Understanding exposure (aperture, shutter speed, ISO)
Exposure is one of the central concepts of photography and is influenced by 3 different (and independent) factors in your camera:
- Shutter speed
- ISO sensitivity
If any of those terms are unknown to you, please be sure to check our article on terminology for a quick recap. What makes things tricky is that each of these factors influence exposure and one more thing:
- Aperture: a large aperture (which means a low aperture number like f2.8) means a greater exposure, but a more shallow depth of field.
- Shutter speed: a longer shutter speed means greater exposure, but increases the chance (or risk) of motion blur.
- ISO sensitivity: high ISO means greater exposure, but more image noise.
Here are a few sample images to make it more clear how DOF (depth of field), motion blur and image noise affect your images:
DOF (influenced by aperture)
Motion blur (influenced by shutter speed)
Image noise (influenced by ISO setting)
Your camera can choose exposure automatically by analyzing the scene you’re looking at and generally aiming for a neutral exposure, making your image neither too dark nor too bright. This will work well in many cases, but an automatic system will quickly reach its limits in certain more extreme circumstances like the following:
- Shooting in very dark situations, e.g. at night,
- Shooting in very bright situations, e.g. during daylight in snow.
- Shooting back-lit subjects, i.e. shooting a portrait of a person with the sun behind them.
Besides, using auto exposure will often result in images being exposed in a slightly different way even when taken in the same place and only seconds apart: If you change your perspective only a little bit, you’ll get a bit more of that bright sunny spot or that dark wall in your image, and your camera will decide to adjust for it to achieve a neutral exposure. If you’re taking series of images, having such inconsistencies can be a huge nuisance.
If you’re a more advanced shooter, most of the time you will want to shoot in manual mode and control all exposure settings yourself. As an intermediate step, you will want to use automatic exposure metering, but add adjust the value chosen by your camera.
Changes to exposure (or EV) are usually measured in f-stops (or just stops): Increasing EV by one stop means allowing twice as much light to reach the sensor, decreasing by one stop means allowing half as much light. This goes on exponentially – “two stops” equal four times (or a quarter) the light, “three stops” represent eight times (or an eighth), etc. In most cases, cameras will allow you to adjust exposure in thirds of steps, e.g. +1/3 EV or -2/3 EV, etc.
If you find your resulting image is too dark, you’ll have to increase exposure by, say, one step. You can do this through only one of the 3 factors, e.g. by doubling the exposure time or by doubling the ISO sensitivity. In some cases though, you’ll want to avoid cranking up a single one of those values to avoid its drawbacks: instead of doubling your ISO and getting a lot of noise or doubling exposure time and getting a lot of motion blur, you could go for a compromise and use +1/3 stop ISO, +1/3 stop aperture and +1/3 stop shutter speed. This will give you a total of +1 stop at the cost of only a little more noise, a little more blur and a little less DOF.
Similarly, halving your ISO (-1 EV stops) while incresing exposure time (+2/3 stop) and opening the aperture by +1/3 stop will give you an identically exposed image (-1 + 2/3 + 1/3 = 0 EV stops) with less noise, more motion blur and less DOF. This can be confusing in the beginning, but getting a good grip on how to balance these three values is a central part of mastering photography and getting your shot look the way you want it.