Pentaprism

Five-sided reflecting prism

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A pentaprism. A perspective drawing showing a roof pentaprism commonly used in a single lens reflex camera (SLR). The image is flipped laterally by the prism.

A pentaprism is a five-sided reflecting prism used to deviate a beam of light by a constant 90°, even if the entry beam is not at 90° to the prism. The beam reflects inside the prism twice,[1] allowing the transmission of an image through a right angle without inverting it (that is, without changing the image's handedness) as an ordinary right-angle prism or mirror would.

The reflections inside the prism are not caused by total internal reflection, since the beams are incident at an angle less than the critical angle (the minimum angle for total internal reflection). Instead, the two faces are coated to provide mirror surfaces. The two opposite transmitting faces are often coated with an antireflection coating to reduce spurious reflections. The fifth face of the prism is not used optically but truncates what would otherwise be an awkward angle joining the two mirrored faces.

In cameras

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An image as seen through a roof pentaprism. This is looking in through the eyepiece plane.

A variant of this prism is the roof pentaprism which is commonly used in the viewfinder of single-lens reflex cameras.[1][2] The camera lens renders an image that is both vertically and laterally reversed, and the reflex mirror re-inverts it leaving an image laterally reversed. In this case, the image needs to be reflected left-to-right as the prism transmits the image formed on the camera's focusing screen. This lateral inversion is done by replacing one of the reflective faces of a normal pentaprism with a "roof" section, with two additional surfaces angled towards each other and meeting at 90°, which laterally reverses the image back to normal. Reflex cameras with waist-level finders (viewed from above), including many medium format cameras, display a laterally reversed image directly from the focusing screen which is viewed from above.

Compared to the pentamirror

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The same optical paths can be realized with three mirrors, in an arrangement called the pentamirror. While substantially lighter, the light enters and exits the mirrors' glass several times, each time losing brightness and instead scattering. The pentaprism is typically much heavier, but only has one entrance and one exit, providing a notably superior optical performance. Additionally, pentamirrors can conceivably go out of alignment while a pentaprism's facets are perfectly aligned until it is destroyed. [3]

In surveying

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Double pentaprism as used in surveying.

In surveying a double pentaprism (two pentaprisms stacked on top of each other) and a plumb-bob are used to stake out right angles, e.g. on a construction site.

See also

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Hensoldt roof pentaprism binoculars

References

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Prisms are usually cut and ground from a solid piece of glass, and then coated with reflective materials on all surfaces except for the two that connect to the focusing screen and viewfinder. This creates a mirror reflection within the prism. As a reflex system for single-lens reflex cameras, penta prisms correct the upside-down image seen on the focusing screen, allowing the viewfinder to display a completely accurate representation of the scene being captured. This enables the user to properly compose and focus the image. When the shutter is released, the mirror flips upward allowing light to pass through the prism, which becomes opaque. In , Pentax released the first camera equipped with a penta prism.

1. Roof Penta prisms

The roof penta prism is a variation of the penta prism and is commonly used in single-lens cameras. The image projected on to the camera body via the lens will be rotated 180°, and therefore the direction of the image will need to be reversed left-to-right when reflected by the plane mirror and passed through the penta prism. This is achieved by replacing one of the reflecting surfaces with a roof-type surface. The two surfaces of the replaced roof-type penta prism are perpendicular to each other, and this changes the chiral properties of the image.

2. Reasons for Using Penta prisms in Cameras

Many diagrams of single-lens reflex camera optical paths show only a side view of the penta prism, giving the impression that there are only two reflection surfaces inside the prism. However, in reality, the penta prisms used in single-lens reflex cameras should actually be called roof penta prisms, one of the various types of prisms optics. Does the camera use surface mirrors? Actually, many low-end single-lens reflex cameras still use surface mirrors instead of prisms. In fact, whether to use prisms or mirrors is also an important criterion for distinguishing between low-end models and higher-end models for many major manufacturers. So what is the difference between a mirror and a prism?

The main difference is cost, as the processing cost for prisms is higher and therefore they are only used in higher-end models, but the prism has a lower optical loss, making the viewfinder appear brighter. In the penta prism camera scheme, there are only two surfaces involved in the path of the light, entering and leaving the prism, while in the surface-mirror scheme there are at least three, which increases the loss of light. In addition, if the surface mirror is coated with a film after the mirror, the parallelism of the surface must also be considered. If the two surfaces of the mirror are not parallel, it will introduce additional aberrations, ghosts, and light loss. Thirdly, surface mirrors are separate components, whereas prisms are a solid entity, with obvious differences in accuracy and durability. A slight impact may cause the surface mirror's light path to deform, but the prism will not. Purely from the optical path, the two can indeed be mutually substituted, but in actual products, which one to use depends on a comprehensive consideration of the above factors.