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I've noticed that some lenses, mainly wide-angle or fisheye (e.g. Samyang 14mm f/2.8, or Nikkor 8-15mm fisheye) have a rounded front element.

Which is the reason for it? It's of course limiting in terms of the opportunity to add external filters. Is it just for protection? But, if so, why is it not utilized for any lenses?

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3 Answers 3

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It can see 180°.
To achieve that it has to be proud of the barrel.

All [or at least most] lenses have a convex front element - but they don't have a field of view so wide that their own barrel will be in shot, so they can be set back into it a little, allowing for a flat filter in front.
Basically, the wider the lens, the further forward it needs to sit.

[The Samyang 14mm 2.8 is not a fisheye, it's rectilinear, but still has a 115° angle of view.]

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    \$\begingroup\$ There are indeed some lenses with concave front elements, but of course, most are convex. \$\endgroup\$
    – osullic
    Dec 4, 2022 at 17:05
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The two basic shapes of lenses are convex (bulge outward) and concave (bulge inward). As light traverses a lens, the lens shape plus its hardness causes the light to change its travel direction (refraction Latin to bend). A ray trace of this action reveals that a convex lens forces light to trace out the shape of a cone. If this cone of light is intercepted at the apex of the cone, allowing it to play on a screen or film or digital sensor, an image is seen.

The distance from lens to image makes known the focal length of this system. Short focal length lenses produce a wide-angle view. To accomplish the lens curvature must be steep. A longer focal length lens requires a more gradual curve. The steepness of this curve is called the lens “figure”.

It is typical that a wide-angle lens has a pronounced figure.

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The front element (or often, successive multiple elements) on fisheye and ultra-wide angle lenses are bulbous so that the lens "sees" a very wide angle of view in front of it.

Typical lens diagrams for explanatory purposes show lenses as symmetric biconvex (front and rear surfaces both convex) or biconcave (both surfaces concave) to explain how they cause rays to converge to the focus point (convex) or diverge from the virtual image point (concave).

But most real word elements are not front/rear symmetric, or even both convex or both concave. The front elements on wide angle lenses are clearly round on the outer surface (hence front convex). But their back surface is hollowed out, concave. In fact, these front elements are thinner in the middle than at the edges, so that on the whole, they are a net concave lens. Thus, light rays entering the lens diverge, bending away from the optical center. These elements are called diverging convexo concave, meaning a combination of convex front, concave rear, net concave.

If you look at the center of the lens from the front, you'll see the tiny entrance pupil, the center of the lens. As you look further and further off-axis, you'll still the entrance pupil. Even at 90º from the optical axis, you'll still be able to see the entrance pupil "right behind" the front element. It's the series of diverging convexo concave front elements that act as "angle of view gathering" elements to allow the lens such a wide angle of view, that causes this.

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