[ivan86]

Hello guys,

I did a little sketch to try to represent the difference between "contrast field" and collimation.
The aim is to show that a pair of binoculars can be collimated and still have the two field of view a little bit different.

The red cross indicate the overlap point of the two images of the two barrels.

We in Italy call "contrast field" the portion of the field of view where the contribution is the sum of the two barrels, or in other words the portion of the field that is seen by both eyes at the same time.

Let me know what you think, is just a sketch with all possible limitations but I think it makes the idea.

click on image for zoom
http://postimage.org/image/bq148go1p/

Sorry for my bad english,

greets
Ivan

[henry link]

Thanks for posting the images, Ivan. I think they convey the idea very well.

We don't seem to have a term for this problem in English. If we did it might be something like "field stop misalignment". I've noticed it in many binoculars. We seem to be highly tolerant of a horizontal misalignment of the field stops since we necessarily experience that at close distances from parallax, but a vertical misalignment looks unnatural and is not so well tolerated.

I think these misalignments are caused mostly by either a tilt in the optical axis of one eyepiece relative to the other or a lateral eccentricity, either of the field stop itself or the eyepiece optics relative to the field-stop. The Swarovision design is one that is prone to the latter because the eyepiece optical cylinder is mounted eccentrically for collimation purposes, but the field-stop is stationary and centered.

Another related problem I notice is a difference in apparent size between the right and left field-stops. I assume that can be caused by a slight difference in focal length between the eyepieces or if the field-stops are installed at slightly different distances from the eyepiece optics.

Henry

Edit: I just noticed the reference to "OK" and "BAD" exit pupils in your images. I think that's a misconception. Exit pupil is really not significantly affected by misaligned eyepiece field stops. The only minor effect would be to introduce a little asymmetry into the vignetting of far off-axis exit pupils near the field edge. The axial exit pupil isn't affected at all.

Also, example 3 appears to show simple miscollimation, but with correctly positioned eyepiece field stops. I don't see why that condition should be labeled "contrast field: BAD".

[looksharp65]

Ivan,

sorry if I have misinterpreted your ideas, but I don't find them entirely correct.
Since the barrels of the binocular "sees" the object from different positions, the images will not be identical unless at infinite distance like star watching.
Same goes for the eyes, that deliver two different images to the brain.
They also share a common, central visual field, but outside of that, the view is monocular, not binocular (= "two-eyed").

Collimation refers to horizontal as well as vertical alignment of the optical axises.
A combination of vertical and horizontal misalignment makes the images appear obliquely separated, like in your 2nd and 4th image. Image #2 is definitely not an example of good collimation! But since there is no double vision (diplopia), you apparently can manage to move your eyeballs enough to obtain visual fusion. This, however, does not necessarily mean that the binocular is collimated, only that the samples of #3 and #4 are even worse!
#3 is quite interesting since the outer edges of the visual fields seem to coincide but the right and the left image show different cutouts.
I would guess that the objective lenses are collimated but a prism has accidentally been moved. Henry would most likely know this better than me.

The human visual system uses the horizontal image shift between the right and the left eye to create a stereoscopic/3D visual sensation. Additionally, the brain receives information from the extraocular muscles that aids the judgement of distance.

At very long distance, the images from the right and the left barrel should coincide like in your first image.
Since the musculi rectus medialis of the eyes are designed for horizontal convergence, a horizontally misaligned image pair like in your 3rd image can be reasonably easy to deal with for the eyes. Depending on what distance you look through the binoculars at, a horizontal misalignment can even be helpful for your eyes since it may allow your eyeballs to maintain a restful position. Ideally, perfect collimation is desirable of course!

In general, ordinary porros (porri? ) require more eyeball convergence than roof prism binoculars at short or medium distance. An object quite close to the observer will appear to the left of the center of the image in the right barrel and conversely.
This will also mean that the part of the image that is seen with both eyes decrease.
Less so with a roof prism, and least with a reverse-porro.

I'll try to find some of my previous posts and link to them here.

Regards,

//L

[looksharp65]

http://www.birdforum.net/showthread.php?p=2248769

http://www.birdforum.net/showthread.php?p=2332624

http://www.birdforum.net/showthread.php?t=181313

http://www.birdforum.net/showthread.php?t=173492&page=2 (post #26 and subsequent posts)

[henry link]

Lars,

As I understand it image#2 is correctly collimated. The eyes would easily merge objects within the common area of the two fields. The problem with image #2 is vertically "misaligned field stops" causing one field circle to appear higher than the other even though objects within the fields are level with each other. I agree that the horizontal separation of the fields would not be so large at long distance, but I've seen a milder form of this kind of vertical displacement of the fields in several examples of Swarovisions as well as many other binoculars. Collimation is fine, but the field circles are not level.

Henry

[Chhayanat]

Originally Posted by henry link

Lars,

As I understand it image#2 is correctly collimated. The eyes would easily merge objects within the common area of the two fields. The problem with image #2 is vertically "misaligned field stops" causing one field circle to appear higher than the other even though objects within the fields are level with each other. I agree that the horizontal separation of the fields would not be so large at long distance, but I've seen a milder form of this kind of vertical displacement of the fields in several examples of Swarovisions as well as many other binoculars. Collimation is fine, but the field circles are not level.

Henry

Is there a remedy for field circles which are not level or for other similar phenomena where the collimation is within tolerances for the manufacturer? The reason I ask is that many binoculars which are sent back for collimation return without visible improvement for the user even though they are 'within spec' for the manufacturer's workshop. Many so-called 'lemons' have problems which are more apparent to persons with vision problems such as loss of accommodation. After having purchased such lemons -usually because there is no opportunity to look through them - is there a remedy for such unfortunates?
Chhayanat

[looksharp65]

Originally Posted by henry link

Lars,

As I understand it image#2 is correctly collimated. The eyes would easily merge objects within the common area of the two fields. The problem with image #2 is vertically "misaligned field stops" causing one field circle to appear higher than the other even though objects within the fields are level with each other. I agree that the horizontal separation of the fields would not be so large at long distance, but I've seen a milder form of this kind of vertical displacement of the fields in several examples of Swarovisions as well as many other binoculars. Collimation is fine, but the field circles are not level.

Henry

I know you know nearly infinitely more about the construction of binoculars than I do.
What would happen if the "Sun Images Method" was used to determine the collimation of that sample? I'm asking because the eyes can sometimes be more forgiving than actual measures of the binoculars (or, for that matter, spectacles) suggest.

For me, the #2 sample is clearly defective, especially considering the vertical image shift. After all, it is only at infinity that binoculars will have identical images in both barrels, and especially with porros the central, joint, binocular image can be significantly smaller than either of the two fields.
But then, it's only in the horizontal direction the lack of overlap shows.

I guess that the binocular in image #2 has its optical axises (objective - ocular system) misaligned, but this has been corrected by moving the prisms instead of aligning the axises.

//L

Edit: At a second thought I realise that the internal fieldstops are not perfectly centered vs the optical axises.
This means that they cut out their respective FOV asymmetrically to each other.
Collimation is maintained but field edges don't coincide.

[henry link]

Originally Posted by Chhayanat

Is there a remedy for field circles which are not level or for other similar phenomena where the collimation is within tolerances for the manufacturer? The reason I ask is that many binoculars which are sent back for collimation return without visible improvement for the user even though they are 'within spec' for the manufacturer's workshop. Many so-called 'lemons' have problems which are more apparent to persons with vision problems such as loss of accommodation. After having purchased such lemons -usually because there is no opportunity to look through them - is there a remedy for such unfortunates?
Chhayanat

Chhayanat,

I doubt that misalignment of the field stops will cause anything more than a mild sense that one field circle is a little higher or lower than the other. I've never seen it any worse than about 2 degrees of apparent field. That much vertical miscollimation of objects within the fields would make the binoculars unusable.

If you have problems merging images with well collimated binoculars there could be some eye condition involved. Lars (Looksharp65) might have some insights about that.

Lars,

Yes, your second thought got it right.

Henry