higher shutter speed is vital.
- For faster shutter speeds (capture movement), use wider apertures and/ or higher ISO.
If you are a visual learner and wish to know more about the exposure triangle, check out this video.