Fixed Focus Lens

A lens is said to have a fixed focus when near and distant objects are in focus on the ground-glass at the same time, thus doing away with the necessity of altering the distance between the lens and the sensitive plate. Only lenses of short focal length, therefore (lenses made to cover plates not larger than 4x5 inches), can be termed fixed-focus lenses. Objects nearer than 6 feet will, however, be more or less blurred (out of focus). Only the cheap forms of cameras are fitted with fixed-focus lenses, and it is seldom that these are made for plates larger than 3 1/4 x 4 1/4 inches.

Fluid Lens

A lens constructed of optical glass filled with distilled water.

Heliar Lens

A name given to an anastigmat lens manufactured by Voigt-lander. Works at a large aperture and is especially adapted to portrait work.

Hypergon Lens

A wide-angle lens, embracing the enormous angle of 1350. Manufactured by Goerz.

Landscape Lens

Achromatic, Single, or View Lens. A lens consisting of a biconvex crown glass cemented to a biconcave flint glass. Corrected for chromatic aberration, but not for curvilinear aberration or distortion.

Meniscus Lens

A lens which is convex on one side and concave on the other, and thicker in the center. A sectional view of the lens has the appearance of the moon in its first quarter.

Negative Lens

A lens having greater concavity than convexity.

Orthostigmatic Lens

A lens whose single combinations are free from astigmatic, chromatic and spherical aberration.

Petzval Lens

One of the earliest forms of portrait lenses. It is very rapid, but gives poor marginal definition and the various aberrations" are quite marked.

Planar Lens

An anastigmat lens manufactured under the Zeiss patents. It embraces a comparatively wide angle, is very rapid, and highly corrected for the various aberrations.

Plastigmat Lens

An anastigmat type of lens manufactured by Bausch & Lomb.

Portrait Lens

A lens specially constructed for portrait work, the principal requirements being that the lens be one of long focus and work with a large aperture, thus giving great speed.

Positive Lens

Any lens which causes rays of light to converge; capable of forming a real image.

Projection Lens

A lens which gives perfectly rectilinear lines and especially adapted to enlarging and lantern projection.

Protar Lens

The name for a particular form of anastigmat lens originated by Zeiss. Manufactured in the United States by Bausch & Lomb.

Rapid Lens

A lens whose working aperture is large as compared with its focal length is termed a rapid lens. In fact, any lens allowing a large number of rays of light to pass through to the sensitive plate is a rapid one. The greater the rapidity of the lens the less is the depth of focus and definition, and for this reason any adjustment which increases the rapidity of the lens necessarily tends to decrease the depth of focus.