For cameras with a 35mm sensor or film, typical circular fisheye camera lenses may have a focal length of 8mm to 10mm. On the other hand, full-frame lenses have slightly longer focal lengths, typically ranging from 15mm to 16mm. Like all other lenses, the focal length effectively increases for cameras with sensors smaller than 35mm. To determine the equivalent focal length, multiply its focal length by the "crop factor" of the camera. For example, a 10mm fisheye lenses on a camera with a crop factor of 1.5 may have an effective focal length of 15mm. This creates a narrower field of view. For this reason, many manufacturers produce fisheye lenses specifically designed for cameras with smaller sensors. These lenses have shorter focal lengths, sometimes as low as 1mm, to create complete 180° images.
A "true" fisheye camera lens is considered to be a lens that can capture 180° at its widest point. However, some lenses sold by manufacturers go further, reaching up to 220°. These lenses are large, heavy, and expensive, typically used for professional technical work. If you want to capture angles greater than 180°, you can use software like Photoshop to stitch multiple fish-eye photos together. This allows you to create images that cover 360°, producing some exciting abstract compositions.
Fisheye camera lens exhibit "barrel distortion," where the subject in the center of the frame appears to bulge outwards and straight lines curve dramatically. This type of image is called a "curvilinear" image. Fisheye camera lens create pronounced barrel distortion, but you can use it for significant artistic effects. In regular wide-angle lenses, you can correct this to generate "rectilinear" images, where the perspective in the scene appears normal, and straight lines are straight. However, the angle of view of a fisheye camera lens is too extreme for this type of correction. This is generally not an issue because most photographers use fisheye camera lenses precisely because they produce interesting, unnatural distortions, which are part of their intentional plan.
The fisheye lens has a significant depth of field when capturing extreme angles. This means that your photo will appear very sharp from front to back, making it ideal for capturing scenes with exciting subjects in the foreground and background. Conversely, this extreme depth of field makes it almost impossible to isolate the subject by blurring the background. Likewise, this is something you need to be mindful of when composing so that you can choose a clean background.