Focal Lengths: Where To Find Them On Your Camera

The focal length of a camera is a fundamental aspect of photography, dictating what your camera can focus on and how your images will turn out. It is the distance between the rear principal point and the sensor, or the space from the centre of the lens to the point where light rays converge to form a sharp image on the camera's sensor. This is typically measured in millimetres. A longer focal length results in a narrower angle of view, while a shorter focal length yields a wider angle of view. The focal length of your camera can be found by using a simple formula: Focal length = the distance from the subject to the lens + the distance from the lens to the camera sensor. This value can also be calculated using the lens equation: 1/Focal length = 1/Image distance + 1/Object distance.

Focal length and lens selection

There are two types of camera lenses: fixed focal length (also called prime lenses) and zooms. Most compact cameras have a non-changeable lens attached to the camera body, so they typically use a zoom lens, which can be quickly adjusted to take in a wide-angle view or zoom in on a smaller portion of the scene. DSLR cameras, on the other hand, have the option of using either zoom or prime (fixed focal length) lenses, which are interchangeable.

When selecting a lens, it's important to understand how focal length works and which focal lengths are best for different types of photography. A short focal length, such as 18mm, will give you a wider angle of view, while a longer focal length, such as 200mm, will provide a narrower angle of view. This is important to consider when deciding how much of the scene you want to include in your frame.

The focal length of a lens is the optical distance, usually measured in millimetres, from the point where light meets inside the lens to the camera's sensor. To calculate the focal length, you can use the formula: Focal length = the distance from the subject to the lens + the distance from the lens to the camera sensor. Additionally, you need to consider whether your lens is concave or convex, as this will impact the focal length calculation.

Different lenses are geared towards specific looks and uses. For example, a fisheye lens produces circular or ovular images and is great for capturing wide cityscapes, while a telephoto lens brings distant subjects closer and is ideal for wildlife photography. A wide-angle lens is perfect for capturing broad landscapes or large groups, and a standard lens with a fixed focal length of 50mm, 85mm, or 100mm is commonly used for portraits, live events, and still lifes.

Experimenting with different lenses and focal lengths will help you develop your style and become more proficient in selecting the right lens for the job.

Focal length and angle of view

The focal length of a camera lens is the optical distance, usually measured in millimetres, from the point where light meets inside the lens to the camera's sensor. Focal length is one of the main considerations when buying and selecting a lens. It determines what your camera can focus on and how your images will turn out.

The focal length of a lens also relates to its field of view (also called the angle of view). Changing the focal length changes the field of view. The angle of view describes the angular extent of a scene that is imaged by a camera. It is used interchangeably with the term field of view.

The angle of view in photography is the area of the scene that is captured by the camera sensor. This area is described in degrees of coverage in front of the camera. The focal length of a lens is measured in millimetres (mm). The longer the focal length, the physically longer the lens will be. Lenses with a wider view will have a shorter focal length and will be physically shorter.

A short focal length (below 35mm) will give you a wider angle of view. An image taken with a 15mm lens will seem abnormally expansive, taking in an entire landscape, for example. Photos taken with short focal lengths show distortion, with foreground elements magnified and background elements diminished.

A long focal length (above 50mm) will give you a narrower angle of view. An image taken with a 100mm telephoto lens will capture a much smaller section of the scene. Photos taken with long focal lengths look more "compressed". They allow you to capture subjects from a greater distance without losing image quality.

Standard focal lengths range from 35mm to 50mm. The field of view provided by standard focal lengths approximates the field of view of the human eye. Images taken with a standard focal length show a natural perspective without distortions.

The focal length of a lens also determines the depth of field. Short focal lengths (or wide angles of view) have a deeper depth of field relative to long focal lengths (narrow fields of view).

The focal length of a lens is usually stencilled on the lens. However, with crop sensor cameras, the stencilled focal length does not provide the same angle of view as on a full-frame camera. For example, an 18-55mm lens on a crop sensor camera set at 18mm will give you an angle of view that is similar to a 28mm lens on a full-frame camera.

Focal length and magnification

Focal length, usually represented in millimetres (mm), is the basic description of a photographic lens. It is not a measurement of the actual length of a lens, but a calculation of the optical distance from the point where light rays converge to form a sharp image of an object to the digital sensor or 35mm film at the focal plane in the camera. The longer the focal length, the narrower the angle of view and the higher the magnification. Conversely, the shorter the focal length, the wider the angle of view and the lower the magnification.

The focal length of a lens determines what your camera can focus on, and how your images will turn out. It tells us the angle of view (how much of the scene will be captured) and the magnification (how large individual elements will be).

The focal length of a lens is determined when the lens is focused at infinity. It can be calculated by adding the distance from the subject to the lens and the distance from the lens to the camera sensor.

There are two types of lenses: prime and zoom. Prime lenses have a fixed focal length, meaning you're locked into shooting at a single focal length with no zooming or magnification. Zoom lenses have variable focal lengths, offering more versatility and reducing the need to change lenses as frequently.

When it comes to magnification in astrophotography, the term "plate scale" is often used instead. Plate scale refers to the image scale or the amount of space in the sky captured by each pixel in the image. It is calculated in terms of degrees, minutes, and seconds.

To calculate the magnification power of a lens, you can use the formula:

Magnification (M) = Focal Length / Focal Length - Distance to the Object

Additionally, the magnification can also be calculated using the lens maker's equation, which takes into account the refractive index of the lens material, the thickness of the lens, and the radii of curvature of its two sides.

In photography, the concept of "magnification" can be ambiguous, as it depends on the display format, sensor format, and viewing distance. A more meaningful term to consider is field of view (FOV), which refers to the area captured by the camera's sensor. The FOV can be calculated by dividing the sensor size by the focal length.

In summary, understanding focal length is crucial for photographers as it informs lens selection and greatly impacts the outcome of their images. By choosing the right lens for the job, photographers can achieve their desired angle of view and magnification, resulting in creative and specialised photographs.

Focal length and image quality

The focal length of a camera lens is an important concept to understand for any photographer. It determines what your camera can focus on, and how your images will turn out. Focal length is an optical property of the lens, and it measures the distance in millimetres between the "nodal point" of the lens and the camera's sensor. The nodal point is where light converges in a lens.

Focal length is important because it relates to the field of view of a lens – that is, how much of the scene will be captured in your photograph. It also determines how large or small the subjects in your photo will appear. Longer focal lengths (e.g. 500mm) are more "zoomed in", while shorter focal lengths (e.g. 20mm) are more "zoomed out".

The focal length of a lens is not the same as its physical length, and it has little to do with its overall size. It is also important to note that focal length is a property of the lens itself, not the camera. For example, a 50mm lens will have a focal length of 50mm regardless of whether it is used on a full-frame or cropped-sensor camera.

However, the size of the camera sensor will impact how your photos look at a particular focal length. The sensor size determines the field of view, or how much of the scene is captured in the photo. A full-frame camera sensor has a size of 36mm by 24mm, while other cameras have larger or smaller sensors, which will result in a narrower or wider field of view, respectively.

When selecting a lens, it is important to consider the focal length that is best suited for the type of photography you want to do. For example, wildlife and sports photography often use longer focal lengths to capture distant subjects, while landscape and architectural photography typically use shorter focal lengths to capture wide angles of view.

Additionally, the type of lens you choose will also impact your images. Prime lenses have a fixed focal length, meaning you are locked into a single focal length with no zooming or magnification. On the other hand, zoom lenses offer variable focal lengths but tend to be heavier and may produce slightly lower-quality images.

To summarise, understanding focal length is crucial for photographers as it determines the focus and overall outcome of their images. By choosing the right lens and focal length for their specific needs, photographers can capture the desired field of view and subject size, resulting in high-quality photographs.

Focal length and aperture

Lenses with different focal lengths offer different angles of view and magnification. A shorter focal length, such as 18mm, provides a wider angle of view, while a longer focal length, such as 200mm, gives a narrower angle of view. The focal length also affects the magnification, with longer focal lengths offering stronger magnification. This is why longer focal lengths are often used for telephoto lenses, while shorter focal lengths are used for wide-angle lenses.

The aperture, measured in f-stops, refers to the opening at the rear of the lens that controls how much light passes through to the image sensor. A smaller f-number (e.g. f/2.8) indicates a larger aperture, resulting in a shallow depth of field, while a larger f-number (e.g. f/16) indicates a smaller aperture and a deeper depth of field. The aperture also affects the brightness of the lens, with smaller apertures resulting in brighter lenses.

By understanding how focal length and aperture work together, photographers can make more informed decisions about their equipment and settings to capture the perfect shot. For example, a wide-angle lens with a large aperture can be useful for capturing a broad landscape, while a telephoto lens with a small aperture might be better for a close-up portrait with a blurred background.

Different types of lenses, such as prime and zoom lenses, also offer different focal length options. Prime lenses have a fixed focal length, while zoom lenses offer variable focal lengths. Prime lenses tend to be more compact and lightweight, with larger maximum apertures, making them ideal for low-light conditions and achieving a shallow depth of field. Zoom lenses, on the other hand, offer more versatility and reduce the need to change lenses frequently.

Frequently asked questions