Final answer:
Capturing a photo where the subject is in focus and the background is blurred is achieved with a fast shutter speed and a large aperture, creating a narrow depth of field. This technique is often used in action photography to freeze the subject's motion.
Step-by-step explanation:
When a moving subject is in focus but the background is blurred, the type of motion you are capturing is known as action or motion photography with a frozen subject. This effect is typically achieved through a combination of camera settings, particularly using a fast shutter speed.
When the shutter speed is fast, such as 1/250th of a second or faster, the camera's sensor captures the image so quickly that it 'freezes' the action of the subject. However, because a fast shutter speed allows less time for light to enter, you may need a larger aperture to compensate for the reduced exposure time. A larger aperture not only provides the necessary light but also creates a narrow depth of field, which results in the subject being in sharp focus while the background is out of focus.
To further ensure your subject is the focus of the image, you can fill the frame with the subject by getting closer or using a longer focal length, such as a 200mm lens. This technique minimizes distractions and directs the viewer's attention to the subject. Additionally, creating leading space can convey the direction of movement, making the subject appear to move across the photo.
In summary, a faster shutter speed is used to freeze action and capture motion clearly, whereas a slower shutter speed may be used to intentionally blur movement for creative effect. A wide-open aperture results in a narrow depth of field with the focused subject standing out against a blurred background, while a small aperture provides a deeper depth of field with more of the scene in focus.