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A pancake lens is colloquial term for a flat, thin lens (short barrel), generally a normal or slightly wide prime lens for a camera.
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Motivation
Pancake lenses are primarily valued for providing quality optics in a compact package. The resulting camera and lens assembly may even be small enough to be pocketable, a design feature which is usually impractical with conventional SLR bodies and lens assemblies. Pancake lenses can be very short and flat because they do not need large amounts of optical correction, i.e. extra lens elements. A lens is generally considered a "pancake lens" when the length of the lens barrel is thinner than 30.0 millimetres (1.18 in).
The problem arises when such lenses have too short a focal length to fit in front of the retractable mirrors used in reflex cameras. In such a situation, a pancake lens focuses in front of, rather than on, the focal plane (film or light sensor) of the camera. This has necessitated the design of retrofocus lenses that refocus the image further back, which is why such lenses are longer and bulkier than their "pancake" equivalents.
Pancake-style prime lenses are generally simpler to manufacture than pancake zoom lenses due to the general lack of an internal micromotor and fewer image correcting elements, allowing for a thinner profile. Because of this limitation, pancake zoom lenses are much less common.
History
An early example is the (Zeiss) Tessar of 1902.
In the 1960s and 1970s the Nikon GN lens was a notable example, while in the 1970s and 1980s pancake lenses were used in compact single lens reflex (SLR) cameras.
Throughout the 2010s, the design has seen a resurgence due to the growth of the mirrorless interchangeable-lens digital camera market. Pancake lenses have increasingly become lighter and feature thinner profiles than years past. An extreme example of this trend would be the Pentax DA 40mm F2.8 XS, released in 2012 and measuring only 9.14 millimetres (0.360 in) long.