A focal distance (focal length) is a measure that defines the field of view of a photographic lens. The greater the distance, the smaller the depth of the scene and the greater its magnification. A short focal length results in a wider angle of view.
The focal length is expressed in millimeters (mm) to inform the distance between the optical center of the lens (point where the light rays of the scene meet) and the camera sensor (or film, in analog cameras). Next, understand how this parameter behaves in each type of lens.
Fixed and variable focal lengths
Lenses can have a fixed or variable focal length. One fixed lens (prime) A 50mm lens has an optical center at that distance from the camera sensor.
Already Zoom lenseswhich allow you to adjust the camera’s approximation in relation to what will be photographed or filmed, have a focal length that varies within a minimum and maximum length.
This is the case with 18-55 mm lenses, which are very common on the market. These numbers indicate that the focal length of the lens can be adjusted between 18mm and 55mm.
If the focal length is extensive, reaching measures such as 300 mm, the camera’s angle of view is proportionally smaller. However, the approximation of what should be photographed or filmed is greater.
Short focal lengths, such as 11 mm or 24 mm, result in a wider angle of view and therefore generate a greater depth of field (area in focus by the camera).
Focal length in different types of lenses
Camera lenses are divided into categories with different focal lengths. These are the main types of lenses:
- Lens wide-angle: with a focal length of up to 35 mm, it typically reaches angles between 80 and 120 degrees. It is often used to record landscape or architectural works;
- ultrawide lens: It’s a wide-angle lens, but with a focal length of less than 24 mm. They are suitable for covering a wide field of view or when there is little space for the photographer to move around;
- Normal lens (standard): has a typical focal length between 35 and 50 mm. Versatile, it can be used for portraits, open-air recordings and even closed environments;
- Short telephoto lens (short telephoto): with focal length between 70 and 100 mm, plus angle in the range of 25-30 degrees, it has zoom effect. It is suitable for images that require a close-up effect;
- Telephoto lens (telephoto): With a focal length of 100mm or more, it has an angle of view close to 25 degrees. It has zoom, so it’s ideal for sports, nature and any circumstance that requires closeness.
The camera market also has lenses whose characteristics make them used for very specific purposes. The best known are:
- Fisheye lens (fisheye): it is a type of ultrawide with a focal length of up to 14 mm, although some manufacturers consider a length of up to 24 mm. With angles of up to 180 degrees, it generates a circular distortion effect appreciated for artistic purposes;
- Super telephoto lens (long telephoto): has focal length above 200 mm. It is often used for sports, motor racing and wildlife;
- macro lens: the focal length usually varies between 35 and 200 mm. It captures details that are not perceived by the human eye. It is suitable for images of insects, plants and electronic components.
Cut factor (crop factor) is a parameter used to calculate the size of the image recorded by the sensor, the component that captures the brightness of the scene to generate a photo or video.
The calculation is important because there are many types of image sensor and these variations change the focal length calculation.
Advanced DSLR and mirrorless cameras often use full frame sensors, whose measurements are close to 36 x 24 mm. This makes them equivalent to 35mm film.
35mm film was popular when SLR (analog) cameras dominated the market. Because these films were a standard size, photographers knew that a lens with a given focal length would have the same field of view on any camera.
The dimensions similar to those of 35 mm films make the industry use full frame sensors as a reference to calculate the focal length of the lenses. However, there are numerous digital cameras based on smaller sensors, which record images with cropped edges compared to the full frame standard.
The most common variation is the APS-C, sensor type with sizes close to 22 x 15 mm. Another is the sensors. micro three rooms (micro four thirds), with dimensions of approximately 17 x 13 mm.
Because of this, the cut factor is used to proportionally recalculate this parameter. Thus, a 50 mm lens placed on a camera whose sensor has a crop factor of 1.5 achieves a recalculated focal length of 75 mm.
The math is done by multiplying the original focal length by the sensor’s crop factor. These are the cropping factors for some popular camera lines:
- Fujifilm X: 1.5
- Pentax DA: 1.5
- Nikon DX: 1.5
- Sony E: 1.5
- Canon EF-S / EF-M: 1.6
- Sigma Foveon: 1.7
Short focal length lenses can cause image distortion. This happens because, the smaller the focal length, the greater the perspective on the elements that make up the scene, which generates the impression that there is more space between them.
There are two types of distortions:
- Barrel distortion (barrel): Generates an outward-curving effect, that is, with the center of the image wider than the edges. It is frequent in wide-angle lenses;
- Pincushion Distortion: results in an inward curl effect when the edges of the image are enlarged more than the center. It is usually found on telephoto lenses.
Barrel distortion can be noticed in wide-angle cell phone lenses. They are used on these devices to allow a larger area to be framed. The consequence is that the edges of the image can be curved. Many phones fix the problem with software tweaks.
Still on a smartphone, distortions can be noticed in selfies, when the person’s face becomes more rounded in the contour and expanded in the middle. This is because the front cameras of these devices are also based on a wide-angle lens.
Distortion is not always unwanted. In fisheye lenses, with very short focal lengths and viewing angles of up to 180 degrees, the circular effect caused by distortion serves creative work.
A chromatic aberration is caused by differences in color refraction. The problem manifests itself when the lens is not able to capture the wavelengths of each color so that they all lie in the same focal plane. The consequence is the appearance of colored halos in the image.
The focal plane is the area on the sensor where the points of light captured by the lens must focus. However, focal length and other factors can cause certain wavelengths to reach points in front of or behind the plane.
Lens aperture or focal length adjustments smooth out chromatic aberrations. Some manufacturers compensate for this effect by building lenses with a low dispersion glass to guide light in the focal plane.
Lenses with a focal length of 18-55 mm are good options for those looking for versatility. This is because they cover the range between 30 and 50 mm, which is closest to the perception of the human eye, in addition to allowing zoom.
The difference between a fixed lens and a zoom is that, in the former, the focal length and the angle of view do not change. In zoom lenses, these parameters vary within minimum and maximum limits.