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An apochromatic optical system is one for which light of three widely spaced visible wavelengths is brought to the same focal point. In practice, this implies that the view through the system will show no color fringing across the full field of view. For wide-field optics, this is an expensive requirement.
Focal length (f) is the fundamental specification of an optical element. For a single lens, the focal length is the distance behind the lens that the image of a distant object will form. Several optical elements (lenses or curved mirrors) can be combined with a single effective focal length.
In a camera, the field of view is determined by the the focal length and the size of the image frame. As focal length increases, field of view decreases. Likewise, as image frame size increases, field of view increases. Most digital cameras have very small image frames compared to 35 mm film, so to provide a digital camera with comparable field of view, the lens focal lengths are small. An f=50 mm lens for 35 mm film format shows the same field size (46 degrees) as an f=20.3 mm lens on a Coolpix 990; at f=23.4 (maximum f), the Coolpix field is 23 degrees. A zoom lens is a camera lens with variable focal length.
A telescope is a device that magnifies an object's image relative to the image without the scope. Strictly speaking, a telescope is afocal i.e., the focal length is undefined. The body (objective) has one defined focal length (400 mm for the Leica Televid), the eyepiece another; the ratio of focal lengths is the magnification. If the eyepiece focal length is 12.5 mm, the Leica will magnify an image 32 times. Combined with the Coolpix at full zoom, this is a 64x magnification over the 35 mm format, f=50 mm lens equivalent; in 35 mm format that would be equivalent to using an f=32 x 115=3680 mm lens. At the Coolpix f=23.4 mm (full) zoom with the 32x eyepiece the effective coupled focal length is 748 mm at f/9.7 (f=32 x 23.4 mm; 77 mm aperture).
The resolution of the scope itself limits the amount of useful magnification, however. For a 77 mm aperture, the resolution is approximately 0.0005 degrees. Increasing the magnification therefore cannot show you any additional detail over what you see at 32x. That is, further magnification makes the image larger, but not more detailed. The image must also become darker as magnification increases because the same amount of light is spread over a larger area.
For a digital camera, resolution is limited by both lens and pixel size, since to resolve two points they must be imaged on different pixels. Coolpix pixels subtend approximately 0.0003 degrees, while the lens resolution (at f=23.4, f/4) is about 0.007 degrees. The maximum magnification benefit to image resolution is therefore about 23x (0.007/0.0003).
The field of view is expressed either as: 1) the angular width visible through the eyepiece; or 2), the linear width visible at a stated distance. Although less intuitive, the angular expression is actually more useful, as one does not need to know the distance to the subject to measure its angular width.
The field of view of the Leica Televid scope, with 32x eyepiece, is 2.3 degrees. That's roughly the same as the angular width of your thumb when extended at arm's length. At full zoom, the Coolpix images the central part of the Leica's field; my estimated value is about 0.7 degrees, about the width of your smallest finger at arm's length. The field can be increased by zooming the camera out, but because the Coolpix and Leica lenses aren't matched with overlapped exit and entrance pupils, vignetting will occur at wider zooms.
It's possible to estimate the distance to your subject, or range, if you know two quantities: the angular field of view and the linear width of any object in the field of view. For a bird, you can use typical head-to-tail lengths. If you don't know your scope's angular field of view, check the sales brochures or web sites. Remember that scope field of view depends on the eyepiece, and camera field of view depends on the zoom setting.
By either looking at the object through the scope or from the camera image, estimate the number of these objects that would span your field of view. Multiply that number by the width of the object. The result is the linear width of the field of view.
For large distances (20+ meters), the linear width is equal to the range times the sine of the angular field. Use the "sin(x)" button on your calculator, with the angular field in degrees. The range equals (linear field width) divided by sin(angular field width). The units will be whatever units you used for linear width.
A camera or telescope focuses a planar image of the subject on a recording medium--an electronic chip (or film) in a camera, or your retina for a telescope. The image plane corresponds to another plane containing the subject; every point on the subject's focal plane will be imaged crisply on the image plane. But because of resolution limits on your lenses, even perfect focus leaves the image slightly blurred, as if the focus were not quite perfect. In other words, your lens will tolerate a slight defocusing without an apparent loss of resolution. Depth of field is the depth of the region before and after the focal plane which is acceptably focused at a single focus setting. What is unacceptable focus? When the defocusing "blur" is larger than the resolution.
Depth of field is a function of aperture; the larger the lens, the smaller the relative depth of field. For scope photography, the depth of field is completely controlled by the largest aperture, that of the telescope, if the camera aperture is set wider than about f/10 (for a 77 mm scope). At long range, depth of field equals twice the range times the resolution, divided by the aperture diameter. Be sure to use consistent units of distance for this calculation--that is, express all lengths in mm, feet, or meters. The subject focal plane is in the middle of the depth of field.
Most Digibird.com photos are taken with the scope atop a Gitzo 1325 Mountaineer tripod and an Arca-Swiss B1 ball head. A ball head is designed to allow complete freedom of rotation of the optical axis around the center of the head. The scope attaches to a ball which rides in a socket, and the head locks by squeezing the socket around the ball. Ball heads are compact, light, and can be very rigid. However, the freedom of rotation can complicate shot framing, as the scope can flop around unless the clamping action is carefully set.
Most birders prefer a variant of the pan head for spotting scopes. A pan head restricts scope/camera rotation to two axes: horizontal motion (panning) and vertical. A video pan head is a pan head designed with variable tension, so that the scope can be swept through its motion smoothly. With either pan design, a handle is usually attached to the head to ease pointing.
At Digibird.com, we prefer the ball head for two reasons: it is very compact and extremely rigid. However, pointing is not as smooth as with a pan head, and thus takes a bit more care, i.e., time. A perfect ball head would be as small and rigid as the Arca-Swiss, with an easily detached handle to aid pointing.
When you change the zoom, what you are really changing is the focal length of the lens. Longer focal lengths reduce the angular field of view. Since moving closer to the subject also reduces field of view, when you "zoom in" you get the illusion of moving closer.
Increasing lens focal length has a price, however. Since the total amount of light emitted by a subject is constant, as you decrease field of view the image must darken. On top of that, the Coolpix zoom is designed so that as focal length increases, the maximum aperture available decreases, one full stop between f=20.3mm (mid-range) and f=23.4 mm (full zoom). When lighting is less than bright, you may really need that extra aperture!
Vignetting is what happens when one optical element (possibly your eye) is passed an image by another optical element with smaller field of view. The image seen by the second element will be surrounded by a black ring where the first element obstructs the larger field. An analogy is the attempt to look at a distant object through a small hole in a sheet of paper. If the hole is very close to your eye, it doesn't obstruct your view of the distant scene; but if the hole is a few inches away, the paper vignettes the view.
With a scope, you are likely to see vignetting as you pull your eye away from the eyepiece (to a distance larger than the eye relief). The camera will see vignetting if either 1) its lens is not very close to the eyepiece, or 2) the zoom is wide. In both cases the camera's entrance pupil (the optic that defines the field of view; usually the lens surface closest to the object) is not within the eye relief. (As you zoom the camera lens out, the lens recesses into the camera). See the Imaging Primer zoom tests for examples.
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