Depth of Field Test - Method and Results (1 Viewer)

[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.

 

[smurf.for.mark]

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