Afocal photography

Source: Wikipedia, the free encyclopedia.
An afocal setup with a digital camera and a finderscope with optical diagram:
1 - Telescope
2 - Camera
3 - Film or CCD plane
L1 - Telescope objective
L2 - Eyepiece
L3 - Camera lens

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]

Overview

Afocal photography works with any system that can produce a

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

angle of view
:

Approximate:
Precise:

Use with optical telescopes

One method of afocal photography is to mount 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]

Spotting scope with a digital camera mounted afocally using an adapter.

Digital afocal photography

The advent of

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.

An afocal astrophotograph of the total solar eclipse of April 8, 2024, taken with an Asus ZenFone 9 and a Celestron C5+ through a 32 mm Plössl eyepiece.

Afocal astrophotography

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.

Kowa spotting scope

Daytime afocal photography

See also: Digiscoping

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

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]

Afocal secondary lenses

Besides combining a Keplerian telescope with a camera, there are also dedicated

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.

See also

References

  1. ^ The British journal of photography, Volume 150, Issues 7410-7422 by The Liverpool Photographic Society
  2. ^ Michael A. Covington, Astrophotography for the amateur, page 75
  3. ^ a b di Cicco, Dennis. "Afocal What?". SkyandTelescope.com. Retrieved 2011-04-16.
  4. ^ Michael A. Covington, Astrophotography for the amateur, page 69
  5. ^ Michael A. Covington, Astrophotography for the amateur, page 243, afocally video imaging from 1997
  6. ^ "The CrystalVue 8X and Kenko 6X Combo". Retrieved May 14, 2012.

External links

Wikimedia Commons has media related to Afocal photography.
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