Telescope Focal Length: Pros and Cons

By Chuck Hawks and Gordon Landers

Focal length diagram
Focal length of a simple lens. Diagram courtesy of Wikipedia.

Astronomical telescopes are typically described by their clear aperture, focal length and focal ratio. The specifications for focal length and aperture are measurements given in inches or millimeters (or both), while the focal ratio is calculated by dividing the focal length by the aperture. Hence, the specifications for a typical telescope, for example the ubiquitous Celestron C8, are as follows: 8" (203mm) clear aperture, 2032mm focal length, f/10 focal ratio.

Focal length, the specification of concern in this article, is defined as the distance from the optical center of a lens (or optical system) to its point of focus when focused at infinity. (See the diagram of a simple lens at the top of this page.) The optical center of a multi-element lens system, like a camera lens, is defined as the "rear nodal point" and usually does not coincide exactly with the physical center of the lens system. As applied to telescopes, this definition works perfectly for refractors, since they use lenses to focus light. The focal length of reflector telescopes, which use mirrors instead of lenses, is measured from the first active optical element (the corrector in an SCT or the surface of the primary mirror in a Newtonian) to the point of focus.

Since focal length is a linear measurement of the scope's light path, conventional refractors and Newtonian reflectors must be physically at least as long as their focal length. Catadioptric (CAT) telescopes fold the light path back and forth internally and thus are typically much shorter, physically, than their optical focal length. This makes for a relatively short, compact optical tube and gives CAT's a big advantage in portability.

The focal length of any telescope (or other lens system) determines its angular field of view. A longer focal length telescope takes in a narrower field of view and a shorter focal length scope takes in a wider field of view. This is easily demonstrated by taking photos through two telescopes of different focal length from the same position with cameras attached directly to the back of their optical tubes at the point of focus. This is called prime focal length photography and the photo from the shorter focal length scope will show a wider field of view, regardless of the aperture or design of the telescopes. In other words, it is focal length that determines a telescope's field of view, regardless of the diameter of the objective lens or primary mirror.

Viewing through an eyepiece is more complicated, because the eyepiece is also a lens system with a focal length that interacts with the prime focal length of the telescope. The focal length of the telescope divided by the focal length of the eyepiece determines the magnification you see. Given eyepieces designed with the same apparent field of view (AFOV), a shorter focal length eyepiece will give a smaller field of view (and greater magnification) in any given telescope.

Argument in favor of SHORTER (f/5-f/7) focal length telescopes (by Gordon Landers)

For a given clear aperture, a shorter focal length results in a larger photographic f/stop; this means shorter exposure times. For example, a telescope with an 80mm clear aperture and a 480mm focal length is an f/6 optical system; a telescope with an 80mm clear aperture and an 800mm focal length is an f/10 optical system.

Short focal length refractors and Newtonian reflectors are easier to store, transport and set-up than their longer focal length counterparts. Short focal length telescopes benefit from an increased field of view. With the excellent color correction of the APO refractor (or any reflector), one of the advantages of long focal length, minimizing chromatic aberration, is obviated.

An optician I'm not, but it seems to me that the other advantage of long focal length is less chance of coma and other aberrations when using shallower curves in refractor lens cross section (or reflector primary mirrors -Editor), resulting in a sharper image. Using my short focal length Stellarvue SV80ST APO refractor (80mm aperture, 480mm focal length, f/6) for comparison, my eye cannot perceive any problem there. I have seen more such problems in some oculars than in my scope.

It seems to me that the only other concern (regarding short focal length) is magnification limits due to the ratio of focal lengths of objective to ocular. I have another scope of greater aperture and focal length for when I want those advantages, but there are times when I really like the wide field and pinpoint stars shown by my short focal length SV80ST. Examples of that are the views of the Double Cluster and the Pleiades. Obviously, the SV80ST will show less lunar and planetary detail than a larger scope of equal quality, but once again, the APO's sharpness and contrast make for rewarding viewing. At star parties, where I can quickly compare views with club members' Dobs, I am still pleased with the SV80ST. In short, some of the disadvantages of short focal length can be overcome with technology and quality.

Argument in favor of LONGER (f/9-f/11) focal length telescopes (by Chuck Hawks)

Photographic f/stops are irrelevant to visual observers. Clear aperture and magnification determine visual brightness (exit pupil).

It is difficult to manufacture the more radical lens shapes required by short focal length refractors; they also typically require more expensive (ED) glasses. The price of such telescopes must consequently be higher or their optical performance will suffer. In catadioptric telescopes and reflectors, shorter focal length for any given aperture results in a proportionally larger central obstruction, which degrades contrast and overall optical quality.

Longer focal length gives you greater magnification with any given eyepiece. This means more magnification from your shortest focal length eyepiece. For the same magnification, you can use a longer focal length eyepiece, and longer eyepieces are generally better and easier to look through than shorter ones. This is particularly important for astronomers relying mainly on (comparatively affordable) Plossl eyepieces, which use four lens elements in two groups. In the longer focal lengths (about 25mm and up), such eyepieces have good eye relief and good sized ocular lenses. Unfortunately, their eye relief rapidly decreases with focal length, as does the diameter of the eyepiece's ocular lens. With Plossl eyepieces of short focal length, it is as if you are peering through a pinhole. Eyepieces of advanced design using ED glasses and six (or more) elements (Tele Vue Radians and Naglers, for example) can overcome these drawbacks, but cost much more than Plossls.

In my opinion, the ocular is often the weakest link in the viewing system. The shorter the ocular, the more exotic, expensive and (usually) physically larger it must be to function adequately. For example, when A&P Online staff member Jim Fleck (Celestron C8, 2032mm focal length) and I (Stellarvue SV115T, 800mm focal length) were comparing views of Jupiter through our telescopes, I had to use a 7mm eyepiece (114x) to approximate the image size he was getting with an 18mm eyepiece (113x). To me, that gave his scope one heck of a built-in viewing advantage. That is why I prefer the longer focal length that accompanies a medium focal ratio of about f/9-f/11, instead of the f/5-f/7 focal ratios so common today in APO refractors and Dobsonian Newtonians.

Summary - Benefits of short focal length

Shorter focal length allows a physically shorter telescope. Shorter focal length means a wider angular field of view at prime focal length or with any given eyepiece. For a given clear aperture, a shorter focal length results in a larger ("faster") photographic f/stop.

Summary - Benefits of long focal length

A longer focal length eases optical design and manufacturing problems, permitting a telescope with superior optical performance and/or a lower price. Longer focal length allows higher magnification at prime focal length and with any given eyepiece. Allows use of a longer focal length ocular to achieve a given visual image size (magnification).




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