Wide angle/fisheye lenses

[kello_there]

At what focal length does wide angle become fish eye?

[maylon_roberts]

It doesn't work that way. You can get one 15mm lens that is a fish-eye and another 15mm lens that is just a super-wide angle. A wide angle lens is rectilinear, while a fish-eye isn't. A fish-eye is only a fish-eye if it says it's a fish-eye.

[Ed_Ingold]

A fisheye lens is designed to have an extraordinary amount of barrel distortion. This has the effect of making the angle of view much greater than a rectilinear lens of the same focal length.

 

The most popular "fisheye" lenses today are so-called full-frame lenses, in which the image extends from corner to corner. The original "fisheye" lenses of the 60's produced a circular image on the film, which is largely responsible for the term "fisheye." If the camera were pointed straight up, the horizon would be a circle around the periphery of the image.

 

Fisheye lenses can be very effective on subjects without rectilinear geometry, like a cave or canyon, or landscapes if the horizon is centered. I've seen some excellent night shots of large groups, where the flash would get swallowed up, or there is not much room to make the shot. Any "artistic" value is rather hackneyed at this point.

[fmueller]

Fisheye lenses do not have barrel distortion, they use a different way of translating a 3-dimensional objects into a 2-dimensional picture. While other lenses, including wide angles, use a rectilinear perspective, fisheyes use a cylindrical perspective, which enables them to have an angle of view of 180 degrees or thereabouts. This means that the reproduce straight horizontal lines above and below the lens axis level as curved, while reproducing straight horizontal lines on lens axis level as straight. There used to be a great web site explaining all of this much better than I could, but I can't find it right now. Does anybody have this link? The site also explained that all fisheye lenses produce a circular image, of which with full frame fisheye lenses only the central part is captured on film.

[Ed_Ingold]

Fisheye lenses have a form of spherical, not cylindrical projection. Even normal lenses have a circular image (unless masked in some way) from which only a central portion is used.

 

Although Zeiss describes their 30mm Distagon as having "special distortion," their data sheet shows that it is simply negative (barrel distortion) that monotonically increases with distance from the image center, reaching 100% at the corners (40mm), compared to 1 or 2% for a normal lens. It's rather convenient that Zeiss publishes data to back up their marketing claims - sadly it's not widely imitated.

 

Straight lines that pass through the center of the lens (sagital lines) remain straight. Circles concentric to the center remain circles, and straight, tangential lines curve more outward with distance from the center, until they become circles at the edge of the field. Except for the extreme limit, the same phenomena describe barrel distortion.

 

If it walks like a duck and quacks like a duck, it's probably a duck.