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home made afocal setup with a digital camera and a finderscope

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 a optical telescope or microscope, with the camera lens taking the place of the human eye[1].

Contents

Overview

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 the camera's lens can focused at infinity, so they are both afocal systems, there is 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 draw back 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:

Approximate: \alpha = {d \over f} \times {180 \over \pi} \equiv {180 d \over \pi f}
Precise: \alpha = 2 \arctan {d \over 2 f}

Film afocal photography

Historically afocal photography with 35 mm SLR or large format film cameras is a very difficult method of photography. With film cameras the bulk and mechanical shake has 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, means film afocal photography can be pretty hit and miss.[3]

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 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 video screen on the back side of the camera so you can actually see the image hitting the image plane.[4] A whole new industry has sprung up selling couplers and other devices for mounting digital cameras afocally. 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 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).[5] Amateur astronomers were early adapters of compact digital still and video cameras, employed them in afocal astrophotography almost from their invention.[6] 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 its self to lunar and planetary objects. Continuing advancements in digital camera and image manipulation have somewhat overcomes this limitation and digital afocal astrophotography has become more popular.

Daytime afocal photography

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

Since the development of compact digital cameras, afocal photography is also widely used by birdwatchers, naturalist, and other photographers. In the birdwatching community it quickly acquired the coined name of “digiscoping”. Birdwatchers and naturalist 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 a plus. It also gives the photographer the use of a very long lens without buying and lugging around extra equipment (except maybe a small afocal adapter).

A "digiscoped" photo of a Goosander, taken with a Fujifilm digital camera and a Kowa spotting scope

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).

References

External links

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