In photography, there are a lot of factors that contribute to the quality of the image. Aperture and shutter speed are two factors that can be controlled within the camera. Aperture is basically the size of the opening that lets the light into the body; a bigger opening result to more light entering and hitting the sensor. On the other hand, shutter speed is the rate at which the shutter opens and closes. When you press the button to take a photo, the sound that you hear is from the shutter opening and closing. A slower shutter speed means that the shutter is open longer and more light would be able to hit the sensor.

These two are used to control the exposure level of the image. Too big an aperture or too slow a shutter speed results in an overexposed image while the opposite results in an underexposed or very dark image. Due to many years of evolution of the technology, the levels to which you can adjust these two have been standardized in relation to each other. Increasing the aperture one stop while reducing shutter speed one step would result in the same level of exposure; making it very easy to change shutter speed or aperture size while still maintaining the correct level of exposure.

Although the correct exposure can be achieved with different values of aperture and shutter speed, it is often desirable to have a specific value. Changing aperture changes the depth of field of the image. A large aperture allows most of the image to be in focus while a smaller aperture blurs objects that are behind or in front of the subject. This is useful if you want to show an entire landscape or eliminate distractions in the background. A fast shutter speed is essential when capturing moving objects in order to prevent blurring due to its motion. On the other hand, slow shutter speeds or long exposures, can result in very dramatic and artistic effects.

If you simply want to use a certain aperture or shutter speed without needing to adjust the other one, you can select a semi-automatic setting for the camera. Shutter priority, often known as Time value (Tv) uses a fixed shutter speed while modifying aperture stop in order to get the best exposure while aperture priority, also known as Aperture value (Av), varies the shutter speed while maintaining a fixed aperture stop.

Summary:

1. The aperture is the size of the opening that lets light into the sensor while the shutter speed is a measurement of the time that the sensor is exposed to the light
2. Aperture and shutter speed are standardized and are conversely proportional with each other
3. Varying aperture changes the depth of field while varying shutter speed changes sharpness
4. DSLRs can prioritize aperture over shutter speed and vice versa