Binocular Evolution I: Field of View (1 Viewer)

[Binocollector]

You can get a good idea of the Fov by shining a phone torch through the eye piece of each and seeing how wide the projected circles of light are on a wall or similar.

I already did that with many binos before -- however not actually directly comparing the different models. Might have to do that.
I am still very impressed with my "skeleton" bino. Wider FoV than the Kowa BDII and for my eyes better panning behaviour. If only the eye-relief were a little better but there is always a price to pay. Until I might one day splurge out the big bucks for a wide angle alpha, the J-B93 will be my "go to" EWA low magnification bino.
The is also a 10x30 version with 7.5° FoV which is also pretty impressive. Might have to get one of those, too.

 

[Troubador]

You can get a good idea of the Fov by shining a phone torch through the eye piece of each and seeing how wide the projected circles of light are on a wall or similar.

Wouldn't you need to shine a light through the objective lens and out of the eypieces, so that it follows the light path used by the light you use for observation?

Lee

 

[William Lewis]

Wouldn't you need to shine a light through the objective lens and out of the eypieces, so that it follows the light path used by the light you use for observation?

Lee

I'm not sure of the logic behind it, only discovered it (for myself- I'm sure many have written about it before!) when messing about owling. My assumption which is bound to be corrected in short order on is that going through the objectives will only give you an idea of the exit pupil and possibly eye relief if the target is moved to the appropriate distance so the edge of the circle of light becomes sharp - but I haven't tried this. I have tried from the eye piece though and it does seem representative.

 

[henry link]

A very careful application of this method can be used to measure the "true" apparent field of any binocular. The circle that forms behind the binocular eyepiece is a projection of the eyepiece fieldstop, not the exit pupil. The projected circle can be set at any distance behind the binocular as long as it's well beyond the eye relief point, which forms the vertex from which the height of the light cone formed behind the binocular is measured. I'll see if I can find a link to the method.

IMO there are easier methods and if all you want to know is how the apparent fields of two binoculars compare just hold one binocular's eyepiece to one eye and the other binocular's eyepiece to the other eye. Level the two field circles, bring them close together and compare their sizes.

If you reverse the method and shine the light through the binocular from the eyepiece end the circle of light that projects from the objective lens is a cylinder with a diameter that corresponds to the effective aperture of the binocular, which is not always the same as the spec.

 

[Binocollector]

Yes, that's exactly what I do to compare. Doesn't always work with bigger binos though.

 

[henry link]

Yes, the eyepieces of the two binoculars need to be brought close enough together to reach your IPD. That's usually possible of you hold the binoculars vertically and play around with their IPDs so that they flatten against each other.

 

[William Lewis]

If you reverse the method and shine the light through the binocular from the eyepiece end the circle of light that projects from the objective lens is a cylinder with a diameter that corresponds to the effective aperture of the binocular, which is not always the same as the spec.

I'm finding puzzling results.

Just tested the method using 2 8x mag binoculars an 8x56 and an 8x32 the '56 has a field of view of 133m @1000 and the '32 has a field of view of 145m @ 1000m. It's important that the small Led is placed at the eye relief distance from the ocular or the results get skewed but the '32 does generate a larger circle of light than the '56. Viewing the same area through the binoculars also seems to show it representing the Fov.

I think I must have misunderstood the appature term - I've always assumed it would relate to objective size and was expecting a larger circle with the '56.

Will

 

[henry link]

Hi Will,

I can't quite follow what you're doing. If you're trying to measure the AFOV the light should be pointing into the front of the binocular. The eye relief point is only used for measuring the distance to the surface on which the light cone coming from eyepiece is projected.

If you're measuring the true aperture of the binocular the light is pointed into the eyepiece end and the circle of light projected onto a flat surface in front of the binocular is the true aperture.

I set up a quick and dirty aperture test of an 8x56 in the photo below. I used the LED flashlight from my i-Phone placed about 15 cm from the eyepiece and placed the objective lens about 10 cm from the white surface. The pieces of tape are about 56mm apart.

Henry

 

[William Lewis]

So to get mildly scientific, I have a degree in psychology so I'm a gardener which says about all you need to know about my academic credentials!

I just set up 2 binoculars with their oculars exactly 5m away from a white wall. One is an 8x56 and the other is an 8x32. I projected the phone torch through the oculars with the light source the right distance away from the ocular lens in respect to their respective eye relief.

I then marked the edges of the projected circles of light ( I say "I" my 5 and 8 year olds like an experiment so they helped, quite accurately with suitable direction actually!). The circle of light from the '56 had a diameter of 65cm (+/-m 1cm) and the '32 circle had a diameter of 70cm.

I then turned the lights back on and looked through 1 optical tube of each bin at the same 5m distance from the wall and the marks on the wall corresponded exactly to the edges of the binoculars fov.


Am I getting something wrong? Shouldn't the '32mm have a smaller circle corresponding to its aperture?

Edit- as mentioned above the '32 has a stated fov of 145m and the '56 133m both at 1000m. The difference as a % measured of fov with the projection matched within a small margin of error the % difference in stated fov from the specs.

Edit 2 (sorry!) I did a smidge of trigonometry this morning over a cup of t and found the angular fov in degrees also closely matches the specs of the 2 binoculars so I'm reasonably confident the methodology works although greater range between binocular and target wall (say 10m) would be beneficial to accuracy. I also tried it using my 8inch reflector with a 6.4mm plossl plugged in, very small light projection diameter.

I think in summary at close range aperture is apparent but at longer ranges angular fov can be ascertained.

 

[GrampaTom]

Hi Will,

I can't quite follow what you're doing. If you're trying to measure the AFOV the light should be pointing into the front of the binocular. The eye relief point is only used for measuring the distance to the surface on which the light cone coming from eyepiece is projected.

If you're measuring the true aperture of the binocular the light is pointed into the eyepiece end and the circle of light projected onto a flat surface in front of the binocular is the true aperture.

I set up a quick and dirty aperture test of an 8x56 in the photo below. I used the LED flashlight from my i-Phone placed about 15 cm from the eyepiece and placed the objective lens about 10 cm from the white surface. The pieces of tape are about 56mm apart.

Henry

Henry, wont the diameter of the light circle, (aperture), on the flat surface, vary as the distance between it and the binocular varies? If I have that right, how does one get true aperture this way?

 

[tenex]

Controlling another aberration may have unwanted effect on distortion, and it may not be uniform.

I don't think Henry was suggesting that complex distortion was a design goal, more a design limitation to satisfy other goals.

What other aberration, if not field curvature? A compromise to satisfy what other goals? Is it just some sort of coincidence that recent flat-field binoculars (EL, NL, SF) all have complex distortion profiles? (These questions are addressed to Henry as well.)

 

[kimmik]

What other aberration, if not field curvature? A compromise to satisfy what other goals? Is it just some sort of coincidence that recent flat-field binoculars (EL, NL, SF) all have complex distortion profiles? (These questions are addressed to Henry as well.)

In a system with 12 optical elements, 10 of which produce CA, coma, petzval curvature, astigmatism, distortion, spherical aberration, it isn’t surprising that residual aberration of anything exists.

Not having zemax and the full optical formula of any model in particular, I am presumably like Lee unable to give you a deeper root cause. Possibly, optimising for a uniform distortion profile in the swaro design will increase field curvature and lateral CA.

In my EL, just like distortion, the lateral CA is also nonuniform, decreasing at the edge. The overall CA and distortion is low though and that is to some preferences more important.

 

[PeterPS]

What other aberration, if not field curvature? A compromise to satisfy what other goals? Is it just some sort of coincidence that recent flat-field binoculars (EL, NL, SF) allhave complex distortion profiles? (These questions are addressed to Henry as well.)

kimmik said:
Not having zemax and the full optical formula of any model in particular, I am presumably like Lee unable to give you a deeper root cause. Possibly, a uniform distortion profile in the swaro design will increase field curvature and lateral CA.

In my EL, just like distortion, the lateral CA is also nonuniform, decreasing at the edge. The overall CA and distortion is low though and that is to some preferences more important.

As I was trying to explain in a post above (#38), pincushion distortion was introduced (by Zeiss, I believe) to prevent the rolling ball effect, and mustache distortion (pincushion followed by barrel distortion) to avoid the rolling bowl effect caused by significant pincushion over a wide FoV. I don't think they have any other design goals besides these. Regarding what they compromise, they affect the AFoV in the sense that AMD reduces the apparent FoV, but I don't think they copromise anything else such field curvature or CA.

 

[tenex]

Possibly, optimising for a uniform distortion profile in the swaro design will increase field curvature and lateral CA.

Is this not just a backhanded way of postulating as I have that nonuniform distortion is a consequence of minimizing field curvature?

and mustache distortion (pincushion followed by barrel distortion) to avoid the rolling bowl effect caused by significant pincushion over a wide FoV

Does it not suffice to simply have a modest amount of pincushioning, i.e. not to overdo it? I've found that to work well for me.

 

[henry link]

Does it not suffice to simply have a modest amount of pincushioning, i.e. not to overdo it? I've found that to work well for me.

This is the approach that Canon uses - just enough pincushion to keep AMD levels to near zero all the way to the field edge combined with excellent corrections for both field curvature and off-axis astigmatism, which are totally unrelated to the distortion choice. Any other manufacturer can do the same if they choose.

 

[Binocollector]

I checked my non-IS Canon 8x32WP, 7.5°, and it has some slight pincushioning. Like Henry wrote, just enough to keep panning behaviour nice and civilized. And it has a very flat field, just like the IS-models.
I also checked a few of my other flat-field models and they all have a small amount of pincushion distortion. Way less than the old super wide models but of course they have smaller FoVs anyway.

 

[lilcrazy2]

I checked my non-IS Canon 8x32WP, 7.5°, and it has some slight pincushioning. Like Henry wrote, just enough to keep panning behaviour nice and civilized. And it has a very flat field, just like the IS-models.
I also checked a few of my other flat-field models and they all have a small amount of pincushion distortion. Way less than the old super wide models but of course they have smaller FoVs anyway.

Sounds pretty similar to my 7x42A WP 7* Canon roofs. Sharp and flat to the edges. Quite a nice older bino.

 

[tenex]

This is the approach that Canon uses - just enough pincushion to keep AMD levels to near zero all the way to the field edge combined with excellent corrections for both field curvature and off-axis astigmatism, which are totally unrelated to the distortion choice. Any other manufacturer can do the same if they choose.

Thanks, but then have manufacturers including Swarovski and Zeiss instead chosen complex/mustache distortion, and if so why? I keep asking because I'd really like to know how it's supposed to be better.

 

[Patudo]

A wide field of view aids rapid scanning ...

I need to note that I do actually agree with this - but I also think that you can scan effectively (capturing a fast-moving target when the binocular is brought to your eye is different) with a binocular that doesn't have a SF or NL-like field of view. I've found that when using binoculars with really large fields of view (Nikon WX), yes the wider FOV covers a larger area but, and I suppose this depends on how obvious the target you are scanning for is, it takes longer for your eye to sweep that wider FOV. Whereas with something more restricted (eg Nikon SE) you quickly take in what's there and move on. You may end up traversing the binoculars more quickly but in essence cover the same area in more or less the same time. A really large field of view (WX) can actually also be distracting at times, in that it can capture other things besides the target you are seeking.

That being said I have never looked through a wide FOV binocular and wanted less of it!

 

[Troubador]

I need to note that I do actually agree with this - but I also think that you can scan effectively (capturing a fast-moving target when the binocular is brought to your eye is different) with a binocular that doesn't have a SF or NL-like field of view. I've found that when using binoculars with really large fields of view (Nikon WX), yes the wider FOV covers a larger area but, and I suppose this depends on how obvious the target you are scanning for is, it takes longer for your eye to sweep that wider FOV. Whereas with something more restricted (eg Nikon SE) you quickly take in what's there and move on. You may end up traversing the binoculars more quickly but in essence cover the same area in more or less the same time. A really large field of view (WX) can actually also be distracting at times, in that it can capture other things besides the target you are seeking.

That being said I have never looked through a wide FOV binocular and wanted less of it!

I use two different binoculars depending on the nature of the coast we are visiting on that day. SF8x32 with 155m fov and Trinovid HD 8x32 with 124m fov. The SF has a fov area 56% bigger and I simply find it easier and quicker to scan an area with the SF than with the Trinovid, which I value highly for its close focus. When I am using the Trinnie's close focus on the coast I naturally also use the Trinnie to scan the sea and coasts and traversing and re-traversing it to ensure I really have covered all the intended area is simply more onerous than flicking my eyes over the image of the wide fov of the SF.

I love both of these binoculars but for sure they have quite different talents.

Lee