Crop factor explained
In the “pre-digital” era of photography, SLR cameras mostly used 35 mm film, and the concept of crop factor was completely absent. This means that they all captured the same part of the image projected by the lens, and the projection given by a specific lens was constant.
Digital cameras are more complex. The film in them was replaced by a matrix, which is usually smaller than a 35mm frame. Since it is physically smaller, it also captures a smaller part of the projected image. As a result, the angle of the lens’s field of view narrows, which gives the impression of being closer.
This raised a certain problem: if the same lenses give different results on different cameras, how can a photographer accurately match them and determine which angle of view will be typical for a particular camera? The crop factor was designed to answer these questions.
Crop factor meaning
The crop factor refers to the difference between 35mm film and sensor size. For example, if your camera has a crop factor of 2, this means that the sensor is half the size of a 35mm frame.
The best digital cameras have sensors that are the same size as 35mm film frames, so they have a crop factor of 1 (also called “full frame”). On the opposite, there are cameras with a very small sensor, so their crop factor can reach 5-6. The higher the crop factor, the greater the zoom effect for each specific focal length.
The crop factor can be calculated by the formula: CF = diag35mm / diagsensor
But usually it is much easier to find out this parameter in the characteristics of the camera – it is always indicated.
Effect of the focal length on the final picture
In simple terms, the longer the focal length, the greater the magnification, and vice versa. Short focal length wide-angle lenses give small magnifications. This means that you have to physically move closer to a medium-sized subject to fill the frame.
It also means that large objects will fit into the frame too, and you do not need to step back to photograph them. Long focal length telephoto lenses provide high magnification, so you can fill the frame with objects that are far from the camera.
Since digital cameras capture a smaller part of the projected image in comparison to cameras that used film, it would be rational to try making the lens smaller without affecting the size of the photo.
As this technology progressed, mirrorless cameras were created. They were specifically made with crop sensors and smaller lenses to be more compact and weight less.
Nowadays, some lenses are made specifically for crop sensor cameras. They have a smaller image circle, so if you want to use them on full-frame cameras, be aware that either they will not work at all or dark corners will appear on a taken photo, giving it a vignette effect.