DOF in binoculars. (1 Viewer)

Please forgive my naive question about the depth of field of binoculars, but I am confused about this.
I am interested in some 10x bins and have been researching possible models, nothing alpha, just looking for a good buy.
One downside of using 10x bins in comparison to 8x bins is that the DOF is significantly less. I had the idea that one , say, 10x42 will have the same DOF as any other 10x42.
I read that the DOF can be improved by manufacturers and that in some way the DOF can be a function of the design of the optics.
This is confusing, in the world of camera lenses, isn't the DOF a function of the focal length and the aperture, which cannot be changed by other design features? Does the DOf of bins include other design parameters that can have an effect.

Dave

Oh boy...

I guess some people see what could be called "perceived DOF" that differ from the actual DOF, which as you say should be a function of the magnification.

Some say that the speed of focusing is involved, I think field curvature has a lot to do with it, etc.

How close was that?

VOP has addressed the issues that confuse people on this issue.

In reality all 10X binoculars have the same DOF. I own enough of them to have come to that conclusion. You can go nuts comparing them. To do the comparisons right you have to first put the object you are focusing on in the exact center (depth wise) of the field of focus. That ain't easy!

Bob

Dave,
A binocular is fundamentally different from a camera, in that it does not produce a focused image. It produces an "afocal" output beam, just like the light coming from an object with no intervening optic, only magnified. So the camera style concept of DOF goes out the window, and only magnification remains, to influence visual DOF.

Higher magnifications have smaller depth of field for a simple reason. They show fine details better than low powers, so it is easier to tell if those details are out of focus.

Findings of different DOF among binoculars of the same magnification are not unusual however. Nobody knows what other factors, besides the simple optical viewpoint I've taken above, might cause this. Possible causes have has been speculated upon here, without any real conclusions. It is a highly subjective finding, and when reported, should be taken with a grain of salt. Not like "they're wrong", only that you may very well not see the same thing.
Ron

Just to expand a little on Ron's post, there are two issues that are easily confused: (1) depth of field, and (2) depth perception. For the most part, people are talking about depth perception, or the perception of spatial depth.

Binoculars have no depth of field because they are "afocal" instruments, i.e., they have no finite focal point. Otherwise put, they focus at infinity. Thinking of the human eye as a camera, binoculars modify the depth of field of the observer's eyes by a factor or 1/M^2. So the higher the magnification the smaller the depth of field.

Depth perception is based on monocular perspective cues and binocular stereopsis. Monocular perspective cues improve depth perception as field of view increases, and decreases it to "tunnel vision" as field of view decreases. Stereopsis is enhanced as the distance between the objectives increases, and decreased as they are brought together.

So, to answer the original question, two binoculars of the same power have the same depth of field. Those same binoculars, however, may produce different depth perceptions due to differences in field of view or differences in separation of the objectives. These factors can work together to enhance or inhibit depth perception, or they can work against each other to produce a mixed result.

Ed

The effective exit pupil is another factor

elkcub said:

Just to expand a little on Ron's post, there are two issues that are easily confused: (1) depth of field, and (2) depth perception. For the most part, people are talking about depth perception, or the perception of spatial depth.

Binoculars have no depth of field because they are "afocal" instruments, i.e., they have no finite focal point. Otherwise put, they focus at infinity. Thinking of the human eye as a camera, binoculars modify the depth of field of the observer's eyes by a factor or 1/M^2. So the higher the magnification the smaller the depth of field.

Depth perception is based on monocular perspective cues and binocular stereopsis. Monocular perspective cues improve depth perception as field of view increases, and decreases it to "tunnel vision" as field of view decreases. Stereopsis is enhanced as the distance between the objectives increases, and decreased as they are brought together.

So, to answer the original question, two binoculars of the same power have the same depth of field. Those same binoculars, however, may produce different depth perceptions due to differences in field of view or differences in separation of the objectives. These factors can work together to enhance or inhibit depth perception, or they can work against each other to produce a mixed result.

Ed

Click to expand...

It is true, binoculars modify the depth of field of the observer's eyes by a factor or 1/M^2. For this reason, the size of the effective exit pupil is the second factor which influences the dof - "effective size" meaning the smaller of both, exit pupil and eye pupil. So it may well be that a 8x42 appears to have less dof in low light than a 8x32, if the latter stops down the effective aperture.

There is no way to 'optimize' dof through any smart optical design - it is magnification and effective exit pupil, nothing else that counts. Of course, a strong field curvature may under certain conditions mimic an increased dof, but that is not what we want

Cheers,
Holger

Since we are on this point of discussion I want to relate a recent experience that is applicable to this issue.

I have two 7x35 "wide angle" binoculars to use as an example that I am hoping some of you gents can help me with (particularly you Ed since you mentioned some components that are appropriate).

One model, the Nikon 7x35 Wide field, has a listed field of view of 9.3 degrees. The other, pick any one of several, but for now lets say a Binolux 7x35 with a listed field of view of 11 degrees. For my eyes the Nikon produces better depth perception. In other words by focusing on a object at a given distance my eyes adjust very easily to objects a considerable distance in front of and behind it. The Binolux, though still very good in this regard because of the 7x35 porro design, does not allow my eyes the flexibility to focus on objects quite as readily as the Nikon. In other words I have better depth perception with the Nikon.

Is this the result of the objective spacing of each design? I have not measured both to see if the Nikon's objectives are spaced farther apart than the Binolux's but would this be a major contributing factor to what I am experiencing? Is there something else about the optical design of each instrument that would be contributing to my experience?

Bob and Holger,
I believe that magnification appearing as a squared quantity results from the fact that longitudinal magnification is the square of the lateral magnification. Is that where ya'll are getting this expression?

I agree that that seems right, theoretically. But it is hard to even show that binoculars of the same magnification have the same DOF, observationally! I don't know of any observational confirmations of that squared result. If you know of any such tests with actual human vision, I'd like to know about them. It would just be kind of cool, as usual, to verify theory.
Ron

Thanks for the replies and explanation, perhaps technically a little over my head.

I understand that binoculars do not provide a focus, this function being carried out by the eye. The exit pupil is a direct function of the aperture and magnification.
Am I correct in thinking that a sort of regulation of the light entering the eye is performed by the eye itself, so a bright scene (exit pupil) may be further stopped down be the iris, effectively reducing the aperture and so increasing the DOF? If so the DOF is a part function of an individual eye as much as the bins?

Dave

Dave,
Exactly right. The eye is the camera, and anything that effectively stops it down, whether the binocular's exit pupil, or its own iris tightening in bright light, increases the visual depth of field.

These pupil effects can happen with any magnification, and don't affect the underlying variation of DOF with magnification. But they could be one of the contrubutors to the perceived differences in DOF with binoculars of the same magnification.

I said "DOF varies with magnification for a simple reason". I didn't say there weren't lots of complicated reasons too!
Ron

My idea, although difficult to practice, would be to make a three-dimensional object with the shape of a bullet or a dome.
The (hyperboloid?) shape has to be very carefully calculated and manufactured to match a certain viewing distance. It must be painted with black rings that become progressively wider the further away from the front they come.
This way, they will match the dioptric influence on DOF and will look equally wide once the central axis of the shape is aligned with the optical axis of the binocular.

The test has to be performed monocularly, in standardized lighting conditions and preferably with an observer who lacks ocular accommodation.

When the binocular is focused on the apex of the target, the observer should count how many circles he can see sharply and if there is one with a sharp front edge and blurred back edge.

Then, the barrel is refocused to find the ideal point of focus where the apex is still sharp while as many circles as possible also are, and then the number of circles is counted again.

To judge a certain binocular against another, it has to be assessed whether they have, or have not, the same magnification. It's well known that 7x binoculars have greater DOF than 8x. But if we assume that there are two specimens marked "8x" but the real magnification is 7.6x vs 8.3x, there has to be a substantial DOF difference as well.

The second test I suggest may possibly reward binoculars with some curvature of field.
Both tests should once and for all show whether or not binoculars with smaller effective exit pupil indeed have a greater DOF.

Exclusion clause:
My explanation is not excellent, neither is my sketch, but in combination they might give you an idea what I'm talking about.

//L

The simplest and most accurate method I've found for directly comparing DOF through different binoculars is to use defocused star points. I've described it a few times, but I can't find any of those threads, so here's a quick description of the test.

Set up a artificial star at a relatively close distance, say 10-15'. Using only one eye, center and focus on a distant object (more than 100') with two binoculars of the same magnification. Again using only one eye, center the artificial star and observe the size of the defocused disk in the two binoculars. If the disks are the same size then the star points are equally defocused and the binoculars have the same DOF. The beauty of this method is that the eye will not struggle to accommodate and no judgements need to be made about what's in focus and what isn't. Only the size of the disk matters.

ronh said:

Bob and Holger,
I believe that magnification appearing as a squared quantity results from the fact that longitudinal magnification is the square of the lateral magnification. Is that where ya'll are getting this expression?

I agree that that seems right, theoretically. But it is hard to even show that binoculars of the same magnification have the same DOF, observationally! I don't know of any observational confirmations of that squared result. If you know of any such tests with actual human vision, I'd like to know about them. It would just be kind of cool, as usual, to verify theory.
Ron

Click to expand...

Yes, theory requires conversion to longitudinal magnification when referring to axial distances.

There is an ancient thread on BF containing monocular measurements taken from viewing a brick wall with different power binoculars. Henry Link probably did the work. I recall fitting the data reasonably well using power squared.

Unfortunately, no visual task prevents the observer from accommodating, which complicates matters enormously. Theory predicts static DOF at a fixed point of regard, pupil size, and accommodation, which is hard to obtain empirically.

Ed
PS. Oops, didn't see the post. I'll have to read that carefully, Henry.

Cluster said:

Thanks for the replies and explanation, perhaps technically a little over my head.
...
Am I correct in thinking that a sort of regulation of the light entering the eye is performed by the eye itself, so a bright scene (exit pupil) may be further stopped down be the iris, effectively reducing the aperture and so increasing the DOF? If so the DOF is a part function of an individual eye as much as the bins?

Dave

Click to expand...

In general, I think you're correct. The primary purpose of the eye's pupil is to protect it from overstimulation, so in bright light it stops down quickly, which will increase DOF — assuming the EP of the instrument is larger, of course.

I would say that DOF is essentially a property of the eye, which is reduced by the squared magnification of the binoculars. It gets more complicated, however, since the power of the binoculars isn't constant either. Generally, magnification increases with decreasing working distance, and that needs to be considered for terrestrial uses since a squaring operation is involved.

All in all, there are too many uncontrolled variables to deal with, and theoretical treatments cling to the ideal zero diopter (infinity focus) situation with an emmetropic eye. After that, we're all out of our depth. :eek!:

Ed