Afocal photography
Afocal photography mounts a camera over an eyepiece for imaging.
Afocal photography, also called afocal imaging or afocal projection, is a method of photography where a 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. It works with any system that can produce a virtual image of parallel light, such as telescopes and microscopes, by creating an afocal system with no net convergence or divergence in the light path between the two devices.
- Focal ratio effect
- high focal ratio, dim image, vignetting
- Field of view approximate formula
- α = d/f × 180/π
- Field of view precise formula
- α = 2 arctan(d/(2f))
- Common use with telescopes
- Keplerian optical telescope
- Digital camera types
- digital SLR, compact point and shoot
- Daytime photography coined name
- digiscoping
Lore & Background
Historically, afocal photography with 35 mm SLR or large format film cameras was a very difficult method, requiring consideration of bulk and mechanical shake, sometimes using a separate tripod for the camera. The general difficulties of focus and exposure with film cameras, along with detailed mathematical calculations and the time lag of waiting for film to be developed, made film afocal photography challenging. The advent of digital single-lens reflex cameras and compact point-and-shoot digital cameras made the afocal method far more popular, as these cameras are small enough to mount directly on telescopes, have auto focus, auto exposure, time exposure capacity, zoom to crop vignetting, and a digital viewfinder. Couplers and other mounting devices are commercially available, and simply holding the camera up to the eyepiece can obtain usable results.
Reader's Guide
Afocal photography is a form of astrophotography long practiced by astronomers, though it is not the preferred system since astrophotographers have many coupling methods, such as prime focus. Compact consumer digital cameras are somewhat problematic for long exposures due to high inherent sensor noise, which can obscure point-objects like stars with hot pixels. The narrow field of view lends itself to lunar and planetary objects. Continuing advancements in digital cameras and image manipulation have somewhat overcome this limitation, increasing popularity. In daytime use, afocal photography is widely used by birdwatchers and naturalists, acquiring the coined name 'digiscoping'. It allows recording subjects by simply holding a camera to the eyepiece, taking relatively high-quality photographs. The light loss and narrow angle of view are not hindrances for single subjects in daylight, and the high effective focal lengths are beneficial. This technique has also been applied to photographing plants, insects, shy or dangerous wild animals, and details in old buildings.
Did You Know?
- Afocal photography works with any system that produces a virtual image of parallel light, such as telescopes and microscopes.
- The camera's lens must be focused at infinity to create an afocal system.
- The field of view can be calculated using either an approximate or a precise formula.
- The technique is the only method available for most popular consumer digital cameras with non-removable lenses.
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