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If that were absolutely true, either it would have zero effect, or you would have to move them both the same amount to preserve the veracity of that statement.
You either have 'is part of the objective' or you have 'alters the focal point of the objective a lot'. You can't have both.

Then think of the focussing element as being analgous to a moving element in a camera lens. They both serve to bring objects at varying distances to a focussed image in the same focal plane. This image is then viewed through the eyepiece, which makes a bin or scope afocal - parallel rays in (for an object at infinity), parallel rays out (for the normally sighted).

Magnification is the quotient of the focal lengths of objective and eyepiece.
A 10x50 f/4 instrument would have an objective focal length of 200 mm and an eyepiece focal length of 20 mm. For an f/6 instrument these focal lengths would be 300 mm and 30 mm respectively.
If you think of magnification as being being the enlargement of the subtended angle of a distant object, then the apparent size of the circle of confusion of a defocussed point is proportional to the magnification and its area is proportional to the square of the magnification. Disregarding the human factors such as accommodation and pupil size, the depth of field in an afocal instrument is inversely proportional to the square of the magnification.

John
 
Magnification is based on one object at a specific distance and in focus.

The question is: how do things de-focus as they move off that point?
The fact that you can have different behaviors in that regard is actually
put to use in many fields. The world is not one lens facing one lens.
At least, not in nicer optical gear. If people discuss difference in depth
of field for just one lens with two surfaces, this is hardly new or speculative.
If extened depth of field systems are used everday for nature cinematography
and extended ranged binoculars, this is not BS. This is optical engineering.
There are tradeoffs, always. That's why many do not have extended range.
It takes from something else.

Let's take something simple, the objective size.
Using a 10-pt-font focus at 15 ft and
22-pt-font illegibility as the center and ends,
and using simple end-movement binoculars, I see this:


pwrXobj near-ft far-ft

6x16 8 33 (audubon)
7x25 12 25 (bushnell custom)
6x30 11 23 (kendon)
7x50 14 21 (daylite)


There are fluctuations, but the trend is clear: the objective size
affects the depth of field. Binoculars generally follow f-stop rules
if there are of fairly plain construction. This isn't f-stop per se:
this is simply the distances in "number of diameters".
Optics is all about relative dimensions.


But what if I alter the opening, change only the
aperature?

near-ft far-ft
7x35 10 24 (bushnell discoverer)
7x(15) 9 33 (same binocs, with foil cover, 15mm hole)

This is the "f-stop effect". The opening relative to the whole system
rather than the target distance.
Sure enough, the depth of field increase as it goes down.
The cost? brightness lost.
Let's say it's a bright day: extra field depth and much less glare
(this is an iris) could be handy. Even better than having a 20mm set,
as someone mentioned the other day.

Additional info:

I didn't get to Alphalund, but I did get to test a pair of the
infamous Steiner Police/Marine 8x30s.

So we have, for depths (of a sharp focus at 15 ft, 22-pt font loss):

Bushnell
Sportview 8x30 11 26

Steiner
Pol/Mar 8x30 10 28

Leupold
Yosemite 8x30 12 21

Store brand
8x42 13 19
(mid-focusing lens)

REI store 12 21
8x25 (focusing lens)
------------
Interesting assortment. The Steiner is just slightly better
than an old 8x30. Since depth of field is non-linear
on the long end though, it doesn't take much for their
"focus at 60ft and you're good nearer to infinity" adage to work.
So...it's kinda true, but they shouldn't be saying "revolutionary".

The Yosemite came up a bit shallow even though it's
end focused. It seems the internal design of either end
of the tube can have an effect. As the cinematographers
and laser welders told me yesterday on the web.

The focusing-lens store unit was not great.
Granted, it was 42mm as well, but it turned in
a range behind an old 7x50. The 8x25 REI focusing lens unit
turned out like the 8x30 Yosemites.

I tried the 8x30 Steiners at 60ft set and out the window
to 200 yd targets. Very slight fuzz. Tried the store mid-focuser:
more fuzz. They were both getting settled into infinity by
then, though.

So...the Steiner sorta works but it's not a big feat.
A focusing lens may make thingsa bit shallower, but
in 25 or 30 mm it may not matter too much.


But: there is no "never affected" or "always the same".
 
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OPTIC_NUT,

John is doing an excellent job of explaining the difference between a camera lens and the AFOCAL optics of a telescope (INCLUDING THE OCULAR). I think you will eventually come around, but not until you stop thinking of telescopes as analogous to camera lenses.

I'm going to give you another link, this time to an old thread from Cloudy Nights on this subject. You may notice that the young pup who goes by the name "Henry Link" is espousing ideas about binocular DOF very similar to yours. He is then taken to school by Jean-Charles Bouget and Holger Merlitz. I hope my Eureka Moment about this will be helpful to you.

http://www.cloudynights.com/ubbarch.../page/0/view/collapse/sb/5/o/all/fpart/1/vc/1

I hope the link works, but you might have to join Cloudy Nights and log in to access it.

Henry
 
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but not until you stop thinking of telescopes as analogous to camera lenses.

I believe you have the sense of my thoughts wrong.
I actually pushed back against a simplistic camera-oriented view in one case.
I do not believe the two are the same.
I do believe they both have considerably more variability in their parameters than
some people do, or understand. An optical system that ends on a retina is what it is.

I have presented data to test theories, and I have received a pedagogical pat on the head
in return, ignoring the facts. I know what a reference to authority without addressing the facts means.
There might be some sort of forced epistemics if you did, so that's best left alone.
I am satisfied with my understanding, tests, and my friend's engineering software. I will try
to steer clear of the depth of field issue after this. I have what I need and that will have to suffice.
This is not the place. Or perhaps, the year.
 
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Disregarding the human factors such as accommodation and pupil size, the depth of field in an afocal instrument is inversely proportional to the square of the magnification.

And not the same-power with different-objective, different-depth results I
observed over and over? With binoculars? Not the same binoculars with
a smaller aperature and far deeper field...observed??
Well, at least I won't suffer Galileo's troubles. This has been quite an
education, in more than Science.
 
O_N

Let me try you with another notion that I hinted at in the other thread. You, like we all do, have simply made a judgement on a limit of acceptable fuzziness to define a close and far limit for different models. A comparison of optical 'sharpness' at different distances to your visual acuity. If a binocular wasn't sharp in the first place those distances would be shorter wouldn't they?

Obviously I don't know how good your test binos are but I have tried the Yosemite, and the Vortex, Opticron, and Kowa clones from the same factory and did not consider any of the 8/8.5x sharp (it's less obvious in the 6/6.5x). The cheaper Steiner porros I've tried were perhaps a little better, but the expensive ones, very good.

Is it possible you have just compared relative optical resolution?

David
 
O_P,

My intention in providing the links was to give you the opportunity to see some of extensive discussions that have already taken place on this subject, including the results of experiments that contradict yours.

I didn't want to directly criticize your efforts, but I see that you consider that to be "ignoring the facts", so I'll offer a couple of objections.

First, as already discussed earlier, greater DOF when a 7x35 is stopped down to 7x15 has nothing to do with a change in the binocular's focal ratio. It results from the exit pupil being reduced to a diameter smaller than the eye pupil, which effectively changes the focal ratio of the eye.

A second more general objection I have is to the method you used generate the precise appearing numbers for near and far defocus. Here I think you have overtaxed the eye with a task for which it is not very competent at making reliable and repeatable fine distinctions. You're demanding a single precise measurement of small differences in defocus from an eye that is dynamically struggling to accommodate. I believe if you repeated these same observations on many different days or added the observations of many others you would get a range of overlapping results which would place the signal essentially at the noise floor. One solution is to give the eye the much easier task of judging the relative sizes of the circles of confusion generated by defocus in different binoculars by comparing the sizes of the diffraction discs produced by out of focus artificial stars placed at fixed distances behind or in front of a common point of focus.

Henry
 
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I have proposed an experient back on the "dioptre adjustment" thread
that almost anyone can perform.

A CONTROLLED experiment. Simple. No hemming and hawing about different
days and objects. If the brightness has some effect, this is far more potent.
If brightness has effects, they are AGAINST me, not for me, in this
semi-bright morning. I overwhelm them. Anyone can. Pick up and look.

I believe the results and the relevence to real experiences will
overwhelm any imprecision or nitpicks. The differencebetween
8x21 and 8x40 seems obvious, whatever standards. The difference
between 7x17 and 7x50 is more blindingly obvious. If it comes to looking
at birds in groups and on the move, I think people can see for themselves.
The theory portion of my discussion is has been wasted.

Now it is time for proof, and replication.
 
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O_N

Let me try you with another notion that I hinted at in the other thread. You, like we all do, have simply made a judgement on a limit of acceptable fuzziness to define a close and far limit for different models. A comparison of optical 'sharpness' at different distances to your visual acuity. If a binocular wasn't sharp in the first place those distances would be shorter wouldn't they?

Obviously I don't know how good your test binos are but I have tried the Yosemite, and the Vortex, Opticron, and Kowa clones from the same factory and did not consider any of the 8/8.5x sharp (it's less obvious in the 6/6.5x). The cheaper Steiner porros I've tried were perhaps a little better, but the expensive ones, very good.

Is it possible you have just compared relative optical resolution?

David


Suffice it to say that at 15 feet, ALL binoculars had more than
enough resolution to focus at 10-pt font, and the fuzzing at
22-pt font is probably many dozens of resolution limits beyond
any limits of the system.

In a nutshell, worries about each binocular's absolute resolution
are orders of magnitude below this test. They mean, effectively, nothing.
From $20 to $500, all easily meet the needed resolution.
 
Suffice it to say that at 15 feet, ALL binoculars had more than
enough resolution to focus at 10-pt font, and the fuzzing at
22-pt font is probably many dozens of resolution limits beyond
any limits of the system.

In a nutshell, worries about each binocular's absolute resolution
are orders of magnitude below this test. They mean, effectively, nothing.
From $20 to $500, all easily meet the needed resolution.

Sorry, but I don't understand the relevance of that.

You need to know your visual acuity, pupil diameter, and the corresponding effective resolution of the binocular for that light condition before you can start to interpret your DOF observations. You've not mentioned it so I guess you haven't done it. (You would also need to incorporate that information in your ray tracing.)

I've attempted to do that stuff a couple of times with different binoculars or varying the aperture but as I have no means to control light level, no convincing way to accurately define blur and I couldn't figure out how to quantitate or eliminate accommodation, I've given up trying. I can't offer you scientific evidence for that reason. I can offer a number of opinions though. Apparent DOF varies significantly with light levels. In high quality optics and optimum acuity conditions. Magnification appears to be the principle determinant of DOF. Crap optics reduce DOF. Using two eyes messes up the results.

Hope that helps

David
 
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If you are close enough it falls off rapidly and you don't need to
worry about the little differences.

This is the debating technique called "radical skepticism".
One side is haggled for the last iota of detail and this distracts from
a lack of the evidence from the other.

Or...wait: maybe I can explain the foundation.

Have you ever looked at an eye chart at the eye doctor?
I have. They have rows of letters in descending font sizes.
Every scrap of criticism you level could apply almost identically to them,
to optometrists. And yet they prescribe lenses based on someone saying this letter is
illegible. Because it happens quickly. They understand focusing at near
field conditions.

The reason is that the test conditions make the degeneration obvious.
I chose 15 or 20 feet for a specific reason. Images fall off-focus much more
quickly at close range. You can be forgiven if you don't know that.
I know this from an optics course and a job writing software for optical lab
equipment. I am practicing fairly standard techniques. While they may have errors,
may differences exceed the errors on the outer cases.
I wonder whether anyone actually checked the numbers at all.

I am happy learning how to translate my experience to binoculars
and having data to prove it. That might have to suffice.
You can try this too...if you have a printer and same-power different-objective
instruments. Just like at the eye doctors, but walk in from ideal, walk out
past ideal. Pick a bigger line and say "hey, I cannot read this".
If you fault me for that you should also fault the eye doctor.



But, finally:

Obviously I don't know how good your test binos are but I have tried the Yosemite, and the Vortex, Opticron, and Kowa clones from the same factory and did not consider any of the 8/8.5x sharp

Look, if you cannot get ANY if those to make out 10-pt font at 15 feet, you must see a doctor now, before the Glaucoma or
diabates gets worse. Seriously. If that complaint is even remotely credible,
for my specific test conditions (if you read them), your eyes are far worse than the binoculars.
I have 40 pair remaining at home, and ALL of them can see 10-pt at 15 ft. The ones that didn't needed cleaning badly
or had smashed prisms. Not even bad nicotine film or mold made them that bad.
Personal aesthetics about greatest focus mean nothing: can you make out words in the font?
That is the question, just like at the Doctor's office.

You see why I am wondering about radical skepticism......something doesn't add up here.
 
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I've tried to be helpful but I think it's time to call a halt on this line of discussion as you clearly don't understand the points I've made and I'm finding your patronising responses offensive.

You need to know your visual acuity, pupil diameter, and the corresponding effective resolution of the binocular for that light condition before you can start to interpret your DOF observations.

Perhaps we have have a sensible discussion on this topic when you've done some research and understand the significance of that sentence for a start.

Best wishes,

David
 
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Since I was responding to patronizing and irrelevent items in the first place,
I may be expressing some shock, but it's just my tryiing to cope with your
understanding of depth of field and especially of metrology.


But to address your "issues":

You need to know your visual acuity,

Clearly it is well beyond that necessary to see 10-point font at 15 feet.
You still do not understand what this means to the test.

pupil diameter,

If this plays a part under the same outer conditions, so be it.
I am measuring the whole system, including my eye.
And....I know, from optics, that my pupil's effect at THIS test is mostly irrelevent.
I am talking about DOF effects and defocusing magnitudes well beyond the effect of the eye's pupil.
This is in the design of the test.
Like the eye doctor's.


and the corresponding effective resolution of the binocular

No: not if the effective resolution is beyond what's needed for 10-point font.
That is metrology: I set about neasure far PAST the effective resolution
if ANY of the tested binoculars. You are telling me I cannot be a dog-catcher
because I haven't considered mice.

This is by no means a 'maximum resolutuion test'. It is
a depth of field test with a specified error (cannot read 22-pt font).
Its standards are far beyond the "corresponding effective resolution"
of all the binoculars. I'm sorry if you do not grasp the meaning of this.


Let's look at some numbers:

----22-point font (that I print) is .250 inch. Illegibility happens at about .1 inch ambiguity gamma.
:: this is about 115 arc seconds.

----Excellent binoculars can do 4-8 arc seconds, including the eyes.
----Very bad ones, call it 25 arc seconds...worse than any I own.
----I know I can see 10-pt font on all units. Checking that is IN THE TEST. That is about 55 arc seconds.
If I can read 10-pt on all binocs, I can make out about 20 arc seconds.
If you cannot, no problem: just shift your fonts up to test.
(that, and get cheaper binoculars and a vacation: higher resolution is wasted)

The resolution of the binoculars is "controlled for" by the experiment.
The worst binoculars will lose 22-pt legibility the same way as the best binoculars.

It was fun calculating all that. My eyes are pretty good.
If your eyes are not so good, you focus at 22-pt for sharpest and go from there.
Your field depths will be different but if you do them all the same way they
are relevant to each other.




before you can start to interpret your DOF observations.

Once again, an optomterist does none of those things because the test
leaves them behind.
As do I.
By the same means.

I can resolve down past 10 point font. Always, in daylight.
Thus: Lower detail than that is obviously not relevant to losing 22-pt legibility.

----------------------------
I can see the 'cloudynights' forum argues as much but they know far more,
about the systems and especially (!!) the metrology, real proof.
I might be in the wrong place.
 
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Since I was responding to patronizing and irrelevent items in the first place,
I may be expressing some shock, but it's just my tryiing to cope with your
understanding of depth of field and especially of metrology.


But to address your "issues":



Clearly it is well beyond that necessary to see 10-point font at 15 feet.
You still do not understand what this means to the test.



If this plays a part under the same outer conditions, so be it.
I am measuring the whole system, including my eye.
And....I know, from optics, that my pupil's effect at THIS test is mostly irrelevent.
I am talking about DOF effects and defocusing magnitudes well beyond the effect of the eye's pupil.
This is in the design of the test.
Like the eye doctor's.




No: not if the effective resolution is beyond what's needed for 10-point font.
That is metrology: I set about neasure far PAST the effective resolution
if ANY of the tested binoculars. You are telling me I cannot be a dog-catcher
because I haven't considered mice.

This is by no means a 'maximum resolutuion test'. It is
a depth of field test with a specified error (cannot read 22-pt font).
Its standards are far beyond the "corresponding effective resolution"
of all the binoculars. I'm sorry if you do not grasp the meaning of this.


Let's look at some numbers:

----22-point font (that I print) is .250 inch. Illegibility happens at about .1 inch ambiguity gamma.
:: this is about 115 arc seconds.

----Excellent binoculars can do 4-8 arc seconds, including the eyes.
----Very bad ones, call it 25 arc seconds...worse than any I own.
----I know I can see 10-pt font on all units. Checking that is IN THE TEST. That is about 55 arc seconds.
If I can read 10-pt on all binocs, I can make out about 20 arc seconds.
If you cannot, no problem: just shift your fonts up to test.
(that, and get cheaper binoculars and a vacation: higher resolution is wasted)

The resolution of the binoculars is "controlled for" by the experiment.
The worst binoculars will lose 22-pt legibility the same way as the best binoculars.

It was fun calculating all that. My eyes are pretty good.
If your eyes are not so good, you focus at 22-pt for sharpest and go from there.
Your field depths will be different but if you do them all the same way they
are relevant to each other.






Once again, an optomterist does none of those things because the test
leaves them behind.
As do I.
By the same means.

I can resolve down past 10 point font. Always, in daylight.
Thus: Lower detail than that is obviously not relevant to losing 22-pt legibility.

----------------------------
I can see the 'cloudynights' forum argues as much but they know far more,
about the systems and especially (!!) the metrology, real proof.
I might be in the wrong place.


You win, as you have the most cutting insult. Bravo.
 
I'm sorry. I just lost it.
I was measuring the shoulders on a phase error curve to estimate the plateau. Just habit.
The prevailing paradigm here is to attempt to measure on the plateau.
You don't need any additional weight on your shoulders with that kind of burden.
Please disregard my experiments.
 
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Since I was responding to patronizing and irrelevent items in the first place,
I may be expressing some shock, but it's just my tryiing to cope with your
understanding of depth of field and especially of metrology.


But to address your "issues":



Clearly it is well beyond that necessary to see 10-point font at 15 feet.
You still do not understand what this means to the test.



If this plays a part under the same outer conditions, so be it.
I am measuring the whole system, including my eye.
And....I know, from optics, that my pupil's effect at THIS test is mostly irrelevent.
I am talking about DOF effects and defocusing magnitudes well beyond the effect of the eye's pupil.
This is in the design of the test.
Like the eye doctor's.




No: not if the effective resolution is beyond what's needed for 10-point font.
That is metrology: I set about neasure far PAST the effective resolution
if ANY of the tested binoculars. You are telling me I cannot be a dog-catcher
because I haven't considered mice.

This is by no means a 'maximum resolutuion test'. It is
a depth of field test with a specified error (cannot read 22-pt font).
Its standards are far beyond the "corresponding effective resolution"
of all the binoculars. I'm sorry if you do not grasp the meaning of this.


Let's look at some numbers:

----22-point font (that I print) is .250 inch. Illegibility happens at about .1 inch ambiguity gamma.
:: this is about 115 arc seconds.

----Excellent binoculars can do 4-8 arc seconds, including the eyes.
----Very bad ones, call it 25 arc seconds...worse than any I own.
----I know I can see 10-pt font on all units. Checking that is IN THE TEST. That is about 55 arc seconds.
If I can read 10-pt on all binocs, I can make out about 20 arc seconds.
If you cannot, no problem: just shift your fonts up to test.
(that, and get cheaper binoculars and a vacation: higher resolution is wasted)

The resolution of the binoculars is "controlled for" by the experiment.
The worst binoculars will lose 22-pt legibility the same way as the best binoculars.

It was fun calculating all that. My eyes are pretty good.
If your eyes are not so good, you focus at 22-pt for sharpest and go from there.
Your field depths will be different but if you do them all the same way they
are relevant to each other.






Once again, an optomterist does none of those things because the test
leaves them behind.
As do I.
By the same means.

I can resolve down past 10 point font. Always, in daylight.
Thus: Lower detail than that is obviously not relevant to losing 22-pt legibility.

----------------------------
I can see the 'cloudynights' forum argues as much but they know far more,
about the systems and especially (!!) the metrology, real proof.
I might be in the wrong place.

O_N

Let me try you with another notion that I hinted at in the other thread. You, like we all do, have simply made a judgement on a limit of acceptable fuzziness to define a close and far limit for different models. A comparison of optical 'sharpness' at different distances to your visual acuity. If a binocular wasn't sharp in the first place those distances would be shorter wouldn't they?

Obviously I don't know how good your test binos are but I have tried the Yosemite, and the Vortex, Opticron, and Kowa clones from the same factory and did not consider any of the 8/8.5x sharp (it's less obvious in the 6/6.5x). The cheaper Steiner porros I've tried were perhaps a little better, but the expensive ones, very good.

Is it possible you have just compared relative optical resolution?

David


This was my "aha" moment.
It looks like certain brands cannot be said to have any field depth, therefore.

But learning from it, I realize, what standards that are used here are for
extremely sharp focus "in the range". This is called trying to measure
at the top of the plateau. As you said....you cannot do it. Nobody really
can. That's why shoulders are used and the plateau inferred.
It's quite impossible to reliably measure the top of a near-flat, and the
top might not mean anything if you do. The sides of the top mean even less.

I'll let you continue now that I have "learned".

My attempt to use cloudynights as an appeal to authority was pointless too, I have learned.
It's like any large Bible scripture: people quote it to support differing positions.
The agreed position uses the agreed references. Matthew 4:12...that sort of thing.
 
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