Field of View
Any given eyepiece has a designed attribute known as its apparent field of view. The apparent field of view is how "wide" the view appears to your eye as you peer into the eyepiece. In contrast, a given eyepiece and telescope combination will also have a true field of view, which describes how much of the night sky is visible at once. The two kinds of field of view are loosely related, but are different things.
Apparent Field of View
An eyepiece with an apparent field of view of 50 degrees means the angle your eye has to move from one edge to see the other edge, is 50 degrees. That is, the circle of light you see has an apparent angular size of 50 degrees. Meanwhile a 100 degree eyepiece shows you a circle of light that is 100 degrees in angular size.
The wider the apparent field of view is, the more immersive the view is. Looking through a 40 or 50 degree apparent field of view eyepiece might feel like looking through a narrow cardboard tube, while looking through a 100 degree apparent field of view eyepiece gives the impression of looking through a giant window on space ship.
Eyepieces come in all ranges of apparent field of view, from narrow 30 degree monocentric eyepieces, to 120 degree hyper wide eyepieces. The wider the apparent field of view is, the larger, heavier, and more expensive the eyepiece tends to be.
Eyepieces with wide apparent fields of view are typically referred to as "wide angle" eyepieces, but there is no hard definition for what constitutes a wide angle. There are other terms like "extreme wide angle" "ultra wide angle" and "hyper wide angle" and "super wide angle". These are nothing more than marketing terms that have no bearing on the stated apparent field of view.
True Field of View
True field of view is not the same as apparent field of view. The true field of view is the angle of measurement of the sky you are seeing in the eyepiece. For example if you were able to just barely fit all of the Pleiades (Messier 45) into the eyepiece at once, then your eyepiece and telescope combination is giving you a true field of view of about 1.6 degrees, even if the eyepiece was measured at 100 degree apparent field of view.
While apparent field of view is a fixed value of the eyepiece, the true field of view depends on what magnification the eyepiece is operating it, meaning it depends on what the focal length of the telescope it is used in is.
Using the above example, you can surmise that if a 100 degree apparent field of view eyepiece is showing you a region of the sky 1.6 degrees across, the rough magnification would have to be 100 / 1.6 = 62.5x. Thus there is a rough relationship between an eyepiece's true field of view and its apparent field of view. If you had a 50 degree eyepiece that was also providing 62.5x magnification, it would only have a 0.8 degree true field of view.
Calculating True Field of View
Calculating true field of view can be done in one of two ways: one being more accurate than another.
Apparent Field of View and Magnification Method
As hinted above, to roughly calculate true field of view, you can divide the apparent field of view, by magnification:
eyepiece apparent field of view / magnification
- 100 degrees / 35x = 2.87 degrees
- 50 degrees / 35x = 1.43 degrees
- 68 degrees / 200x = 0.34 degrees
It should be noted that this method is only approximate. The more accurate method is below:
Using Field Stop
Most eyepieces have a field stop or an effective field stop (depending on their design). If the manufacturer has supplied this field stop value, you can use it to more accurately compute the true field of view using the following formula:
eyepiece field stop diameter in mm / telescope focal length in mm x 57.3
- 42mm / 1200mm * 57.3 = 2 degrees
- 6mm / 1800mm * 57.3 = 0.19 degrees
Unfortunately, not all manufacturers supply field stop information. In some cases, an eyepiece has a Smyth field lens (essentially a built-in barlow) that renders the given field stop value inaccurate. High end eyepiece manufacturers like Tele Vue or Explore Scientific will provide accurate field stop information for all of their eyepieces, regardless of design.
Apparent Field of View Considerations
AFOV and Barrel Size
Most modern eyepieces come in one of three standard barrel sizes: 1.25", 2", and 3". The reason for this it accommodate different apparent fields of view at longer focal lengths. Since the apparent field of view is really governed by the field stop of the eyepiece, it means there are limits to how much apparent field of view you can get out of a barrel of a certain size.
For example, a 40mm Plossl eyepiece in a 1.25" barrel will typically have a maximum apparent field of view of just 42 degrees. A 24mm eyepiece in a 1.25" barrel can have a maximum apparent field of view of 68 degrees. A 16mm 1.25" eyepiece can have an apparent field of view of 82 degrees, and a 13mm 1.25" eyepiece can have an apparent field of view of 100 degrees. Thus the longer the focal length of a 1.25" eyepiece, the narrower the apparent field of view must be, just due to the inherent geometry of light passing through the eyepiece.
If you wanted lower magnification and wide apparent fields of view, a 2" barrel is required. For instance, a 40mm eyepiece in a 2" barrel can have an apparent field of view of up to about 70 degrees. A 30mm 2" eyepiece can have an 82 degree apparent field, and a 25mm 2" eyepiece can have an apparent field of view of 100 degrees. A 3" barrel would allow a 30mm eyepiece to have a 100 degree apparent field of view.
AFOV and Abberations
Very wide apparent fields of view can have downsides - they can manifest aberrations near the edges of the field of view more egregiously than eyepieces with narrower apparent fields of view.
The first aberration to consider is coma. Coma is an inherent aberration present in Newtonian reflectors with parabolic mirrors. While the eyepiece itself does not cause coma, it can reveal coma. The wider the apparent field of view is, the worse the coma will be revealed. This relationship is linear. A 100 degree AFOV eyepiece will show 2x larger coma at the edges than a 50 degree AFOV eyepiece will. This mean that if you have a short focal ratio Newtonian reflector (which has strong coma), you will notice it more easily in wide angle eyepieces than eyepieces with narrower fields of view.
Astigmatism in an eyepiece is not the same as astigmatism in your vision. Telescopes with short focal ratios (regardless of their design), send light into the eyepiece at steeper angles than telescopes with longer focal ratios. A wide angle eyepiece then needs to accept these steep light rays and bend them to form the wide apparent field of view. The wider the field of view, and the steeper the entrance angle of light from short focal ratio telescopes, the more strongly this light has to be bent. This can cause astigmatism, where stars appear cross-like or streak-like the closer you get towards the edges of the field of view.
To correct for this astigmatism requires more glass elements and complex shapes, which increases the cost of the eyepiece. This is why some wide angle eyepieces are significantly cheaper than others. While they are able to form a wide apparent field of view, they don't always do so cleanly, and stars near the edges look badly distorted. More expensive wide angle eyepieces are able to keep stars looking like points all the way to the edges, even in telescopes with short focal ratios (assuming a coma corrector is used where appropriate).