Afocal photography, also called afocal imaging or afocal projection is a method of photography where the camera with its lens attached is mounted over the eyepiece of another image forming system such as an optical telescope or optical microscope, with the camera lens taking the place of the human eye. [1]
Afocal photography works with any system that can produce a virtual image of parallel light, for example telescopes and microscopes. Afocal photographic setups work because the imaging device's eyepiece produces collimated light and with the camera's lens focused at infinity, creating an afocal system with no net convergence or divergence in the light path between the two devices. [2] 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, with a correspondingly dim image, and some vignetting. A high focal ratio also means the field of view will be narrow. Field of view can be calculated using:
Focal field of view/angle of view:
One method of afocal photography is to mount a camera with its lens attached behind the eyepiece of a Keplerian optical telescope, the combination giving the photographer a long focus lens. 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. [3]
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 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. [3] Couplers and other devices for mounting digital cameras afocally are commercially available[ citation needed ]. 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 photography is a form of astrophotography 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). [4] Almost from their invention amateur astronomers were adapting compact digital still and video cameras for use in afocal astrophotography. [5]
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. 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.
Since the development of compact digital cameras, afocal photography is also widely used by birdwatchers, 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 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[ citation needed ]. 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).[ citation needed ]
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, technically called teleside converters. These lenses are usually Galilean telescopes 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 focal ratio. 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. [6] Like their Keplerian counterparts these can be universally adapted to most camera lenses with the proper type of adapter.
A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system that permits the photographer to view through the lens and see exactly what will be captured. With twin lens reflex and rangefinder cameras, the viewed image could be significantly different from the final image. When the shutter button is pressed on most SLRs, the mirror flips out of the light path, allowing light to pass through to the light receptor and the image to be captured.
A camera is an instrument used to capture and store images and videos, either digitally via an electronic image sensor, or chemically via a light-sensitive material such as photographic film. As a pivotal technology in the fields of photography and videography, cameras have played a significant role in the progression of visual arts, media, entertainment, surveillance, and scientific research. The invention of the camera dates back to the 19th century and has since evolved with advancements in technology, leading to a vast array of types and models in the 21st century.
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 was taken in 1840, but it was not until the late 19th century that advances in technology allowed for detailed stellar photography. Besides being able to record the details of extended objects such as the Moon, Sun, and planets, modern astrophotography has the ability to image objects outside of the visible spectrum of the human eye such as dim stars, nebulae, and galaxies. This is accomplished through long time exposure as both film and digital cameras can accumulate and sum photons over long periods of time or using specialized optical filters which limit the photons to a certain wavelength.
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 electronic image sensors.
A zoom lens is a system of camera lens elements for which the focal length can be varied, as opposed to a fixed-focal-length (FFL) lens.
Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a size ratio called optical magnification. When this number is less than one, it refers to a reduction in size, sometimes called de-magnification.
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 an optical device to observe an object or sample. The objective lens or mirror collects light from an object or sample and brings it to focus creating an image of the object. The eyepiece is placed near the focal point of the objective to magnify this image to the eyes. The amount of magnification depends on the focal length of the eyepiece.
Macro photography is extreme close-up photography, usually of very small subjects and living organisms like insects, in which the size of the subject in the photograph is greater than life size . By the original definition, a macro photograph is one in which the size of the subject on the negative or image sensor is life size or greater. In some senses, however, it refers to a finished photograph of a subject that is greater than life size.
In photography, a viewfinder is a small window the photographer looks through to see what a photo will look like before they capture it.
Digiscoping is a neologism for afocal photography, using a (digital) camera to record distant images through the eyepiece of an optical telescope.
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses (dioptrics) and curved mirrors (catoptrics). Catadioptric combinations are used in focusing systems such as searchlights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes, and telephoto lenses. Other optical systems that use lenses and mirrors are also referred to as "catadioptric", such as surveillance catadioptric sensors.
An astrograph is a telescope designed for the sole purpose of astrophotography. Astrographs are mostly used in wide-field astronomical surveys of the sky and for detection of objects such as asteroids, meteors, and comets.
The following outline is provided as an overview of and topical guide to photography:
Polar alignment is the act of aligning the rotational axis of a telescope's equatorial mount or a sundial's gnomon with a celestial pole to parallel Earth's axis.
A telecompressor or focal reducer is an optical element used to reduce focal length, increase lens speed, and in some instances improve optical transfer function (OTF) performance. It is also widely known under the name “Speed Booster”, which is the commercial name of a line of telecompressors by the manufacturer Metabones. Popular applications include photography, videography, and astrophotography. In astrophotography, these qualities are most desirable when taking pictures of nearby large objects, such as nebulae. The effects and uses of the telecompressor are largely opposite to those of the teleconverter or Barlow lens. A combined system of a lens and a focal reducer has smaller back focus than the lens alone; this places restrictions on lenses and cameras that focal reducer might be used with.
A digital microscope is a variation of a traditional optical microscope that uses optics and a digital camera to output an image to a monitor, sometimes by means of software running on a computer. A digital microscope often has its own in-built LED light source, and differs from an optical microscope in that there is no provision to observe the sample directly through an eyepiece. Since the image is focused on the digital circuit, the entire system is designed for the monitor image. The optics for the human eye are omitted.
An autoguider is an automatic electronic guidance tool used in astronomy to keep a telescope pointed precisely at an object being observed. This prevents the object from drifting across the field of view during long-exposures which would create a blurred or elongated image.
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 where the physical distance d between the elements is equal to the sum of each element's focal length fi (d = f1+f2). A simple example of an afocal optical system is an optical telescope imaging a star, the light entering the system is from the star at infinity (to the left) and the image it forms is at infinity (to the right), i.e., the collimated light is collimated by the afocal system. Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam, increasing magnification. The magnification of such a telescope is given by
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 increasing as longer focal length lenses are used. A long-focus lens is one of three basic photographic lens types classified by relative focal length, the other two being a normal lens and a wide-angle lens. As with other types of camera lenses, the focal length is usually expressed in a millimeter value written on the lens, for example: a 500 mm lens. The most common type of long-focus lens is the telephoto lens, which incorporate a special lens group known as a telephoto group to make the physical length of the lens shorter than the focal length.
