# Thread: Field stop and FOV relationship.

1. ## Field stop and FOV relationship.

I havent ran across an answer to this...

I notice different eyepieces all seem to have different field stop diameters. Is the field stop taken into account already in figuring up how large the AFOV and TFOV will be??

I was looking some plossls i have and the field stops are different. You can hold say a 7.5mm plossl and 20mm plossl up to your eyes, the 20mm has a larger field stop and you can see it easily just by looking into the eyepiece at a wall or something. Both these plossls have a 50* AFOV.

My question is... is it just really that simple of taking the AFOV of an ep and dividing it by the magnification to get the TFOV??? and not really pay any attention to the size of the field stop hole? Im just confused how the plossl designs have the same AFOV when you look into them and the longer FL plossl has a larger field stop circle to see though. I know i am misunderstanding something here......
Last edited by brettecantwell; 02-26-2012 at 10:42 PM.

2. Originally Posted by brettecantwell
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My question is... is it just really that simple of taking the AFOV of an ep and dividing it by the magnification to get the TFOV??? and not really pay any attention to the size of the field stop hole? Im just confused how the plossl designs have the same AFOV when you look into them and the longer FL plossl has a larger field stop circle to see though. I know i am misunderstanding something here......
No, it really is not that simple. We should really be using the field stop to calculate the TFOV.

This is especially true on some of the longer focal length Plossls where they will sell you things like a 40mm Plossl with a field stop which is actually slightly smaller than that of the 32mm. Net effect is that the 40mm gives you lower magnification with no increase in TFOV - and an actual decrease in the AFOV.

I think one of the best places for info on eyepiece calculations is: Tele Vue Optics: Calculator

Note that toward the bottom of the page they show the calculation for TFOV and the field stop.

Ignoring the field stop is a mistake we commonly make.

3. ## The Following User Says Thank You to OleCuss For This Useful Post:

JuanM (02-27-2012)

4. Ive noticed that orion sirius plossls the advertised AFOV does go down on their 40mm lense.

5. Originally Posted by brettecantwell
Ive noticed that orion sirius plossls the advertised AFOV does go down on their 40mm lense.
Right. The field stop on their 40mm is actually a tiny bit less than on their 32mm. So with the Sirius Plossl you can think you are going to a wider FOV by going from the 32mm to the 40mm and you are happy to step down to a lower magnification to do that.

Thing is, your TFOV actually decreases in that particular case and you've dropped magnification to boot.

There may be a good one, but I can't think of any good reason to get a 40mm Plossl with a 1.25 inch barrel if you can get a 32mm instead.

6. Don't forget -- it's also a way to make lower cost glass perform better -- by covering up the edges.

Personally I don't worry about such things as most of the major eyepiece brands are of good quality... and perform reasonably well. Much better than the "old" days ---

7. This thread is very interesting...

It seems that we all tend to obviate EP field stop and use AFOV / Magnification as the formula to calculate TFOV. However, following OleCuss advice and looking at Televue's website and other places, TFOV must be calculated using the field stop of the eyepiece.

The accurate formula for TFOV seems to be :
Eyepiece field stop (in mm) / Telescope focal length x 57.3

I was very intrigued by this, as I was mistakenly using the proxy formula based on AFOV and magnification, so I made some calcs to see how different are both figures. TFOV can be upto 13% less when using the correct formula !

So my next question was: how bad can be our calcs if we only rely on manufacturers' quoted AFOV and magnification, instead of using the EP field stop ?

Well, using Televue's Plössl specs (up to the 40mm) as an example, TFOV differences from one method to the other range from -2% to -10%, all in negative, while the Orion's range from -13% to +13% (???), assuming field stops are correctly quoted on their respective websites.

Conclusion: I will never use AFOV and Magnification as TFOV calculation again !!!

8. Originally Posted by OleCuss
No, it really is not that simple. We should really be using the field stop to calculate the TFOV.

This is especially true on some of the longer focal length Plossls where they will sell you things like a 40mm Plossl with a field stop which is actually slightly smaller than that of the 32mm. Net effect is that the 40mm gives you lower magnification with no increase in TFOV - and an actual decrease in the AFOV.

I think one of the best places for info on eyepiece calculations is: Tele Vue Optics: Calculator

Note that toward the bottom of the page they show the calculation for TFOV and the field stop.

Ignoring the field stop is a mistake we commonly make.
On the other hand.....

Just as the perfect ep is designed for a specific scope while all eps fit in all scopes, the "field stop" is often not even built into the ep. As a result the only stop is the aperture stop.

A field stop must be at or near the focus of the ep. Most mid priced eps that I've seen don't have one. The focus is well outside the lens and there is no stop at the focal plane. The result is that the aperture is the only limit to the TFOV. In that case it is good enough to use the simple calculation AFOV/magnification = TFOV.

9. Generally, the AFOV is all we get from the manufacturer. Getting true field stop measurements can be tricky. Also, moving mirror scopes, such as SCTs make knowing magnification and scope FL difficult since the FL changes with the focal plane which is dependent on eyepiece, diagonal type/size, visual back, etc. The best way to measure TFOV is by the star drift test. For me, AFOV/mag is quick and dirty and really close enough.

10. I've no problem with using the AFOV/mag if one remembers that those numbers can be very misleading.

There are many different EP designs and everything has its limitations. But when the field stop is available, pay attention to it. It will sometimes give you warnings that you may be making a mistake.

At this point, if a Plossl vendor/manufacturer is not telling me the field stop I'll probably go elsewhere.

If I'm going with high-end eyepieces from manufacturers I trust, I won't worry so much about the field stop because I am pretty sure there are no games being played.

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