Rifle Scopes Technical question scope specification

[Subwrx300]

I'm fairly versed in scope language/specs (have been shooting as main hobby for 15+ years) but I realized I don't know the name for the following term. I'm trying to find the name of the specification that describes how big the image appears to the shooter (NOT MAGNIFICATION/POWER). Not easy to describe though so hang with me while I explain:

When you look through two or three scopes side by side set to the same magnification level, using proper amount of eye relief for scope to have clear image (no shadowing), some scopes look like they have a bigger image.

I'm NOT referring to field of view; I am referring to some scopes feel like looking through a pipe that's 2' with a 2" image and others look like a 6" pipe with a 10" image (overall image feels closer/larger).

Example: looking through a Bushnell at 20x or Sig Tango 6 (34mm tube 56mm objective) at 20x vs a NF ATACR at 20x, the NF seems much larger. FOV is about the same for most of them but the image in the NF is much closer to your nose and easier to resolve.

I would like it looking at your phone from arm's length verus 10" from your eyes. Or like The closer image is easier to see overall.

Can someone tell the which specs affect this? Make @koshkin can chime in? Trying to find scopes that "feel" more like NF/XTR2 image size. Sig and a few others I've oogled feel very different.

Happy to clarify if I can.

 

[koshkin]

Apparent Field of View.

 

[spife7980]

Apparent Field of View.

So how can we arrive at this number? Ive never noted its advertisement. My googling says its a combination of anglular/linear field of view and magnification?
Or is it a bit more direct in its relationship and a larger field of view at a certain magnification provides a larger apparent field of view than a scope with the same field of view at a lower magnification?

 

[Old_Longhair]

This does about as good a job of explaining it as I've seen.

 

[Discogodfather]

From what I gather, it's easier to understand in terms of universal telescope design:

"An eyepiece's apparent field of view is the angular diameter, expressed in degrees (°), of the circle of light that the eye sees. It is analogous to the screen of a television (not the picture seen through it). Eyepiece apparent fields range from narrow (25° - 30°) to extra-wide angle (80° or more). The true field (or real field) of view is the angle of sky seen through the eyepiece when it's attached to the telescope. The true field can be approximated using the formula:

True Field = Apparent Field ÷ Magnification"

I think most of it has to do with the ocular. In telescopes, the eyepiece is usually advertised with an apparent FOV, which is upwards of 70 degrees on the really expensive stuff. Most rifle scopes have very low numbers on apparent FOV comparatively because they cannot get away with any field curvature for obvious reasons. It's interesting, because that new March PRS scope seems to have gotten something in the 20 degree area and it's supposed to be a big deal, hopefully without field curvature.

Most rifle scopes fall in at around 10-15 degrees right?

I have noticed in higher magnification scopes, apparent FOV seems to be a lot less than in a mid or lower mag scope.

When I look through my 82mm APM binocular telescope with a 100 degree AFOV with a Televue Ethos eyepieces it doesn't feel like the image is in front of you, it feels like you are inside the image. It's breathtaking to look through an optical instrument unconstrained by weight, size, and ability to take recoil like a riflescope.

 

[Subwrx300]

Apparent Field of View.

I knew you would have the answer! So to mirror @spife7980 question, is there an easy way to quantify the apparent field of view from stats/specs given by scope manufacturers?

Would be awesome if they could be easily calculated for reference sheet to use here when comparing scopes.

 

[koshkin]

Numerical definition of the Apparent FOV (AFOV) is correctly stated a couple of posts above: magnification * real FOV (in degrees or radians; you have to go with angular units for this). The term is commonly used with binoculars and spotting scopes, but not so much with riflescopes. There are a couple of reasons for that .

Weirdly I had this same discussion with Burris' engineering manager back when I visited in December. They went through great pains to make sure that the AFOV of XTR3 scopes stays consistent through the entire magnification range.

Because a riflescope has a couple of focal planes inside and because of variable magnification, depending on what magnification it is set at, there might be a couple of different field stops in there that come into play at different magnifications.

Depending on which field stop is in use, the black ring around the image will appear to be of different thickness and that messes with how you perceive the image. A thicker ring will make a large apparent FOV look smaller and vice versa.

Then, there is tunneling to consider which messes with you further.

For binoculars and spotting scopes that do not have to deal with recoil, apparent FOV is larger simply because your eye is so much closer to the eyepiece. The only variable riflescope made with the same idea in mind is MTC's Viper Connect that I use on my airgun.

ILya

 

[koshkin]

This does about as good a job of explaining it as I've seen.

View attachment 7047315

Conceptually, a riflescope is closer to a Keplerian telescope, than a Galilean one.

I'll see if I can find a simpler illustration. If not, I'll make one. I have a couple of videos that talk about FOV up on Youtube where I sketch this out on a white board, but I should probably make a proper illustration.

ILya

 

[Old_Longhair]

Conceptually, a riflescope is closer to a Keplerian telescope, than a Galilean one.

I'll see if I can find a simpler illustration. If not, I'll make one. I have a couple of videos that talk about FOV up on Youtube where I sketch this out on a white board, but I should probably make a proper illustration.

ILya

 

[spife7980]

So am I doing this right?

How would I go about and justifying these to one another? 20 afov at 21x is a big difference than 23 afox at 25x

Is it as simple as just scaling that percentage wise to get everything to the same spot?
So a atcr with 23.42 afov @ 25x would be 23.4286*(25/20)=29.285 afov at 20x? Seems weird that the atacr would provide an apparant fov of 29 at 20x where as the bushnell would only be getting an afov of 21@ 20x
But I guess the atacr really does seem that much larger like subwrx said in his original post.

 

[Discogodfather]

Yeah, that is very cool, a refactor telescope is a little bit easier to understand because there is no extra mechanical system to adjust a reticle, so you can see the basic optical design. In astronomy, most people don't care if the image is upside down or switched left to right so no real image erection is necessary, although many get what's called a diagonal to flip right to left (usually a mirror but can be a prism).
It would be interesting to rate scopes based on this AFOV phenomenon. I'd say of the scopes I have looked through Hensoldt/Zeiss and Kahles have some impressive engineering to make the image appear larger. I always thought my March scopes were on the worse side- kind of small and distant, but with great optical quality in other ways.

 

[Subwrx300]

So am I doing this right?

View attachment 7047381



How would I go about and justifying these to one another? 20 afov at 21x is a big difference than 23 afox at 25x

Is it as simple as just scaling that percentage wise to get everything to the same spot?
So a atcr with 23.42 afov @ 25x would be 23.4286*(25/20)=29.285 afov at 20x? Seems weird that the atacr would provide an apparant fov of 29 at 20x where as the bushnell would only be getting an afov of 21@ 20x
But I guess the atacr really does seem that much larger like subwrx said in his original post.

View attachment 7047389

Spot on. My two easy reference points are the Sig Tango and NF ATACR, both of which I own. The Sig is significantly smaller field of view.

Here are photos taken with 15x on three scopes.

7-35x 56mm ATACR has 47MOA visible from center to bottom of reticle.

Sig Tango 4-20 x 50mm has 40MOA visible below center

Burris XTR 2 3-15 x50mm (same objective size as Sig) has 42MOA visible below center.

While this is not exactly what I'm referring to (I believe this would be straight FOV), if you look carefully, you can see the size of the image on the Burris and Sig are much more closely aligned. All images were shot in middle of eyebox (no shadowing) as if you looked through it.

Even with the same "size image" between Sig and Burris, the Burris has slightly more visible (4 moa total - 2moa in each direction from center)

The NF does have a larger Objective; is this part of the reason for bigger image but the Sig vs Burris shows even for same size image, the XTR has bigger actual standard FOV.

This is what I'm trying to capture; larger apparent field of view/image size AND larger actual field view.

 

[SageRatSafaris]

Love the pics and science, but I laughed out loud when I saw your creative use of Excel!

 

[Subwrx300]

Love the pics and science, but I laughed out loud when I saw your creative use of Excel!

Easy to zoom in and out with lines and "unlimited" canvas size.

And it was already open.. lol

 

[koshkin]

To answer a question a couple of potsts above: yes, for the same eye relief, a large eyepiece will usually (not necessarily) make for a larger FOV and AFOV.

ILya

 

[Quarter Horse]

Subwrx300, I fully understand what causes you to ask the question. I bought my first Alpha scope some months ago. I spent hours comparing it to the SS 5-20x50HD of which I have three. I appreciate all the work posters put into evaluations of scopes comparing various details but my evaluation was simplistic. The PMII was simply easier to use. One of the most striking differences is what you are referring to.

Apparent field of view seems to be the proper term. Actual FOV is easy to measure. AFOV is the impression you mind has about what it is viewing through the scope. In the case of the SS it is an image in the case of the PMII it is more panoramic. Again simplistic but the PMII is simply easier to use.

 

[Discogodfather]

Another optical issue to consider is exit pupil. If the size of the exit pupil is larger the image will appear to be much easier to see through the scope relative to the center optical axis. Commonly incorrectly referred to by people reviewing the scope as "eyebox", which is a marketing term. A good "eyebox" is actually the cumulative effect of good FOV, good AFOV, and large exit pupil. The image appears bigger and more immersive as opposed to further away and staring down a tunnel.

Exit pupil is largely a mathematical relationship to the objective size, so a smaller objective will always have a smaller exit pupil (and vice versa) in my understanding. For instance, all 56mm scopes usually have around a 2mm exit pupil at 25x regardless of manufacturer or price level. It's simply objective size divided by the magnification.