Afocal photography, also called afocal imaging or afocal projection is a method of

photography Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employe ...

where the

camera A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with a ...

with its lens attached is mounted over the

eyepiece An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks through the device. The ...

of another image forming system such as an

optical telescope An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through elect ...

optical microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsc ...

, with the camera lens taking the place of the human eye.

Overview

Afocal photography works with any system that can produce a

virtual image In optics, an ''image'' is defined as the collection of focus points of light rays coming from an object. A '' real image'' is the collection of focus points made by converging rays, while a virtual image is the collection of focus points ma ...

of parallel light, for example telescopes and microscopes. Afocal photographic setups work because the imaging device's eyepiece produces

collimated A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction p ...

light and with the camera's lens focused at infinity, creating an

afocal system In optics an afocal system (a system without focus) is an optical system that produces no net convergence or divergence of the beam, i.e. has an infinite effective focal length. This type of system can be created with a pair of optical elements wh ...

with no net convergence or divergence in the light path between the two devices. In this system the device is focused on the object and the camera is placed above the eyepiece as close as possible. The drawback is the system will have a high

focal ratio In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...

, with a correspondingly dim image, and some

vignetting In photography and optics, vignetting is a reduction of an image's brightness or saturation toward the periphery compared to the image center. The word '' vignette'', from the same root as ''vine'', originally referred to a decorative bord ...

. A high focal ratio also means the

field of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Human ...

will be narrow. Field of view can be calculated using: Focal

/ angle of view: : Approximate:

\alpha =  \times  \equiv

: Precise:

\alpha = 2 \arctan

Use with optical telescopes

One method of afocal photography is to mount a

with its lens attached behind the

a Keplerian optical telescope, the combination giving the photographer a

long focus lens In photography, a long-focus lens is a camera lens which has a focal length that is longer than the diagonal measure of the film or sensor that receives its image. It is used to make distant objects appear magnified with magnification increas ...

. Historically afocal photography with 35 mm SLR or large format film cameras was a very difficult method of photography. With film cameras the bulk and mechanical shake had to be taken into consideration, with some setups employing a separate tripod for the camera (adding the complexity of setting up the camera in relationship to the eyepiece). The general difficulties of focus and exposure with film cameras, along with the detailed mathematical calculations, combined with the time lag of waiting for the film to be developed, made film afocal photography challenging.di Cicco, Dennis
"Afocal What?"
SkyandTelescope.com. Retrieved 2011-04-16. Nikon digiscoping set

Digital afocal photography

The advent of digital single-lens reflex camera and, moreover, compact point and shoot digital cameras has made the afocal method far more popular since this type of camera is small enough to mount directly on to telescopes or other devices, is for the most part a

solid state device Solid-state electronics means semiconductor electronics: electronic equipment using semiconductor devices such as transistors, diodes and integrated circuits (ICs). The term is also used as an adjective for devices in which semiconductor elect ...

with minimal moving parts, has auto focus, has auto exposure adjustment, has some capacity for time exposure, usually has a zoom mechanism to crop vignetting, and has a digital viewframe that allows the user to see the image hitting the viewing plane. Couplers and other devices for mounting digital cameras afocally are commercially available. Simply holding the camera up to the eyepiece and snapping a picture can obtain usable results. Most popular types of consumer digital cameras have non-removable lenses so afocal photography is also the only method available for these types of cameras.

Afocal astrophotography

Afocal photography is a form of

astrophotography Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1840, but it was no ...

long practiced by astronomers. Afocal setups with film and digital cameras are not the preferred system for astrophotography since astrophotographers have many ways of coupling a camera to an astronomical telescope, the simplest being prime focus (using no camera lens and allowing the image to fall directly onto the film, or image plane of a digital single-lens reflex camera or purpose built astronomical CCD camera). Almost from their invention amateur astronomers were adapting compact digital still and video cameras for use in afocal astrophotography. But since most celestial objects require a long exposure, compact consumer digital cameras are somewhat problematic due to their high inherent sensor noise. This noise limits their usefulness, especially since point-objects, such as stars, can be obscured by even one "hot"

pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the ...

. The narrow field of view for this type of photography lends itself to lunar and planetary objects. Continuing advancements in digital camera and image manipulation have somewhat overcome this limitation and digital afocal astrophotography has become more popular.

Daytime afocal photography

Since the development of compact digital cameras, afocal photography is also widely used by

birdwatchers Birdwatching, or birding, is the observing of birds, either as a recreational activity or as a form of citizen science. A birdwatcher may observe by using their naked eye, by using a visual enhancement device like binoculars or a telescope, by ...

, naturalists, and other photographers. In the birdwatching community it quickly acquired the coined name of “ digiscoping”. Birdwatchers and naturalists found a digital camera used afocally with a

spotting scope A spotting scope is a compact high-power telescope optimized for detailed observation of distant objects. They are used as portable optical enhancement devices for various outdoor activities such as birdwatching, skygazing and other naturali ...

to be a particularly effective technique since it gave them an easy way to record their subjects (sometimes by simply holding the camera up to the eyepiece) as well as allowing them to take relatively high quality photographs. Since these types of photographs are usually single subjects (narrow field) in daylight hours, the light loss and narrow angle of view are not a hindrance, and the high effective focal lengths are beneficial. It also facilitates the use of a very long lens without buying and transporting extra equipment, other than perhaps a small afocal adapter. This technique has lent itself to many other types of photography including photographing plants (for example, wild orchids growing in the canopy of a jungle), insects (for example, wild bees near their beehive), other shy or dangerous wild animals, or details in old buildings (for example, statues/gargoyles on the roof of old churches/castles).

Afocal secondary lenses

Besides combining a Keplerian telescope with a camera, there are also dedicated secondary lens afocal attachments that mount on the front of a camera lens to work in the role of a

teleconverter A teleconverter (sometimes called tele extender) is a secondary lens mounted between a camera and a photographic lens which enlarges the central part of an image obtained by the lens. For example, a 2× teleconverter for a 35 mm camera enl ...

, technically called

teleside converter A teleside converter (also known as a telephoto conversion lens or a front mount teleconverter) is a secondary lens which is mounted on the front of a photographic lens to increase the effective focal length of the lens they are attached to. They ...

s. These lenses are usually

Galilean telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and a ...

s that alter the width of the entering beam of light without affecting the divergence of the beam, so they can change the effective focal length 1 to 3 times without increasing

. There are models that are 6x or 8x and even Russian made 12x to 14x Gregorian Maksutov designs that can be used as long lenses and microscopes. Like their Keplerian counterparts these can be universally adapted to most camera lenses with the proper type of adapter.

References

External links

barrie-tao.com/afocal.html Afocal photography
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