Understanding the Focal Length of Fisheye Lens

What is fisheye lens?

Fisheye lens is a special lens belonging to the ultra-wide-angle lens category. Its field of view strives to reach or exceed the range of human vision. Therefore, the images produced by fisheye lenses are very different from the real world seen by human eyes, because the objects we see in real life have fixed and regular shapes, while the images produced by fisheye lenses go beyond this category. The fisheye lens is an ultra-wide-angle lens that can produce a 180° view, with the aim of creating panoramic or hemispheric images. Its name was created by American physicist and inventor Robert W. Wood in 1906, who compared the effect of fisheye lenses to the underwater world view of fish.

You will experience this strong visual distortion, which is due to its unique optical shape. The fisheye lens is affected by the so-called "curvilinear barrel distortion". This is where straight lines around the center of the frame appear curved. Most of the time, this is not the effect that photographers want to achieve when taking pictures. In fact, distortion is often described as a defect of the lens rather than an ideal feature when examining the lens. When we use a wide-angle lens, we usually want the straight lines to look straight. However, the curved distortion effect makes the fisheye lens so unique and distinguishes it from the "straight-through lens" - the produced image makes straight features look straight.

What is the focal length of fisheye lens?

There are two types of fisheye lenses: circular and full-frame. The difference between the two is that when you use a circular fisheye lens to take a picture, the entire 180° image is projected onto the camera sensor, resulting in a small world effect. The full-frame fisheye lens also has a 180° field of view, although the image projected onto the camera sensor is much larger than that of the circular fisheye lens, resulting in an image that fills the entire frame.

A typical fisheye lens has an ultra-wide-angle focal length of about 24mm or less. They can be used as fixed-focus lenses (with a fixed focal length) or zoom lenses (with a variable focal length). Some fisheye lenses, such as the Canon 8-15mm f/4L fisheye lens, cover a wide range of focal lengths and are compatible with full-frame and cropped sensor cameras. There are also fisheye lenses specifically designed for cameras with APS-C sensors, which can produce similar effects, such as the Tokina 10-17mm f/3.5-4.5 fisheye lens.