Aperture
The aperture controls how much light your lens lets in. A lower number indicates more light. Each "stop" higher, for example from f/2.8 to f/4, reduces light by one half. Each stop lower doubles the light. The aperture is varied by an iris diaphragm located in the lens. Its behavior is analogous to the iris of your eye.
Aperture also affects focus range, also known as depth of field. This is the range between the nearest and farthest objects in a scene that remain in focus. A lower, brighter aperture has a shallow depth of field. A higher, darker aperture has more depth of field but also lets in much less light.
Depth of field is also affected by the focal length of the lens. A "longer lens" like a 200mm will have
Shutter speed
The shutter controls how long the shutter lets light in. A lower number is slower and allows more light to collect on the sensor. Each doubling of shutter speed reduces light by one half. There are several types of shutters. The common focal-plane shutter just in front of the image sensor has two "curtains" that follow each other rapidly to adjust how long the sensor is exposed to light. One opens to let light in and the other follows it to block light.
Shutter speed also affects how well you can "freeze" motion in a scene. An image taken at a slow shutter speed like 1/30 of a second will blur if the subject moves. A fast shutter speed like 1/500 of a second will freeze motion but allows less light and must be compensated by a wider aperture or higher ISO to let more light reach the sensor.
ISO
ISO is an indication of how sensitive the film or digital sensor is to light. A higher number is more sensitive to light and allows you to shoot in lower light and at faster shutter speeds or higher (darker) apertures. There is a tradeoff, however. High ISO results in pictures that look more grainy and have more color noise, especially in dark areas of the scene. This may be acceptable for small pictures, but the grain and color noise become more apparent as the picture size increases.At small sizes the differences between a low ISO of 100 and a high ISO of 6400 aren't very apparent. Using a high ISO lets you capture pictures in low light that you wouldn't be able to with a low ISO.
At larger sizes, the grain and color noise are easy to see.
Exposure Triangle
Aperture, shutter speed and ISO interact and affect each other. The three-way interaction is often called the exposure triangle. To get a correctly exposed image in different light you have to balance the settings of all three. Shooting in auto mode takes care of this for you, but shooting in other manual modes gives you more creative control.
Equivalent Exposure
If you increase the shutter speed from 1/125 to 1/250, you let in half as much light. In order to get the same amount of light as at 1/125, you need to increase the aperture by one stop which doubles the amount of light let in. Here is a table that shows equivalent exposures for different shutter speeds and apertures at the same ISO.| Shutter Speed | Aperture |
|---|---|
| 1/2000 | f/1.4 |
| 1/1000 | f/2 |
| 1/500 | f/2.8 |
| 1/250 | f/4 |
| 1/125 | f/5.6 |
| 1/60 | f/8 |
| 1/30 | f/11 |
| 1/15 | f/16 |
| 1/8 | f/22 |
Assuming the ISO was set at 100 while using the above combination of shutter speeds and apertures, if you double the ISO to 200 (twice as sensitive to light) you have to either double the shutter speed (half the light) or decrease the aperture 1 stop (half the light) to achieve the same exposure.
For example, at ISO 100 and shutter speed 1/250 at f/4, if you double the ISO to 200, the equivalent exposure would be 1/500 at f/4 or 1/250 at f/5.6.
Here are a couple exposure calculators that let you play with settings to see the relationship.
Available light exposure calculator
Exposure calculator to compare two camera exposures
Helpful Resources and Tutorials
Basics of Photography: Your Camera's Manual Settings (lifehacker)
A Comprehensive Beginner's Guide to Aperture, Shutter Speed, and ISO (PetaPixel)
Shutter Speed Chart and Tips on How to Master It (BorrowLenses)
Basics of Photography: The Complete Guide (lifehacker)
Video Tutorials
Photography Tutorial: ISO, Aperture, Shutter Speed (YouTube)
Aperture, Shutter Speed, ISO, & Light Explained–Understanding Exposure & Camera Settings
Cheat Sheets
Digital Camera World publishes cheat sheets that have a wealth of information consolidated into one graphic that you can download or print. Here are some articles with useful cheat sheets in them.How to understand f-stops
Which shutter speed should you be using?
How to understand ISO settings
Depth-of-field decisions
