By Ernie DeMaio, Program Coordinator, Liberty Science Center (excerpts from several sources)
Many specifications and concepts are shared in common between cameras, binoculars and telescopes. Here is a brief rundown on those commonly shared concepts.
Camera lenses have two major specifications, focal length (28mm, 50mm, 135mm etc.) and f/Stop. (f:2, f:4, f:5.6, etc.)
1. FOCAL LENGTH
The focal length determines the size of the image on the film. If we consider a zoom lens you know that the 35mm setting is “wide angle”, 50mm is” normal” and 135mm is “telephoto.” Focal length, for a given film size (such as 35mm) determines the “field of view”, a specification usually ignored or unstated directly. ” Field of view” is of
course the angle of view seen in the real world and is inversely proportional to focal length. The 135mm setting has the highest magnification and smallest field, while the 35mm setting has the lowest magnification and the largest field, in fact almost 4 times the field size (135/35) and 16 times the field area!
Many people of course confuse f/Stop and focal length and think that a change in the f/Stop setting affects the image size in some magical way. It’s only the focal length that affects image size and field size.
2. FOCAL RATIO, OR f/Stop
Usually written as a ratio, (1:2 for example, on the lens barrel means f/2), this gives the “fatness” of the cone of light that reaches the film. “Fatter” cone, more light, faster exposure. f/# is actually the ratio of the focal length to the effective diameter of the lens. A 50mm f/2 lens has an effective lens diameter of 25 mm. ( Remember that 25.4
mm equals 1 inch – a useful fact when we discuss telescopes, where lens diameter is sometimes given in inches and the focal length in millimeters.)
Most people instinctively appreciate that an f/2 lens is faster than an f/4, but may not know that it’s 4 times faster! Also, there’s a misconception that the f/2 lens is sharper. (Theoretically possible, but rare in practice.)
While f/Stop is a critical specification for cameras, where f/Stop, film speed and subject brightness are exposure variables, we shall see that it has no direct effect on brightness of images viewed through a telescope. A shocking statement to my photographer friends who’ve recently become “telescope aficionados.”
A big thank you now to the binocular industry, which has developed meaningful product specifications, never, ever mentioning f/ratios.
Not everyone thinks of binoculars as 2 parallel telescopes, but it sure is handy when explaining how a telescope works. Neglecting the prisms which turn the image right-side up, the binoculars consist of an objective to form an image just like a camera lens, and a magnifier to view the image directly instead of film to capture the image. We call the magnifier an eyepiece, or ocular.
The ratio (there’s that word again) of the focal length of the objective to focal length of the eyepiece gives the magnification or power of the binocular or telescope.
6 x30, 7 x35, 7 x50, 10 x50 etc. How nice to have meaningful specs without confusing f/#’s. A 7×35 means 7 power having a 35mm objective lens aperture (diameter). Notice that nobody said it’s 7 power because the objective has a focal length 7 times that of the eyepiece, or that maybe it came from a 140mm focal length objective used with a
20mm focal length eyepiece. Or, that the 140mm objective with a 35mm diameter means it’s an f/4 (who cares? But these are interesting facts when we discuss telescopes more fully.)
The aperture specification is very meaningful but indirectly, because the aperture divided by the magnification gives the “exit pupil” diameter: 35mm aperture divided by 7 power = 5mm exit pupil. A 7×50 binocular has a 7.1 mm exit pupil. So what? Well, while f/# gives a relative measure of image brightness for a camera, the exit pupil compared to your own eye’s pupil, determines the image brightness in binoculars. The exit pupil is the little circle of light you see when you hold the binocular away from your eye. The circle is actually the image of the objective, that is formed by the eyepiece.
1. PUPIL SIZE (EYE RELIEF)
The human eye pupil is 2 to 3mm in diameter in daylight and goes up to 7mm at night when the eye is dark adapted. If the binocular exit pupil is at least as large as your pupil, the image will be about as bright as a normal view. If smaller, then the brightness is diminished by the ratio (there I go again) of the area of the exit pupil to that of the eye. A 7 x35 binocular will be just as bright as a 7 x50 in the daylight when your eye pupil is smaller than 5mm, but at night, the 7×50 will appear (50/35) squared = twice as bright!
2. FIELD OF VIEW
Another binocular specification describes how much field you actually see. More complex eyepieces allow a greater area of image to be seen, almost like a camera using a bigger piece of film. For example, a simple eyepiece will show 390 ft. at 1000 yards, while a more complex one might reveal 496 ft. at 1000 yards, under the same magnification. Beware though. The larger field usually shows fuzzier images toward the edge. For viewing astronomical subjects, the field size can hardly be measured at 1000 yards, so we have to convert the specs to true field angles: 390 ft. at 1000 yards equals 7 degrees. That’s the true field. Since the binocular magnifies 7 times, the field appears about 50 degrees in the eyepiece. The larger field eyepiece produces an apparent field of 65 degrees in the eyepiece: Every eyepiece has its own fixed apparent field.
We can see that a binocular has magnification, aperture, an exit pupil, a real field of view and an apparent field, all interrelated mathematically.
What does a telescope have that half a binocular doesn’t: Flexibility – with interchangeable eyepieces with different focal lengths so almost any power can be obtained. Brightness: while most consumer binoculars have between 1 and 2 inches aperture, telescopes have 3, 4, 5, 6, 8, 10 or more inches aperture, so there’s ample exit
pupil for bright images at higher powers. Resolution: higher power, up to a point, shows greater detail. Detail is limited by the quality of manufacture, aperture, and turbulence in our atmosphere. In practice, about 50 power per inch of aperture is the upper practical limit.
For telescopes larger than 4 inches, the atmosphere generally limits detail, so powers above 200 or 300 largely magnify the turbulent fuzzy images. So why have large aperture telescopes? Because the “light bucket” capability shows stars vastly brighter and more numerous. Serious amateur astronomers and astrophotographers are usually aperture hungry to see faint galaxies, gas clouds and star clusters. For the less dedicated or beginning crowd, a small scope, up to about 5 inches, will show the rings of Saturn, belts of Jupiter, lunar craters and many “double stars” extremely well.
Unfortunately, many small telescopes are mere toys, but are promoted as high power instruments. A 250 power telescope with a 2″ (50mm) diameter lens, on a flimsy mount is guaranteed to frustrate the beginner.
Generally, the lowest power is most often used. Since the magnification is given by the objective focal length divided by the eyepiece focal length, the longest focal length eyepiece possible will give the lowest power, widest field, largest exit pupil and brightest image for use as a spotting scope. The lowest useful power is about 3 or 4 per inch of telescope aperture.
In general, telescopes with fast (f/4 to f/6) objectives require more sophisticated eyepieces for sharp imaging, particularly at the edge of the field. Modern multi-coated eyepieces come in a wide range of focal lengths. The best eyepieces have at least 4 elements. Plossl types are excellent for apparent fields up to 50 degrees, while more
complex types have fields of 65 degrees, or even more than 80 degrees apparent field. To compare apparent fields, simply hold 2 eyepieces close to your eye so you can see the circular edge of the field, called appropriately, the field stop. Position the eyepiece so the circles overlap, as in binoculars. Whichever appears larger, is actually larger. The widest apparent fields offer eye-opening ” space walk” viewing.
There are 3 standard eyepiece diameters common today; .96″ O.D., 1.25″ O.D. and 2″ O.D. The .96″ is most used in small imported refracting telescopes. There’s much more variety and quality in the increasingly universally accepted 1.25″ standard size. Many larger or more advanced instruments accept 2″ O.D. eyepieces for the very largest field possible. Of course, smaller eyepiece sizes can be adapted to larger, but not the other way round, except for 1 1/4″ to .96″.
With a long tube and objective lens up front, the refractor is and” looks” like a traditional telescope. It generally never needs alignment and usually has fine resolution. Typical amateur sizes are 2.4″, 3″ and 4″ aperture. Below 2.4″ you’re better off with binoculars or a spotting scope. Because refractors usually are designed for f/12 – f/15, they can get rather large and expensive for the amount of light gathered.
I strongly recommend having one low power eyepiece with the widest apparent field in a focal length of 20mm or longer to avoid frustration when using refractors. An equatorial type mount is highly recommended, especially for 3″ or larger aperture scopes used for astronomy. New refractor designs from f/5 to f/8 using 3 or more elements are more portable, have wide fields and are simple to use.
Often called” Newtonians” after the inventor, Isaac Newton, these instruments use a concave mirror at the bottom of the tube to focus the light cone. A small flat mirror at the top reflects the light to the side where the image is viewed with the eyepiece. The 200 inch mirror telescope at Mt. Palomar is a Newtonian reflector.
A 6″ Newtonian compares in size and cost to a 3″ refractor, while an 8″ compares to a 4 inch. You’re gaining about four times the light gathering power but must contend with alignment adjustments and occasional cleaning of the mirrors in the open tube. f/5 models offer wider fields and are more compact than f/8 models which usually have slightly better image quality.
These modern instruments combine lenses and mirrors to make a very versatile, compact telescope. More expensive than equivalent size reflectors, they are called “Schmidt-Cassegrains”, or” Maksutov” types depending on the nature of the correcting lens.
They’re great for travel, and relatively light weight. Like refractors, their closed tubes keep optics clean and prevent air currents in the tube, which sometimes disturb images in reflectors. While most are in the f/10 to f/15 range, photographic versions are produced as fast as f/5.6. When a catadioptric is made this fast, the secondary mirror must be rather large which results in a large black spot in the middle of the exit pupil.
This causes some resolution problems at high powers and annoying shadows at very low powers. So consider these f/5.6 models primarily telephoto lenses, with moderate effectiveness as a telescope.
4. “RICH FIELD” Telescopes
Most scopes in the f/4 to f/6 range can be classified as “Rich Field”, which means their power is low enough, field wide enough and exit pupil large enough to see wide areas of the Milky Way, with literally thousands of stars in view. Of course, this makes the scope ideal for spotting and photography if 35mm adapters are available.
The very best ” Rich Field Telescopes” ( RFTs) should be capable of using 2″ type eyepieces for the widest field.
RFTs are made as refractors, Schmidt-Newtonians, ” Newtonian” reflectors, and” Dobsonians” (John Dobson, a west coast amateur pioneered a style of Newtonian on a simple wooden altitude/azimuth mount-like a gun turret). Most RFTs are not quite as good for high power, high resolution planet viewing as f/8 or longer telescopes.