Depth of Field Test - Method and Results

[Matt_RTH]

I wish I had some time to do some actual birding but instead I had to get my fix by doing a simple test. There have been a couple threads relating to depth of field lately so I thought I'd use something from photography. For those of you with several sets of binoculars, or just for fun, you can do this simple test to check for depth of field. This is a subjective test, meaning that you are not shooting for atomic clock accuracy or clean room-quality controls. However, it is quantitative and if done with consistency, should be quite repeatable.

I'm defining depth of field as the distance nearest and furthest from me from a given focal point that I can focus view with "acceptable sharpness" - ie, not perceptibly out of focus. This depth is shortest at closer distances, nearer the minimum focus distance of the binoculars. As a subjective test, there are no geiger counters, spectrum analyzers, gamma rays, cathode rays, heavy metal toxic waste or other drama. This takes a measuring tape and binoculars.

For those who don't want to see the methodology , here are my results (binoculars and depth of focus).

* Pentax 8x32 SP - 12 inches
* Eagle Optics 8x32 Platinum Class - 14 inches (noticeably more than the SP surprisingly).
* Nikon 7x35E - 25 inches. Significantly deeper than the others.

I also have several other sets of bins but not accessible to me presently. I'll try my 8x32SE when I get a chance.

Conclusion - depth of field is a real issue with binoculars at closer focal distances. At the same magnification, with my limited testing , the depths are very similar. However, with just a 1x reduction, I see a significant increase in depth.

Methodology:

Requires a tape measure of at least 25' in length (sorry, not using metric system here). Ideally the testing length is a bit longer than the minimum focal distance of the binoculars. If you are testing multiple sets, use the longest minimum focal distance as the target.

1. Stretch out the tape measure in a well lit area.
2. Straddle the tape measure so that you can see a distance as far as the minimum focal distance (or slightly longer - consistency is important, not exact length - this is a subjective test).
3. Focus the binoculars on the desired distance (yes, this is slightly longer because you are looking at a length but also height - this isn't critical due to the subjective nature).
4. Now focus the binoculars such that the target number is JUST still in focus but focus continues in back of it (everything between you and the target number is out of focus). This takes trial and error but use that "acceptably sharp" standard above.
5. Now scan to see how far back the numbers are JUST acceptably sharp.
6. Write down the distance between the target number and the furthest sharp number.

Now you've have a sense of the depth of focus. Do this for other binoculars in your possession and compare them. Just be sure to use the same target distance to be consistent!

Why does this matter? I've had many occasions where it did. Most will. Why bother with a test like this? I like to think of it as practicing with a speed bag prior to the fight.

It's also easy to do this in a store while comparing binoculars as well using strategically selected or placed objects.

Matt

[orbitaljump]

FWIU, DoF is dependent on more than magnification.

[OwenM]

WHAT? What's wrong with you people? Why do you have to start this up again? Why can't you be narrowmindedly arrogant enough to ignore the reality of multiple variables that simple equations cannot compensate for? Why can't you just pump a few numbers into your little scientific calculator, and think it makes you superior to all those poor idiots who rely on their eyes to tell them what they see instead of asking you? Don't you want to speak condescendingly to people who are smarter than you? Don't you want to be an internet authority on something, an expert?
edit: Ha. Sorry, don't know what happened. All I meant to say was: You're right. Obviously.

[mark22c]

i've noticed that dof is almost the same in a 10.5x70 and a 7x50 likewise 10x50 and 15x70 so is exit pupil a factor here in determining dof? apparent fields of view are similar as well i'v noticed but not always

[Alexis Powell]

Users often mistake field curvature for increased or decreased DOF, depending on the circumstances. Binoculars with fast focus ratios are also often mistakingly perceived to have narrow DOF. That aside, the real DOF that a user experiences is strongly affected by one's accommodation (eye focusing) ability. The ability of one's eye to bring the world into acceptable focus is itself affected by pupil size, which varies according to the brightness of the image. Some complicated interactions w/binocular specs result.

I'm not sure anyone has been arrogant here, but I do wish more Birdforum participants would make better use of the message archive. DOF in binoculars does _not_ conform to the preanalytical expectations that most folks have based on their experiences/knowledge from camera lenses. DOF has been explored in great detail by Birdforum members in archived threads. I, arrogantly, think the best summary of those results is that the only significant contributer to DOF at a given distance and eye accomodation is magnification (lower mag = greater DOF).

--AP

[orbitaljump]

Alex, I participate on several forums. And on technical forums like this, its foolish to think that no one should ever discuss DoF or other attributes, just because they have been discussed in the past in depth and detail by others....even others that are much more knowledgable. If that were the case, then the orignial participants should have read a book themselves instead of discussing it.

Cordially

orbitaljump

[orbitaljump]

PS - I also think that talking about a subject with others interested in the subject of varying levels of knowledge helps one to thoroughly understand it better. Much better than just reading it in a book.

[Matt_RTH]

I was thinking exit pupil could have a key to this but I tried my 7x50 binocular and it too had about 25" that I could measure. I think the field curvature that Alexis mentions makes sense. The EO is notorious for field curvature, while the SP has a relatively flat field.

As I thought about this, and just doing this, I realize one significant element of depth of field is where I focus. When I focus binoculars, I focus such that my subject just comes into focus. Therefore, it is not in the center of focus. This means that if it goes just forward or just backwards, depending on where I focused , my DOF will appear narrow while not being so.

Matt

[Kevin Purcell]

To avoid accommodation issues don't use the human eye use a digital camera (like digiscoping).

Perhaps using a resolution target (and moving that) rather than moving the focus on the bin is a more accurate way of experimentally measuring the DOF. And avoid the issue of "fast focus".

I'd also rather measure at "real" distances. For some reason I have a dislike of close in observations (but it's probably irrational).

Combing the two should give an accurate experimental measure of the bin resolution (from the image of the resolution target ... camera folks do this all the time: software is already written) and an accurate curve.

You could even extend this to looking in different parts of the field so you could isolate the effect of field curvature.

Any property I should control for that I missed?

I have my own bin DOF test: focus on a 7m target (with spiders webs for pinpoint focusing) then read a sign 40m away. Then ABCD a group of bins and rank them on how defocused the sign is. The sign is black on white text (with sharp edges) about 0.15 degrees tall so no field curvature effect with the sign in the center of field. I can rank bins this way. With the same magnification. So I conclude they differ. And in fact, in real life, they do. The worst one has a problem keeping a whole wren in focus at close range ;)

Oh and for the record my 47 year old eyes have about 2D or so of accommodation (from the close focus for my corrected distance vision). So I don't have a lot of accommodation. But it could still be an issue.

So I'm not convinced the simple model that says magnification is the only effect actually applies to real bins.

e.g. "Consider a spherical horse ..." (as the joke goes).

http://everything2.com/index.pl?node_id=1674920
http://my.opera.com/feldgendler/blog/2005/06/21/let-us-consider-a-spherical-horse-in-a-vacuum

And yes, I was an experiemental chemist/physcist a while back so I love testing theoretical models.

So I have a method for making accurate measurements. Now if the weather would improve. ;)

[Alexis Powell]

Alex, I participate on several forums. And on technical forums like this, its foolish to think that no one should ever discuss DoF or other attributes, just because they have been discussed in the past in depth and detail by others....even others that are much more knowledgable. If that were the case, then the orignial participants should have read a book themselves instead of discussing it.

Cordially

orbitaljump

I've no objection to discussing any topic, but I do think there is some value to not reinventing the wheel. These discussions are much more interesting when folks read up on the basics, get past the bulk of their ignorance (not meant pejoratively), and then ask questions etc (if they still have any) to explore the topic further or refine its explication. But that's just me. And yes, I'm the sort of person who tends to look up technical things I'm curious about in the literature (whether paper or electronic) and only ask about things that I either don't understand or about which I cannot find references. I'm not so foolish as to think that folks won't ask the same old basic questions, but I am so foolish as to think that the world might be a slightly better place if they didn't. Likewise, it might be a better place if those answering the questions first asked themselves if they have any reason to think that they know anything solid about the topic. Ah small talk, it is the stuff of life, no?

--AP

[ronh]

Give up.

Here is a classic case where the messy zoo of binoculars, eyes, and usages will never conform to scientific reason. If you took careful measurements of depth of field using impartial instruments in a controlled setting, the results would not conform to people's subjective impressions, so what good are they? The "confusers" of true field depth, including stereo effect, field curvature, and sweet spot size, cannot just be thrown out. What it "looks like", not what it "is", is really what's important here!

Depth of focus, in the case of diffraction-limited astronomical telescopes focused at infinity, has been ably analyzed by Roger Gordon and Chris Lord.
http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/forum/depthoffocus.pdf
It might seem tempting to use geometrical optics to make the translation of their results from image space to object space and call it quits. But related to binoculars, the concept is too complicated, subjective, and illusory to have any hope of success with such a sensible approach.

Personally, I appreciate people's personal depth of field impressions, and measurements, too. They all add to the rich pastiche which is depth of field. I just grow weary of the endless truth-seeking.

My military style Fujinon 7x50 has TERRIFIC depth of field! Must be the magnification or something like that? If I focus my 16x70 at infinity, everything over a mile away looks pretty sharp too, as far as I can tell, but as a daytime binocular it sucks in the first place so I'm not real sure, but still not bad, huh? If an astronomical object comes closer than a mile, I have worse problems than my binocular's depth of field. But my Leica 8x42, woah, it takes the cake. It has a "Focus Knob", so who cares---it doesn't NEED depth of field!

I hope I have offended nobody, or at least everybody equally,
Ron

[Matt_RTH]

Kevin, I also considered digiscoping but I wanted to do it very informally, although I think digiscoping would be help. As I look at it I agree that it's not something to obsess about. I did do it as a way to observe directly rather as I have done the reading in the past but I usually try to balance my reading with some degree of observation. I'll leave the mathematics to the physicists but I do know what I see for the most part, although this test has also taught me something about optical illusion.

BTW, the test I did was a variation of a proven back/front focus technique used with digital cameras. DOF and acute focus accuracy are incredibly important with 1.6x crop factors of modern dSLRs. Again, it's to be familiar with the limits of my gear, although in the case of photography, now that I'm using Canon, problem solved.

[henry link]

Rather than trying to locate the close and distant points at which an image loses acceptable focus I've found that it is much easier to determine differences in DOF between two binoculars by using out of focus glitter points.

There are many possible variations, but the basic idea is to focus both binoculars on the same sunlit object, then compare the size of defocused artificial stars (little shiny round things reflecting tiny images of the sun), which are placed far enough in front or behind the focused object so that the glitter point of the sun dissolves into a diffraction disc. The size of the disc indicates how much the glitter point is defocused, so larger discs = more out of focus = lower DOF. The eye's natural tendency to try to accommodate and judgements about acceptable sharpness are eliminated. Only the size of the disc matters. I should mention that only one eye should be used for this test and the diffraction discs should be carefully centered to occupy the same spot in the field as the original focused object. Low light tests can also be done with pinholes in aluminum foil stretched over a flashlight.

BTW, very far out of focus glitter points have other interesting uses since they are images of either the exit pupil or the pupil of the eye, whichever is smaller. For instance, they can be used to measure how much of a binocular's objective is actually being used or observe how much off axis vignette is present.

[Alexis Powell]

Personally, I appreciate people's personal depth of field impressions, and measurements, too. They all add to the rich pastiche which is depth of field. I just grow weary of the endless truth-seeking.

Hmm....your comments help clarify for me that I appreciate the sharing of opinions about things that are matters of opinion, but that for technical topics I much prefer thoughts and speculations that have been tempered/honed by familiarity with the hard-won accumulated knowledge of others (the understanding the community, one's predecessors, the "scientific" record). I don't see that there need be any conflict between use of mathematics/optical equations and "facts" acquired through observation/empiricism, and I like for discussions to include both approaches used together.

The "confusers" of true field depth, including stereo effect, field curvature, and sweet spot size, cannot just be thrown out. What it "looks like", not what it "is", is really what's important here!

For sure, for sure, but discussions of apparent depth of field will be much more useful if commentators are aware of all these factors, have some sense of the role that they play, and discuss the effects of these factors rather than their subjective composite impression of the amount of DOF a bino has.

Example 1: It is common for reviewers to conclude that binos with fast focusing ratios have a narrow DOF. By recognizing the basis for the illusion, one can learn to turn the focus knob more slowly and avoid overshooting the point of best focus--magically, the bino now has normal DOF! What it looks like can thus be strongly affected by knowledge of what it is.

Example 2: Reviewers 1 and 2 evaluate the DOF in a binocular with strong field curvature, but neither is aware of its influence. Reviewer 1 looks at a prairie landscape and notices that when centered on the distant horizon that the scene seems quite sharp from very close in the foreground all the way to the horizon and sums up the experience with the conclusion that the bino has tremendous DOF (in comparison to another model, with a flatter field, tested against it). Reviewer 2 looks at a bird and notices that when focused on its head in the upper part of the FOV, that its tail, which extends into the distance, is very blurry as seen in the lower part of the FOV, and concludes that the bino has very shallow DOF (in comparison to another model, with a flatter field, tested against it).

Because of these sorts of confusions, unless the circumstances of testing are described in detail, comments on DOF tend to be meaningless and of no practical use.

--AP

[henry link]

Ron,

I have to agree with Alexis about this. One trouble with using DOF so loosely is that the term actually has a legitimate and specific meaning when it's applied to binoculars of different magnifications or exit pupils smaller than the entrance pupil of the eye. When the same term is appropriated to describe an "impression" that could be caused by several different things or a combination of things the reader really can't know what's going on or why.

Henry

[elkcub]

... Here is a classic case where the messy zoo of binoculars, eyes, and usages will never conform to scientific reason. If you took careful measurements of depth of field using impartial instruments in a controlled setting, the results would not conform to people's subjective impressions, so what good are they? The "confusers" of true field depth, including stereo effect, field curvature, and sweet spot size, cannot just be thrown out. What it "looks like", not what it "is", is really what's important here!


Ronh,

Adding to Henry's comment, however, I do think you've put your finger on an important point, namely, that the observer's perceptions don't always have (perhaps most often) directly measurable physical counterparts. I mean, in particular, those perceptions that we have a phrase for, but for which we struggle to find a unique physical index. Try to find an index of beauty, for example — even a complicated one.

So, in this case we have physical optics, and we also have visual perceptions, which, at the end of the day, underly our overall appreciation of the "view." In this instance, visual depth of field is a mental construct that results from a large number of sensory cues and personal variables. Optical depth of field has a specific theoretical meaning (or a limited number, anyway:^) ), but doesn't conveniently correspond with the overall perception. In fact, it my be only a small part of the perception. We always need to know, therefore, which domain is being discussed, and should probably say "visual" DOF or "optical" DOF, if the context doesn't make it clear.

I must admit to being somewhat complicit in this conflation of terms, particularly with regard to advertisements suggesting that advances have been made in optical DOF. For those who are confused, however, there is no question but that the same optical parameters that affect DOF in a camera also apply to binoculars, i.e., f, f/#, and A. The only difference is that other than contributing a scaling factor based on magnification, the parameters all belong to the observer's eye not the components of the binocular. (Conditioned on the exit pupil being smaller than the eye's pupil.)

Ed

PS. Thanks for the Gordon & Lord paper. At first blush I suspect they are using a different definition of DOF. Keep in mind that astro observations are all made at optical infinity, and the eye is a completely different instrument when dark adapted.

[ronh]

Thanks for being so nice after I heckled you. It is natural to want to take the view apart, like a clock, to find out how it works. Good luck in your quest for a description of depth of field. Maybe you guys can do it. But, if the eye is a vital part of the chain of events for this parameter, well I mean, really, really good luck.
Ron

[Pileatus]

FWIW...

Pete's observations summarize the DOF question quite nicely:
http://featheredgeoptics.com/productreviews_nikonedg.htm

[Tero]

I immediately went to the optics case here at the Center for Research and Education and pulled out a 7x42 Zeiss FL and a 7x42 Leica Ultravid HD–two of my favorite binoculars of all time.
Now I have a name for the top shelf in my closet as well. But I do not any 7x pairs there, only one I had was a 7x35 porro for a short while. The Gold Finches were miniscule, it never got used.

I know what the guy means by brightness. I have all kinds of diopter issues with dim binoculars. DOF also. But even at 10x, you can get the brightest for your money, that always helps.

The neck strap is double-adjustable and commendably long–long enough for us bandolier birders. I just wish that companies would stop putting non-skid rubber on the inside of the neck band portion of the strap. The functionality of the bandolier method is contingent on the strap sliding smoothly, not catching on your clothing as the glass is slid/lifted into position.
Well, for a free demo $2000 bin, he can get his own strap. I would have put the lens caps on for the ruggididety test.

[henry link]

Thanks a lot, John! All I need on a fine Sunday morning is to have my blood pressure soar from exposure to some of Pete Dunne's writing on optics.

I'm going birding, so I'll have to respond to this later, but meanwhile here's a quiz for you "boys and girls". How many bone-headed ideas about binocular optics did Mr. Dunne manage to cram into a few paragraphs under the heading "EDGing forward"?

Henry

[henry link]

Ok, I've simmered down a bit now. I think it's best to take on only one Dunnism at a time, so I'll confine my comments to the claim that "stands to reason" from the quote below.


..."If you are a photographer, you know that when your aperture is wide open your photo has a shallow depth of field (so only the subject, or part of the subject, is in focus). When the aperture is narrow, you increase depth of field (so more of the world remains in focus).

It stands to reason that a binocular that is very bright will result in the user’s pupil constricting, thus increasing depth of field."...


Notice that this claim is for a real increase in optical DOF (not a subjective impression of DOF), caused by a change in the aperture and focal ratio of the eye from a rather small increase in light transmission.

Does this really make sense? If an increase in light transmission simply stimulates the eye's pupil to compensate by constricting then virtually all binoculars would appear equally bright. There wouldn't be any "very bright" ones. To use a Dunnish phrase, Pupil Constriction Peter would rob Higher Light Transmission Paul. Anyone who has compared an old single layer coated bin to a fully multi-coated one knows this doesn't happen, but never mind, let's assume it does. The supposed increase in optical DOF from a brighter image can be put to the test using the defocused star method I described earlier.

I picked two 7x50 binoculars with very different light transmission, a Nikon 7x50 Prostar with state of the art transmission in the 90-95% range and an old Leitz 7x50 with transmission around 75%. The difference in brightness between the two is quite obvious, but when I tested them for DOF using the defocused artificial star method I could see no difference at all. I should mention that I even increased the sensitivity of the test by using two closely spaced star points at a distance that caused the edges of the defocused discs to barely touch in both binoculars. Even a very slight difference in the size of the discs would have been visible.

Alas, this isn't the first time I've found myself dismayed by Pete Dunne breezily disseminating an optical myth of his own invention, without testing it or bothering to think it through.

[Surveyor]

These are some of my thoughts on Depth of Field calculations.

1. There are two lines to be considered. Optical DOF, calculations that are well defined and have a specific meaning and published algorithms from various sources, one set of which is attached as a jpg. For those who remember the pre-auto focus lenses that had the DOF scales printed on them.

2. The other is the mechanism of the human optical and perception system. These include such things as eye pupil diameter, depth perception and accommodation.

3. Since this is a binocular forum, and not a human interface-engineering forum, I have only concerned myself with the optical DOF of the instrument itself. Each individual will have to find out how they react to the instrument specifications as they apply to him or her.

For those of you who are familiar with photography, which appears to be many from comments on the forum, take your favorite DOF calculator for cameras and examine the outputs for the following conditions; 50 mm f:/4 (1x), 400 mm f:/4 (8x) and 500 mm f:/4 (10x). Notice the hyperfocal distances are 32.9m, 2105.7m ((400/50)^2 X 32.9) and 3290 m ((500/50)^2 X 32.9). Also check that the difference of the near and far focus distances when converted to diopters also matches the 64:1 and 100:1 ratios, allowing for round off errors in the calculator.

A binocular objective system is not much different than a camera except the focal plane replaces the film position and is a real image. The eyepiece functions to enlarge this real image by scaling the image (and DOF) to the desired magnification, maybe, adding aberrations. I have found the camera programs to be fair predictors of the optical DOF of binoculars when certain adjustments are made. The COC, or blur ratio, has to be modified because of magnification and the paper size has to be modified in the form of an aperture ratio. Once this is fit to observed data (i.e. scaling the camera data to fit field measurements) the resulting figures are approximately in the range of focus range uncertainty. I have checked this in the past by using a method similar to one posted in another thread and adjusting a camera program to match those measurements and then substituting binoculars of different powers.

This works well when the optical DOF is the limiting factor, like for young people who can dilate to 5 or 7 mm and have accommodations of > 2-3 diopters. For us older folks that can only dilate maybe 4 mm and have accommodations <2-3 diopters, then our own eyes are probably the limiting factor a lot of the time, probably limited to our accommodation. These considerations apply to other measured specifications of the optics also, for instance, someone measures the field curvature at 2 diopters then someone with an accommodation of greater than 2D will see the field a fairly flat and in focus but someone with 1 or 1.5D will see the edges with considerable defocus.

The optic system can be engineered/calculated to a desired status for the average observer, but can anyone predict how any given individual will perceive the image, no. If you are expecting any subjective or objective review to tell you what you will see, I think you will be unhappy with any result. At least with good objective measurements, if you know your limits and visual capabilities you will be able to make a better estimate of the expected performance. The same can apply to subjective reviews also, if the conditions the results were obtained under are included.

Just my thoughts and by no means definitive. Everyone have a very good rest of the year.

Ron

PS. Henry, I have occasionally used one other modification of your procedure to verify that the optical DOF is similar for a given power. I use one sphere with two light sources, one either side of the line of sight, so that I can adjust distance from, and the angle between, the defocus spots to bring them tangent. Great idea, your test

[Tero]

Henry, even with his faulty logic, dunne comes up with bright is good. And i have to say, even at mid price, brightness is good for all my viewing problems. It evens out some problems, such as my two eyes not being the same.

[ronh]

Ron,
Thanks for sharing your work in applying photographic DOF to binoculars. After my saying there's no way to do this, I see that you were already well along the path! I don't doubt the results that you give, so don't think that I'm arguing with you, but I was kind of surprised that the correlation between photography and binoculars is anything like good, because the systems seem very different to me. But I think I see how it can be. I'm just going to lay out how it seems to me, please correct me where I am wrong. I'm not pontificating, just testing my understanding, okay?

A camera forms a real image with a single, or "objective", lens. According to geometrical optics, there is but one focal distance for a given image distance. So, how does depth of field arise? In photography, it arises from the imperfect defining powers of the lens, and the medium. Photographic lenses are compromised to be sharp at the edges, and do not approach diffraction limitation even in the center. This is acceptable, because the grain/pixel structure of the medium could not resolve, at typical photographic focal lengths, diffraction limited sharpness. As long as the image of a point does not exceed that grain size, the image will appear as sharp as can be. The image can in fact be substantially defocused without the point spread function exceeding the spatial resolution of the medium. So, there's a range of object distances for a given image plane, called "depth of field", that will satisfy this criterion. To the extent that lenses and film/pixel resolution don't vary much, DOF is a useful and well-defined concept in photography, to the point where lenses are typically engraved showing depth of field about any setting.

Binoculars are different. If you consider the system independent of a "medium" (which would be the eye), the origin of depth of field is the inherent blurriness of the optic itself. This is limited by diffraction, but I am led to believe that in most binoculars, optical flaws dominate the diffraction blur. As long as enlargement of the image of a point due to misfocus does not exceed the optical blur, the image in the focal plane will be as sharp as it can be.

Folding in the eye may seem a can of worms, but is essential to complete a realistic picture of the situation. In the absence of visual astigmatism, an eye might resolve an angle of 1 arcmin through a well focused eyepiece. The more the eyepiece magnifies, the more visible the blur from the objective becomes, which qualitatively explains the perceived decrease in DOF with increased magnification. But, the fact that photographic and binocular DOF should agree (you say approximately, so I don't know how close you have found them to) is at first surprising. It suggests that binocular and camera lenses are of similar quality, which is not too surprising, but also that the angular resolution of the eye somehow is somehow comparable to the spatial resolution of film/CCD.

I should not be surprised. After all, photography gives results which look good to the eye at a casual glance, but on very close inspection often appear imperfect. Similarly, good binoculars give images which look very sharp in normal viewing, but flawed under boosted magnification. In other words, the quality of photography and binoculars both accurately match/barely exceed the eye's resolving power. Any better performance would be wasted, and poor engineering. So, photographic DOF might very well reasonably match binocular DOF.

Could you tell us what all the symbols in you equations represent? I'd like to work a couple of cases out and see what I get.
Ron

[Surveyor]

Hi RonH;

I am not going to try a full answer to this today, it would take too long and I would probably have to do some reading to get a coherent answer together. Today is going to be full.

I did take a simple test I did on a pair of compacts the other day and just threw the numbers into a DOF calculator this morning so you could see the correlation. A caution, do not expect the numbers to be exact. There is some focus error and I can not estimate a 100 micron blur diameter very well, it may be twice or half that.

http://www.birdforum.net/showpost.php?p=1355636&postcount=6 (http://www.birdforum.net/showpost.php?p=1355636&postcount=6) is were I made the field test. In the attached jpg, the first two columns are an approximation of those observations. Note, the calculator shows about a 1.7 diopter DOF and what I observed was about 1.9D.

PM me your email address an I will send you the full PDF that goes with the DOF calculator I use.

Hope this helps some.

Ron

[Surveyor]

For those interested in trying this you can download the free calculator at http://www.vanwalree.com/optics/vwdof.html also download the PDF manual, it is very useful and contains the formulas used.

Below is the way I modify the input to get around not knowing the focal length or f number. I do not use much anymore since I have found some quick estimates that suit my needs but would be interested in any feedback from real data as compared to the calculated values.
------------------------------------------------------------------------------------------------
Set focal length to aperature * power
Set f number to aperture

Set format to x,y size of aperture i.e. 14.142x14.142=20 mm (aperture * 0.707107).
Set COC to percentage of FOV in mm (.085% of 42 mm objective = .036 mm----- Note: change this COC value to match field data.
Note that the COC used so far is the default film values * 0.707107

[ronh]

Thanks Ron,
That is overwhelming and sufficient. The article linked at the bottom of the page is good reading. It is simple matter to transform spatial blur in the image plane to angular blur as viewed through an eyepiece. Then I guess one has to try on a few cases, experiment vs calculation, to discover his own personal tolerable blur criterion. I don't know if I'll ever get there, but this was a good learning experience already.
RonH

[Surveyor]

RonH and others;

During lunch I took the time to fit the field data a little better (first and last column) and included the input fields as an example.

[ronh]

Ron,
I think I'm on the right track. Computers give me the willies, so I used the equations for near and far points. I tried to reproduce your result with your 8x20 for an object distance of 30m. Not understanding your substitution, I simply guessed the focal ratio to be 4, and used your .019mm circle of confusion. I got:

near point: 22.1m, vs your 20.8m
far point: 46.5m, vs your 53.9m

I suppose the difference is due to my erroneous focal ratio. I am led to believe that roof binos are usually faster than f/4. I can get your results almost exactly with a focal ratio of 3.3. Then, 8x requires an eyepiece with a focal length of 8.3mm. Through such an eyepiece, the .019mm COC would appear to subtend 8 arcmin. A good eye can barely resolve 1 arcmin under the best conditions, 3 is usually more like it, and 8 is reasonable in this "stretch" situation. I presume you accepted a slight apparent blur.

So I think I get it, but please straighten me out if I don't.
Thank you,
RonH

[Matt_RTH]

Wow, this thread has a lot of great info. Kevin's "defocused sign" and Henry's "glitter points" sound great. I like the idea of an easy to use test and those are pretty simple. One thing about "larger disks" in that message, higher magnifications would show larger disks, correct? So it may be hard to compare say a 7x and 8x? I'll have to try.

To summarize the thread (let me know what I missed):
- Perceived DOF (the very generic and broad sense, not in a optical science sense) is dependent on a lot of variables, such as field curvature, eye dilation, light level, but is most dependent on magnification.
- DOF in an optical sense would be far more difficult to observe but can be calculated.
- Focus habits can also affect apparent depth of field (front/back focus)
- Tape measurement has calibrated numbers but the actual readings are too prone to error for even a subjective test.

BTW, could binoculars be designed at F4. That's a pretty fast focal ratio. I'd suspect F5.6 (like most scopes) or F8. An 8.3mm eyepiece in astronomy has to have a pretty advanced design to have any eye relief at all. I understand that eyepieces in bins are often the earlier, low eye relief design so wouldn't the actual eyepieces have to be in the 15-25mm range? I would be glad to see some published specs.

Thanks much.

Matt

[henry link]

Matt,

If you compare 7x and 8x bins using the a glitter point the defocused disc will be larger in the 8x because it's more out of focus, not because it's enlarged by higher magnification. If you see different sized discs in binoculars that are supposed to have the same magnification it's an indication that the magnifications are actually not the same.

Most binoculars have focal ratios around f/4 or lower. That's why the image usually looks pretty poor if the magnification is boosted very much. They're just not very good telescopes. However, in daylight the effective focal ratios are higher. For instance a 5mm exit pupil binocular with an f/4 objective will be stopped down to f/6.7 if the eye's pupil is closed to 3mm.

Specs for objective and eyepiece focal lengths are rare. I've measured a few directly and occasionally you can employ a trick or two to figure them out. If two binoculars in the same series use the same eyepiece and prism, but have different magnifications you can measure the difference in physical lengths to determine the focal lengths of the objectives. For example, the Nikon 8x30 and 10x35 EIIs use the same EP and prism. The difference in length measured from the objective glass is about 27mm, so you could say that each 1x of magnification in these binoculars equals about 13.5mm of objective focal length. The 10x35 has a 135mm f/3.86 objective and the 8x30 is 108mm f3.6 and, of course, the EP focal length is about 13.5mm.

Henry

[Tero]

As long as we are dragging this on, is it true that a porro in the same size as a roof, say 8x42, has more DOF? Yes or no, plus any math you want to add.

[ronh]

Tero,
I can't see why there would be difference for prism type. No math needed, prism type simply is not a parameter in the equations. There might be a DOF-like illusion from the Porro's increased stereo effect, like "the view has more depth", but not DOF in the strict sense of what distance range appears fairly sharp.
Ron

[Tero]

I have two 8x42 pairs, and I was not able to see one either, other than the porro was sharper, confusing me a bit.

[ronh]

Tero,
Well, an overall sharpness difference could sure throw you off. Some say the sharper bino will appear to have greater DOF, since, being sharper to begin with, it has more "headroom" for defocus tolerance. I'm not so sure. With a not-so-sharp binocular, a bit of additional blur might not stand out like it would when added to a very sharp image. Duh.
Ron

[BobinKy]

I just ran the numbers on some binoculars with the VWDOF Calculator that Surveyor posted.

But first a word of caution.

The DOF calculator was developed for cameras, not binoculars. So a bit of tweaking and guesswork is required for the calculator to output DOF estimates for binoculars.

As Surveyor suggests, here is the tweaking that is required to run your own numbers.

Set format to x,y size of aperture, such as 22.6 x 22.6 = 32mm (objective size x 0.707107)

Set COC to percentage, such as .085% x 32 = COC (.085% x objective size = COC). [I know this is a bit of a tweak!]

Set focal length, such as 6 x 32 = 192mm (power x objective size = focal length). [Another tweak!]

Set F number to objective size.

Second, as stated many times above, depth of field is used exclusively for terrestial use, particularly in close-up viewing. It has no place in astronomy viewing when all binoculars are focused upon infinity.

Next, binocular depth of field is influenced by prism design, eyepiece design, perceived sharpness, and eyeball dimensions of the observer, to mention a few of the variables that have been reported in the threads. Consequently, field tests have shown repeatedly that two different binoculars of the same dimensions may easily produce different DOF results for different situations and with different users.

Finally, DOF calculation should be done primarily to identify trends (or DOF gaps) in binocular collections.

. . .

Having said the above, here are a few estimates [er... tweakimates] on some common binocular sizes.

6x32 . . . size
20 meters . . . object distance
14 meters . . . near point of focus
38 meters . . . far point of focus
24 meters . . . depth of focus

7x50 . . . size
20 meters . . . object distance
15 meters . . . near point of focus
30 meters . . . far point of focus
15 meters . . . depth of focus

8x32 . . . size
20 meters . . . object distance
16 meters . . . near point of focus
27 meters . . . far point of focus
11 meters . . . depth of focus

8.5x44 . . . size
20 meters . . . object distance
16 meters . . . near point of focus
26 meters . . . far point of focus
10 meters . . . depth of focus

10x50 . . . size
20 meters . . . object distance
17 meters . . . near point of focus
24 meters . . . far point of focus
7 meters . . . depth of focus

. . .

6x32 . . . size
50 meters . . . object distance
23 meters . . . near point of focus
Infinity . . . far point of focus
Infinity . . . depth of focus

7x50 . . . size
50 meters . . . object distance
27 meters . . . near point of focus
360 meters . . . far point of focus
333 meters . . . depth of focus

8x32 . . . size
50 meters . . . object distance
30 meters . . . near point of focus
147 meters . . . far point of focus
117 meters . . . depth of focus

8.5x44 . . . size
50 meters . . . object distance
32 meters . . . near point of focus
120 meters . . . far point of focus
88 meters . . . depth of focus

10x50 . . . size
50 meters . . . object distance
35 meters . . . near point of focus
86 meters . . . far point of focus
51 meters . . . depth of focus

16x70 . . . size
50 meters . . . object distance
43 meters . . . near point of focus
60 meters . . . far point of focus
17 meters . . . depth of focus

. . .

WOW, the lower power sizes are really DOF-friendly. If you do a lot of nature viewing during the day, I recommend you add one or two lower power models to your collection. My DOF gem is the Leupold Katmai 6x32 roof CF--where an extended DOF contributes greatly to obtaining quick, one-handed views. The Fujinon Polaris FMT-SX 7x50 porro IF is DOF friendly as well--although I must use both hands for this big guy.

Surveyor, thank you for posting the VWDOF Calculator. I may not have used it properly, but it did get me thinking about how important DOF can be in certain situations.

--Bob
Kentucky, USA

[Matt_RTH]

Even 7x is way more pronounced than 8x. A real advantage in my book. I may get a 6x eventually but I haven't found a problem yet that a 6x would solve. Though I am a hopeless gearhead so why need an excuse.

[Kevin Purcell]

Even 7x is way more pronounced than 8x. A real advantage in my book. I may get a 6x eventually but I haven't found a problem yet that a 6x would solve. Though I am a hopeless gearhead so why need an excuse.

Woodland birding.

Especially when they're in brush, they're singing or scolding, you are close but still can't see them so you have to scan depthwise in the brush.

I remember having problems like this using a Pentax WP 8x32 looking for a wren in a thicket. I wanted to ID it particularly because I'd seen lots of Bewick's Wrens in this location, a couple of Winter Wrens and I was curious if this was going to "complete the set" with a House Wren. I was perhaps 6 feet or a little more away from it.

I was sorry I hadn't brought the Yosemites.

[Tero]

I was birding at sun down with 10x42. Depth of field was poor. Is it because my eye is dilated?