Focal length is the distance between the optical centre of your camera lens and your camera's image sensor or film when the lens is focused on infinity. It is usually measured in millimeters (mm). The focal length of a lens determines its field of view and magnification. A lens with a shorter focal length has a wider field of view, while a lens with a longer focal length has a narrower field of view.

For example, if you're shooting with a telephoto lens and using a wide aperture, like f/2.8, the subject will be in sharp focus while the background will be blurry. This is ideal for portraits, where you want to isolate the subject from the background.

Focal length is determined in part by the size of the camera's sensor. For instance, because a full-frame camera has a larger sensor than a crop-sensor camera, a 50mm lens on a full-frame camera will have a wider field of view than the same lens on the crop-sensor camera.

If you're shooting with a wide-angle lens and using a smaller aperture, like f/11, the entire scene will be in focus. This works well for landscape photography, where you want everything from the foreground to the background to be sharp.

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Focal length determines numerous important elements of the photographic process, including field of view, magnification, and depth of field. Understanding focal length is essential if you want to create compelling and captivating photographs. Let’s take a closer look at focal length in photography.

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For example, if you take a portrait with a wide-angle lens, the subject's nose and face will appear larger than they are in reality. This is because the lens exaggerates the distance between the nose and the ears, making the nose appear larger. A telephoto lens, on the other hand, compresses the distance between the nose and the ears, making the face appear flatter.

Focal lengths range from 8mm to 2000mm. When it comes to focal length, there are three main type of lens: wide-angle, normal, and telephoto. Wide-angle lenses have a focal length of less than 35mm. They are great for capturing landscapes, architecture, and interiors because they provide a wider field of view, meaning they can capture more in the frame. Normal lenses have a focal length of between 35mm to 70mm. They are also known as standard lenses. Normal lenses capture the subject without distorting it, which makes them perfect for everyday photography, portraits, and street photography. Telephoto lenses, which have a focal length greater than 70mm, are ideal for capturing distant subjects, such as wildlife, sports, and events. They magnify the subject and compress the background. Macro lenses, which offer significant magnification (1:1, for instance), come in a variety of focal lengths. With these lenses, focal length determines how close you need to be to your subject to capture a sharp magnified image. The longer the focal length, the further away your camera can be from your subject without losing focus.

The focal length of a lens affects the perspective of the image. A wide-angle lens exaggerates the distance between the foreground and the background, making the background appear further away than it really is. A telephoto lens compresses the distance between the foreground and the background, making them appear closer together.

Focal length determines the angle of view, magnification, perspective, and depth of field of the image. Understanding focal length is essential in choosing the right lens for your photography needs. Whether you're shooting landscapes, portraits, or wildlife, selecting the right focal length will help you capture the perfect shot. To learn more about focal length, download our free 90-page eBook, An Introduction to Photography.

The focal length of a lens also affects the depth of field of an image. (Depth of field refers to the range of distances that appear acceptably sharp in the image.) A longer focal length results in a shallower depth of field, while a shorter focal length results in a deeper depth of field.

The crop factor depends on the camera's sensor size. For example, a crop factor of 1.5x means that a 50mm lens on a crop sensor camera will have an effective focal length of 75mm.