A stop of light is a way to calculate light, how much is coming into the camera. Increasing by a stop of light, you are doubling the amount of light that comes into the camera, when decreasing by a stop of light, you are cutting the amount of light in half. For example: talking about f-stops, going from f-stop F/2 to f/2.8, you are decreasing by one stop of light, meaning that it is cutting the main light in half. If you go for a shutter speed from 1/30 to 1/15, you are doubling the amount of light because in shutter speed one stop of light increase. If you go from 400 ISO to 200 ISO, you are reducing by one stop of light which is cutting them out of light in half.
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How Light Works
How much exposure your camera gets depends on something called the exposure triangle. This is made of three things – shutter speed, aperture, and light sensitivity (ISO if you’re shooting digital). All three of these can compensate for each other so that if you need a faster shutter speed or a lower ISO you can adjust accordingly and still get a properly exposed image. Each of these three is equally important and directly related.
Thankfully, cameras have numeric values assigned to each of these three things so that you can adjust relatively and expose correctly.
Exposure also relates to time. Back in the days of film, when the compounds coating the film were exposed they would imprint the amount of light allowed in that time e.g 1 second. If the same film were exposed in exactly the same way but for 2 seconds it would receive double the amount of light and be twice as bright. If the same experiment were done with two pieces of film being exposed for 1 second but one of the film strips was twice as sensitive the results would be the same as in the first experiment.
The pattern here is that increasing any of the things – exposure time, or sensitivity to light affects the exposure and can be manipulated to create the same exposure with different settings. It doesn’t matter which of the three settings you’ve used, you’ll still get the same exposure on the film if it’s done correctly. Twice the exposure means twice as bright of an image. It’s really that simple.
The correlation between the three is what an exposure stop is. Each stop is double the exposure of the stop before. Adjusting any of the exposure triangle will mean twice as much light captured or half as much depending on the adjustment.
The Exposure Triangle
Adjusting your exposure and getting the numbers right means learning what each numeric exposure value means so that you know whether you’re getting half as much light or double the light.
Aperture
The aperture of your camera is the opening that allows light inside. It is comprised of a series of blades called a diaphragm. Aperture is measured in f/ stops, where the smaller the number the larger the aperture and the more light can be let in. So an aperture of f/1.4 has a large opening, while f/32 would be a small opening.
The actual size of the aperture is only relevant to the size of the lens rather than a physical size. To double the amount of light coming in through the aperture the opening size must be doubled, not the size of the aperture itself. This is rather tricky because it makes an exposure equation harder and entirely dependent on the lens used.
Most common aperture sizes are between f/1.4 and f/32, and while there are exotic and specialized lenses outside of those apertures they’re rarely used and very specific to medium or large format.
The simplest way to explain the exposure stop regarding aperture is that every time you go one stop you’ll expose twice as much. So f/1.4 is twice as much light as f/2 then either times more light than f/4 and so on. Moving up the scale f/2 would be twice as much light as f/4 etc.
The measurements are not always whole numbers, as this can be measured in both half stops and third stops depending on the aperture size and the amount of light entering from the lens. Each full aperture stop essentially doubles the amount of light being let in while halves and thirds allow a noticeable change without going a full stop.
Shutter Speed
Shutter speed is pretty self explanatory – it’s the amount of time the shutter stays open and allows light in. The speed is usually measured in seconds of fractions of a second and, just like aperture, you’ll find half stops, full stops, and third stops in those measurements. The shutter stops with 1 second or more of exposure are marked using an “ symbol to differentiate. That means any stop without the “ symbol actually means a fraction of a second so 125 would be 1/125th of a second shutter speed.
The range of shutter speed is huge, including those manually done using triggers and photographers often use speeds as low as 30 seconds for astrophotography and as high as 1/80000 in nature or sport photography. A thirty second exposure is 5 stops more than a 1 second exposure meaning that you’ve got much more flexibility to adjust the exposure using shutter speed than aperture. Calculating shutter speed is a lot easier than aperture because every stop is a multiplication of 2.
ISO Sensitivity
ISO relates to how sensitive the media is to light. When film was still used, this related to how sensitive to light the film would be, but now it relates to your camera sensor. The image sensor can change sensitivity without having to change film which is more convenient. It’s the most important, and most expensive, part of a camera and relates to how well the camera can gather and “understand” light to form an image.
A more sensitive sensor will be able to capture low light images without flash but because much of the sensitivity becomes a “guess” the camera adds noise or grain. The noise is essentially where the guess isn’t quite right, so while you may see black the camera adds a pixel of pink or green to try and understand the blackness.
Each sensor has a base ISO where the exposure requires no adjustment. This is dependent on the camera and the sensor but is usually around ISO 100-200.
Calculating ISO is similar to shutter speed because one stop is also a multiplication of 2. The stops can range from 100 all the way up to 51,200 or more. These are considered the most useful and usable, though the sensor may be able to pick up higher the photos begin to become unusable because of the amount of noise before that amount. The ISO range in the camera also includes half and third stops so you will likely have a 100 ISO, 125 ISO, 160 ISO etc before ISO200.
Inconsistency
All three of these measurements have full stops, half stops and third stops, but if you look at the numbers they aren’t always consistent. For example, a one third stop between ISO100 and ISO 200 would be ISO133, yet most cameras are marked at ISO125.
From a practical standpoint manufacturers only standardize the full stops, meaning that while they try and stay somewhat consistent there is some rounding up going on between the smaller numbers.
Different manufacturers also round up differently so while the first stop on one lens may be f/5.6 on another camera it may be f/6.3 or f/6.4. The margin of error is entirely based on multiplication created to make that number because it uses the square root of two, something that is not a round number. In reality, this inconsistency doesn’t matter, the difference in numbers is fractional at this point as long as you realize it’s an approximate half or third of a stop.
Compromising on the Triangle
All three of these adjustments (ISO, Aperture, Shutter Speed) are relevant so adjusting any one of them affects the exposure by 1 stop. Increasing any number by 1 stop will double the amount of light let into the sensor while decreasing halfs this. This means that you can compromise, and by changing the exposure triangle avoid blurry images or create blur when needed.
For example, if you are focusing movement and need a faster shutter speed but don’t want to risk under exposing you can increase all of the settings and get four times the amount of light. This will allow for two full stops while keeping the image exposed properly, with less blur, and a compensation for a shallower depth of field and a little more noise. It’s essentially a trade off to get a sharper image.
How much to increase of decrease is wholly dependent on the situation. Even two thirds of a stop can be a lot in some cases and much of it is personal preference to the style of the photographer and whether they think they can fix it in post processing or not if the image is underexposed. In fact, most photographers would prefer to under expose than over expose and risk losing the information in brighter areas.