Actual Field of View
By Paul Markov
Knowing the actual field of view for all your eyepieces can be very helpful if you rely on star hopping for finding deep sky objects. It is also very important if you are trying to determine the pointing accuracy of your computerized GOTO telescope.
Two fundamental eyepiece specifications are focal length in millimeters and apparent field of view in degrees. Eyepiece manufacturers do not state the actual field of view because it changes from telescope to telescope (since it is a function of a telescope’s focal length). The commonly used mathematical formula for determining the actual field of view of an eyepiece in a given telescope is as follows:
Actual field of view (deg) =  Apparent field of view (deg) 
Scope Focal Length (mm) / Eyepiece Focal Length (mm)  
OR =  Apparent field of view (deg) 
Magnification 
For example, if a telescope has a focal length of 2000 mm and the eyepiece is a 20 mm with a 50 degree apparent field of view, then the actual field of view is expected to be 0.5 degrees. I have always been told that this formula will only give an approximation of the actual field of view, but until now I never bothered to find why it is not very accurate.
Recently I decided to use a much more precise method for determining the actual field of view of my eyepieces – the drift method. It entails pointing the telescope at a star near the intersection of the celestial equator and the meridian (if a telescope is equatorially mounted then the star just has to be close to the celestial equator) and timing with a stopwatch how long it takes for the star to drift across the field of view of each eyepiece (from edge to edge). The mathematical formula for calculating the actual field of view using the drift method is:
Actual field of view (deg) = time in seconds / 240 seconds
For example, if the timed crossing of the field of view is 120 seconds, then the actual field of view is 0.5 degrees. I performed the drift method for each of my eyepieces and found a relevant discrepancy when compared to the "calculated" actual field of view. The "timed" actual field of view was 9% to 20% less than the "calculated" actual field of view! I was puzzled by this finding and wanted to get to the bottom of it, so I searched the Internet and asked a few questions on a couple of email lists and found that this delta can be attributed in part to something called "pincushion distortion". Optics is not my area of expertise so I will not attempt to explain pincushion distortion here, however I found the following definition for this undesired optical effect: "pincushion distortion is an aberration of optical systems in which magnification increases with distance away from the optical axis". Therefore, if magnification increases offaxis this implies that the field of view must decrease accordingly, which explains why my "timed" actual fields of view are less than the "calculated" actual field of view. The first formula noted above does not account for pincushion distortion, therefore you can expect the "calculated" actual field of view to be larger than the "timed" actual field of view. I suspect there is a formula for compensating for pincushion distortion, but it probably contains too many variables and unknowns for an amateur astronomer to tackle.
There are two other reasons why the "calculated" actual field of view may not be very accurate. Although all eyepieces are marked with a specific focal length, most are not exactly as marked. A 26 mm eyepiece could be off by a few millimeters. The same applies to the focal length of a telescope; the specification may be 1600 mm for given scope, but it could vary by several millimeters. With a moving mirrorfocuser telescope (such as a SchmidtCassegrain) the actual focal length of the scope is rather difficult to determine because it changes slightly as the main mirror moves during focusing and also because the use of a diagonal will "push back" the focal plane, thus increasing the telescope’s focal length. In fact, someone noted that the addition of a diagonal in a SchmidtCassegrain increases its focal length so much so to causes approximately a 10% increase in magnification when compared to the same telescope without a diagonal. Since both the eyepiece and telescope focal lengths are crucial in the first formula noted above, if either is inaccurate, the "calculated" actual field of view will not be correct.
The table shows the "calculated" versus "timed" actual field of view for my eyepieces. My telescope’s focal length is 2500 mm. Note that my telescope is a SchmidtCassegrain and I did use a diagonal for the measurements below, thus the "% delta" is most likely a cumulative effect of pincushion distortion and increased focal length due to the diagonal. In the future I will try the "drift method" without the diagonal and determine how much of the decease in actual field of view is attributable to pincushion distortion.
Focal Length (mm) 
Apparent FOV (deg) 
Calculated FOV (deg) 
Observed FOV (deg) 
% delta 






Meade Super Wide Angle  40 
67 
1.07 
0.925 
13.7% 
University Optics Konig  32 
50 
0.64 
0.512 
19.9% 
Meade Super Plossl  26 
52 
0.54 
0.454 
16.0% 
TeleVue Plossl  15 
50 
0.30 
0.254 
15.3% 
Meade MA Reticle  12 
40 
0.19 
0.175 
8.9% 
Therefore, if you want to know the actual field of view of your eyepieces with a fair degree of accuracy, your best choice is to use is the drift method as it is independent of focal length variances of your eyepieces and telescope, as well as pincushion distortion.
Copyright (C) 2001 by Paul Markov
Back to Articles Page.
Back to Main Astronomy Page.
Back to Main Page.