# Thread: Equation for AFOV of 1.25 Eyepiece

1. ## Equation for AFOV of 1.25 Eyepiece

This question is focused only on eyepieces in the 1.25 format. According to what I have read on this forum, Every 1.25 barrel eyepiece has a Max AFOV no matter what the manufacture advertises it at. I recently purchased a 40mm plossl eyepiece that claims to have a 52 AFOV. But from what I have read, the claimed "52 AFOV" is impossible with a 40mm eyepiece in the 1.25 barrel format. People say that a 32mm eyepiece CAN have a 52 AFOV. I have searched for the equation for the 1.25 barrel Max AFOV but cant find it. I dont want to get ripped off anymore. Can some one please list this equation so that in the future Ill be able to spot false advertisement when it comes to eyepieces. Again this is focused on the eyepiece and not the telescope. Thanks in advance.

2. ## Re: Equation for AFOV of 1.25 Eyepiece

It is not the apparent FOV per say but the size of the field stop of the eyepiece.
32mm Plossl usually have a field stop of 27mm which it the maximum you can have in the 1.25" size.
A 40mm Plossl at 50° AFOV has a field stop of 35mm

3. ## Re: Equation for AFOV of 1.25 Eyepiece

Their 40mm EP could have an AFOV of 52 degrees. The AFOV is made by the manufacturer and is not subject to any equation. BUT their 52 degree 40mm EP would be limited by the 1.25" barrel and would have the same TFOV as a 32mm 52 degree EP which fills the 1.25" barrel. Most likely the AFOV of the 40mm EP is 44 degrees.

4. ## Re: Equation for AFOV of 1.25 Eyepiece

The easy equation for AFOV is: 57.3 * <field stop diameter> / <eyepiece focal length>. This formula is an approximation that works well for small angles, less then 5 degrees. As such it works well for calculating a scope's max TFOV (replacing eyepiece focal length with objective focal length), but breaks down at the large angles found in an eyepiece.

The more accurate formula is: 57.3 * 2 * atan(<field stop diameter> / (2 * <eyepiece focal length>)) This formula assumes that your ATAN function returns a value in radians, as most do. If you have an ATAN function that returns degrees, omit the 57.3 from the formula.

5. ## The Following User Says Thank You to KathyNS For This Useful Post:

Brodavid1953 (08-28-2017)

6. ## Re: Equation for AFOV of 1.25 Eyepiece

The field stop is not at the end of the 1.25" mounting barrel.

If the mounting is extended around 15mm beyond the field stop location, how would this affect the AFOV?

7. ## Re: Equation for AFOV of 1.25 Eyepiece

Originally Posted by watson1
The field stop is not at the end of the 1.25" mounting barrel.

If the mounting is extended around 15mm beyond the field stop location, how would this affect the AFOV?
It doesn't. The image is located at the focal plane (the focal plane of the scope's objective and the focal plane of the eyepiece coincide when the eyepiece is in focus), and that is where the field stop needs to be. Moving the field stop to a different location will give a fuzzy, vignetted edge to the field of view. Only the width of the field stop matters, since its location is fixed.

The length of the tube has no bearing on the FOV.

8. ## Re: Equation for AFOV of 1.25 Eyepiece

As has been pointed out, the apparent field of view (AFOV) for an eyepiece is determined by its design. There is no reason why a 40mm focal length eyepiece couldn't have a 52 degree apparent field of view, except perhaps for design practicality. Notice that for the Orion Sirius Plossl series, the focal lengths range from 6.3mm to 40mm and the stated AFOV for all eyepieces except the 40mm is 52 degrees. The specs for the 40mm eyepiece give an AFOV of 43 degrees. Notice that for the Explore Scientific line of 82 degree AFOV eyepieces, the focal lengths range from 4.7mm to 30mm. For Plossls the field stop lengths are often shorter than the eyepiece focal lengths. For most wider field eyepieces, the field stop diameter is larger than the focal length. If you do some calculations for AFOV based on the formulas given above, you will see some variability between the calculated and stated AFOV. I assume the stated AFOV would be more accurate since the eyepieces were designed to meet the stated specifications. All of this applies equally to a 2 inch barrel format as well. Of course, the true field of view (TFOV) takes into account the magnification factor of the eyepiece-telescope combination and is smaller than the AFOV. I hope this helps.
Last edited by stargazer55; 08-28-2017 at 05:40 PM.

9. ## Re: Equation for AFOV of 1.25 Eyepiece

Originally Posted by soulreaper
This question is focused only on eyepieces in the 1.25 format. According to what I have read on this forum, Every 1.25 barrel eyepiece has a Max AFOV no matter what the manufacture advertises it at. I recently purchased a 40mm plossl eyepiece that claims to have a 52 AFOV. But from what I have read, the claimed "52 AFOV" is impossible with a 40mm eyepiece in the 1.25 barrel format. People say that a 32mm eyepiece CAN have a 52 AFOV.
To your point, yes you would need to go to a 2 inch barrel size, which would allow a larger field stop, to have a 52 degree AFOV with a 40mm focal length. But check the specs for a specific eyepiece rather than the stated AFOV for the whole series. The AFOV for one or two eyepieces may differ.

10. ## Re: Equation for AFOV of 1.25 Eyepiece

Thanks guys for all the answers. You guys are the best.

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