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Depth of field, depth of focus: relevant to binoculars?

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#1 jrbarnett

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Posted 24 November 2017 - 10:34 AM

Okay, I have a notion as to what they might mean in the binocular context, and a couple of examples, and would like conformation that I have it right.

 

Depth of focus example:  The 7x50 Nikon OceanPros seem to have "depth of focus" because...the target appears to be satisfactorily focused over a range of focuser travel, rather than at a single point.  Focusing is very forgiving.  These have the largest amount of this characteristic (whatever the proper term) of any binoculars I have.

 

Depth of field example:  The 10x70 Fujinon FMT-SXs seem to display significant depth of field because...when focused optimally on a target at distance x, focus for targets at both distance x+1 and x-1 can be accommodated by my eyes without refocusing the instrument.  My other binoculars do not behave in this manner.

 

Are these the correct terms for these phenomena?  What gives a binocular (design-wise) one or the other characteristic (deep field/deep focus)?

 

Best,

 

Jim



#2 Pinac

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Posted 24 November 2017 - 12:10 PM

Jim,

 

I don't believe there is something like "depth of focus" - unless you mean by that term that a slower focus travel per turn of the focus wheel results in more "depth of focus" than a faster focus mechanism. But would that really say anything relevant?

 

And as to depth of field, just a few quotes from Holger Merlitz' book "Handferngläser", 1st edition 2013 (tentative translation by me):
p. 123: "The depth of field which an observer can attain is primarily determined by the accomodation capacity of his eyes, then also by the magnification of the binocular, and to a lesser degree by the diameter of the effective exit pupil..."

p. 30: "In internet forums there have been statements that the depth of field of a binocular can be affected or optimized by the optical design of the instrument. This is not the case... in the practice of observation, the illusion of an increased depth of field can be created by field curvature, because object near the observer which would not be sharp in the middle of the field of view appear still sharp in the lower part of the field ... this, however, is a side effect of an optical aberration and should not to be confused with real depth of field... the depth of field of an optical system scales with the reciprocal square of the magnification."

 

Pinac


Edited by Pinac, 24 November 2017 - 12:12 PM.

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#3 Jon Isaacs

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Posted 24 November 2017 - 12:17 PM

Jim:

 

I think they are the correct terms.  

 

Depth of focus depends on focal ratio so if the binos are operating at F/4, the depth of focus of the binocular is very shallow, about 0.0013".  That's not possible with binocular focusers.  But the exit pupil is large and your eye is not diffraction limited with a 7 mm exit pupil,  that probably requires a 1 mm exit pupil.. 

 

Depth of field depends on focal length and focal ratio. 

 

If you're doing these tests during the day,  you pupil diameter affects both depth of focus and depth of field. 

 

A 7x50 @F/4 with a 2 mm pupil becomes an F/14 with a much greater depth of focus,  about 0.016". A 10x50 at F/4 becomes an F/10 with about 0.008" depth of focus. 

 

Edit:

 

Reading Pinac's post,  I am basically saying the same thing though I did want to add the importance of the accomdation of the eye but forgot to.. 

 

Jon


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#4 Rich V.

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Posted 24 November 2017 - 12:58 PM

Past threads about this have agreed that magnification is the primary factor in DOF, with the bino's effective focal ratio coming into play as well.  7x binos inherently have more DOF than 8x or 10x but between different binos of the same mag, the effective focal ratios can make a difference.  If your eye's pupil stops down a 7x50 to 7x25, the stopped down view should have more DOF because of the increased effective focal ratio.

Another effect we can see is the "apparent DOF" caused by the bino's FC.  This is not related to actual DOF but it creates the illusion of increased DOF to the viewer.   For example, foreground areas of the field outside of the center can show sharp focus right out to the edges.  I can see this effect clearly in my Nikon E2 binos when watching quail feeding out in the yard. The view is amazing sharp from the center to the foreground under these circumstances.  FC clearly increases the overall sharp focus of the FOV.

Our individual focus accommodation may also be a minor factor in our perception of DOF; older eyes won't have the same range of focus as younger ones.

Here's an old archived CN discussion that could be helpful.  JCB, Henry Link and Holger Merlitz make some particularly good points:

https://www.cloudyni...depth-of-field/


Edited by Rich V., 25 November 2017 - 11:25 AM.

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#5 MartinPond

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Posted 24 November 2017 - 01:18 PM

Depth of focus  is an informally coined term, but it is a good term to avoid confusion.

Depth of field, as it applies to all optics, and especially cameras, is correct,

 but ends up mangled in with "apparent 3D depth" which is totally different.

Field depth should be used for focus, but is mangled by some using binoculars.

So...I like depth of focus!

 

Sometimes, focusers with a fine pitch can seem like they have a greater depth of focus,

but that would be some confusion...

 

Still, depth of focus meaning "the range over distances over which things look sharp"

    is vey high in 7x50 binoculars. Since marine binoculars are rarely used for less than

75 ft,  they hardly ever need to be focused...for one person.  So...indpendent focus

is no problem, and it's far easier to seal and has better alignment.

You just focus for something far away, and everything from ~70ft to inifinity

is focused well enough for your visual acuity...and your eye's ability to adjust a bit.

 

There is no debate that the lower the power, the greater the depth of focus.

I take depth-of-focus to apply to the outside world, not the focuser travel....as with cameras.

(the two depths might mean the same thing though)

 

There are arguments over whether the optical design or the f-ratio (stopped) have an effect.

 

Experimeting with many binoculars and masks (lens covers with reduced diameter holes in them),

I have found that the depth of focus can be extended, but nowhere near as much as with cameras...

...not entirely sure why.

 

Experimenting with dozens of models, I have found that some models do stand out

from others.   B&L 7x35 Zephyrs go from infinity down to 60 feet, and the current

 best 7x35 is a pair of Limer IFs, with a stunning 45 ft to infinity, box marked "XXFD".

They use an aspheric EP that can do 55 degrees but it field-stopped to 50deg.

 

The deepest are a pair of 6x30 Kendons Imarked "XXXFD", which can take you from

a mind-boggling 353ft (approximately) to infinity.   Not sure exactly how this happens,

but there is a very slight defocusing at about 100ft, and then things crisp up again coming in.

I suspect the makers played the objectives and prisms and eyepiece off against each other...somehow.

 

So, empirically, I must conclude that some binoculars of the same power and apreture can have 

a different depth of focus than others.   I have them.  I can't say why this happens, though.

Many cognoscenti say this isn't possible......but here they are.

 

One experiemwnt I haven't done yet:

   to take extra-wide-field (and not mushy) binoculars and field-stop them narrower,

   to see if that increases the depth-of-focus.  The idea would be that the phase response

   of the extra-wide can translate into a better distance response if the whole field isn't assembled.


Edited by MartinPond, 24 November 2017 - 01:25 PM.

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#6 hallelujah

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Posted 24 November 2017 - 01:20 PM

Past threads about this have agreed that magnification is the primary factor in DOF, with the bino's effective focal ratio coming into play as well.

 7x binos have more DOF than 8x or 10x but between different binos of the same mag, the effective focal ratios can come into play as well.

Rich,

 

True, however, I'm still amazed how much DOF that I see in my discontinued Minox 10x44 Porro prism & my vintage Nikon 10x50 Gold Sentinel wide field Porro prism binoculars. like-button.jpg

 

Help me out... I can't remember what CF is. tongue2.gif

 

Stan



#7 Rich V.

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Posted 24 November 2017 - 01:54 PM

HI, Stan, if by CF you mean FC, I'm referring to field curvature.  It's something I can't help noticing its presence/absence anymore.  wink.gif

 

Rich



#8 hallelujah

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Posted 24 November 2017 - 02:07 PM

HI, Stan, if by CF you mean FC, I'm referring to field curvature.  It's something I can't help noticing its presence/absence anymore.  wink.gif

 

Rich

Thanks, it's my dyslexia at work again. bigblush.gif



#9 Jon Isaacs

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Posted 24 November 2017 - 02:43 PM

Depth of focus  is an informally coined term, but it is a good term to avoid confusion.

 

Depth of focus is concise term with a precise definition and one that can be calculated. 

 

DF= 4 x lambda x f where lambda is the wavewave length of light in question (550nm typically) and f is the focal ratio. 

 

The basic concept is that this is the range over which an image is diffraction limited. At binocular exit pupils,  the eye is not diffraction limited..  

 

Jon


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#10 MartinPond

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Posted 24 November 2017 - 03:29 PM

 

Depth of focus  is an informally coined term, but it is a good term to avoid confusion.

 

Depth of focus is concise term with a precise definition and one that can be calculated. 

 

DF= 4 x lambda x f where lambda is the wavewave length of light in question (550nm typically) and f is the focal ratio. 

 

The basic concept is that this is the range over which an image is diffraction limited. At binocular exit pupils,  the eye is not diffraction limited..  

 

Jon

 

 

 

Now that's special! 

I remember that as being the term as it applies to microscope objectives or electron microscopes...

 

 

Would would be your concise definition for:

 

A) "the range of distances over which a camera lens produces a spot size less than the specified diameter"

 

     (almost all optical instrument refs call this "depth of field") ....in which case I can simply call the 3D term "bogus"

 

or

 

B) "the ranges of distances over which a telescopic system (including a standard eye)

         produces a spot less than twice a given eye's acuity"

 

      (seems almost identical to the concept A) above)


Edited by MartinPond, 24 November 2017 - 03:34 PM.


#11 Jon Isaacs

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Posted 24 November 2017 - 06:05 PM

Martin:

 

"In astronomy, the depth of focus, Delta f  is the amount of defocus that introduces a +/-lambda /4 wavefront error, and can be calculated using 

 

DF= 4 x lambda x f"

 

https://en.m.wikiped.../Depth_of_focus

 

I think that's pretty what I had written.. Lambda/4 being "diffraction limited"

 

jon


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#12 GlennLeDrew

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Posted 24 November 2017 - 07:50 PM

Defining depth of field as the range of distance in object space wherein  image sharpness meets some specified standard of sharpness...

I

For an afocal instrument, where essentially collimated light is delivered to the eye and upon which the eye performs the focusing, the f/ratio of the instrument's objective *by itself* plays no role in depth of field. Rather, the two relevant factors are magnification and exit pupil diameter (or the observer's iris if smaller than the exit pupil.)

 

For a fixed iris diameter, any instrument that delivers an exit pupil as large or larger, the depth of field scales inversely as the magnification.

 

At given magnification, the depth of field scales inversely as the exit pupil of iris diameter, whichever is the smaller.



#13 MartinPond

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Posted 24 November 2017 - 08:19 PM

Whether or not the barrel alone can be called "afocal",

the barrel plus the eye is necessarily a -->focal<--- system,

to point out the obvious.   

Your eye has a near focus.

Your eye has a far focus.

You and the barrel form some focal system at or between those points.

A system that braings a point of light in space to a point of light on your retina.

Collimated light comes into the eye --->at the far focus only<--- where the virtual image seems to be at inifinity.

So...your approxiation (collimated light entering the eye) only applies to one distance, at one focus,

and it assumes the eye cannot focus on anything but infinity. This does not describe the range of eye focusing

as it applies to the range of sharply focusable targets.  It ignores eye focusing from the start.

 

So.....there is a wide range of distances your assumption does not cover at all.

The eye focuses, and that affects the range it sees sharply, with or without an instrument.

We are talking about not touching the focuser and seeing what the eye can focus.

 

On to your distance case:

If you assume an eye that sees perfectly collimated light, the entrance or exit pupil (whichever is smaller)

  would make up the f-stop of your eye as 'camera'.  And less power would mean a bigger exit pupil,

untill it trips up on your eye's entrance pupil.

 

But: for even this case, it is trivial to see that the exit pupil of my 7x50, with a 25mm opening masking it,

 shows half the diameter.   I "eye not allow to focus" depth of field with that aperture (3.5mm) as my entrance pupil,

 and that would change the F-ratio  --->of the eye<----  If I use a smaller mask, my eye wold in fact be seeing through

 a smaller pupil, regardles of  eye dilation.

This would explain the observation "the f-ratio changes the depth, but not as much as expected by the instrument".

It's safe to say that

  "as you change the instrument's f-ratio, you can alter your in-focus range,

   when it alters the exit pupil to be less than that of your eye,

  because that can alter the entrance to the eye".   

      That fits the real obcservations I have made.

 

The majority of the range the eye can focus in has still been tossed out, though.

The eye focuses, with or without binoculars, telescopes, or microscopes.

Assuming perfectly infinity-focused eyes and perfectly infinity-focused binoculars

erases most of the range and history and common practice of using nautical binoculars.

 

The world is round, your grace.   It's fine to call it flat near Venice, but the approximation breaks down.


Edited by MartinPond, 24 November 2017 - 08:24 PM.


#14 Jon Isaacs

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Posted 24 November 2017 - 08:48 PM

For an afocal instrument, where essentially collimated light is delivered to the eye and upon which the eye performs the focusing,

 

That's probably a good model for someone who is 45.. For someone well in their senior years with pretty much fixed focus eyes. I am more dubious. 

 

Jon



#15 Philip Levine

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Posted 24 November 2017 - 08:49 PM

Posts by Rich V and Martin Pond regarding 7x binoculars confirm what I experience when viewing through a pair of Swift Sport King 7x35 binos.  Using during the daytime, viewing lake scenes, the views are wide and have dramatic depth of field in focus.

Phil


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#16 Jawaid I. Abbasi

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Posted 25 November 2017 - 04:14 AM

Jim,

Asking too many question now you pay 10c to me of each question and answers bow.gif

seriously,

very good question asked but here is the answer I may know:

DOF is the focus range or you may called the effective focus range where the optics only focus one and this is done by f#

 

Check your inventory and find out the ratio along with the magnification and you would see longer f# and higher magnification gives more DOF as I experienced myself.

 

Basically, it is a photographic term and what we see is the curvature that induce (bit softer farther image then nearer or vice versa)

 

 

Jon,

the formula you put where did you get it.



#17 Jon Isaacs

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Posted 25 November 2017 - 06:52 AM

Jon,
the formula you put where did you get it.

 

It's something I'd looked up some years ago.  The Wikipedia link I provided has a long discussion,  it's at the very end. 

 

As far as focal ratio not being part of the focusing equation for afocal instruments..  I'm thinking about focusing fast scopes versus slow scopes and the need for two speed focusers at high magnifications. My experience is that at equal magnification and equal aperture,  focus is easier with a slower scope.  Say a 4 inch F/10 versus a 4 inch F/5.4 at a 0.5 mm exit pupil. 

 

I understand the black box concept of afocal instruments but I am not sure it extends to something this,  the amount of focuser travel necessary to span the diffraction limited range. 

 

At binocular exit pupils,  typically the eye is not diffraction limited, I've said this a few times,  but a perfect sensor would,  it seems to me,  detect that the physical motion required to span the diffraction limited range of focus was a function of focal ratio. 

 

The defocused images at the ends of the range would appear identical,  that's the black box afocal optical principle.  The mechanical displacement required to achieve that range would seem to vary.. 

 

Jon



#18 MartinPond

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Posted 25 November 2017 - 07:47 AM

Jim,

Asking too many question now you pay 10c to me of each question and answers bow.gif

seriously,

very good question asked but here is the answer I may know:

DOF is the focus range or you may called the effective focus range where the optics only focus one and this is done by f#

 

Check your inventory and find out the ratio along with the magnification and you would see longer f# and higher magnification gives more DOF as I experienced myself.

 

Basically, it is a photographic term and what we see is the curvature that induce (bit softer farther image then nearer or vice versa)

 

 

Jon,

the formula you put where did you get it.

 

Aha....you are on the other side of me.

I actually have the seen range rated on many binocs.

It's extremely difficult to find production binoculars with much varience,

especially in the F12----F16--F22 range, yes?

 

But I did use masks, on 10x50s and 7x50s...

I'm to the right of you...that is, I do not see the depth camera logic would dictate.

But....I'm to the left of them: I do see a little variance.

Put a 25mm mask on 10x50s and you can see.



#19 MartinPond

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Posted 25 November 2017 - 07:51 AM

Posts by Rich V and Martin Pond regarding 7x binoculars confirm what I experience when viewing through a pair of Swift Sport King 7x35 binos.  Using during the daytime, viewing lake scenes, the views are wide and have dramatic depth of field in focus.

Phil

I think you mean....when the view is bright and your eyes close down a bit?

Makes sense....your "eye camera" has a small entrance pupil..

 

Right...Jon; the idea of diffraction limiting is several ballparks away from binoculars.


Edited by MartinPond, 25 November 2017 - 07:52 AM.


#20 GlennLeDrew

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Posted 25 November 2017 - 09:24 AM

 

For an afocal instrument, where essentially collimated light is delivered to the eye and upon which the eye performs the focusing,

 

That's probably a good model for someone who is 45.. For someone well in their senior years with pretty much fixed focus eyes. I am more dubious. 

 

Jon

 

Even a person who's had a lens replacement, where the solid thing cannot be altered in shape at all and hence is fixed in focus, the eye does the final focusing. That's what I was emphasizing, in the context of an afocal system. I was not implying the act of lens shape alteration at all.

 

For those folks who can accommodate by focusing their lens, that's an additional variable. The fundamental 'rules' I laid out still apply, with the proviso that additional eye re-focus increases the range of distance in depth over which the image is sensibly sharp.

 

*At any one instant*, the depth of field is constrained by magnification and exit pupil/iris diameter alone, irrespective of the eye's accommodation.

 

It bears mentioning that the object distance plays a role, where, for fixed magnification and pupil, decreasing subject distance results in decreasing depth of field.


Edited by GlennLeDrew, 25 November 2017 - 09:26 AM.


#21 GlennLeDrew

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Posted 25 November 2017 - 09:34 AM

In my first post I was concerned only with depth of field, not depth of focus.

 

For the former, the 'black box' model for an afocal system applies, where only magnification and exit pupil/iris diameter apply.

 

For the latter, indeed does the objective f/ratio have an impact on the acceptable drift in focuser position.



#22 Jon Isaacs

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Posted 25 November 2017 - 10:00 AM

In my first post I was concerned only with depth of field, not depth of focus.

 

For the former, the 'black box' model for an afocal system applies, where only magnification and exit pupil/iris diameter apply.

 

For the latter, indeed does the objective f/ratio have an impact on the acceptable drift in focuser position.

 

waytogo.gif

 

If I had read your first post more closely and realized you were discussing depth of field rather than depth of focus,  I would have save myself a lot of time and effort.. 

 

foreheadslap.gif

 

Jon



#23 jrbarnett

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Posted 25 November 2017 - 10:01 AM

I like both sensations - i.e., of being able to focus "good enough" quickly and not having to fine tune due to atmospherics or initial focus errors on infinity targets, and also the seemingly enhanced accommodation permitting a large range of field distanced by day to appear to be adequately focused without adjusting mechanical focus.  I don't find them in a single binocular, though, among those I have.  Could be the sample size, but for my sample it seems to be either-or, or neither.

 

Best,

 

Jim



#24 MartinPond

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Posted 28 November 2017 - 09:49 PM

We can only get so worked up over the in-focus range, since in astronomy......we are all

   looking at optical infinity  ;-)

 

For extremely stable mechanics, extra field edge quality,

   easy maintenance, and the ability to tweek the focus now and then,

   I do love independent focus.   Sure, it plays havoc when you follow around 

   a birdy flitting from 25ft to 75ft in the daytime, but...I have 6x30s and 7x35s for that..

    Seems silly using 10x50s for that.



#25 jrbarnett

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Posted 28 November 2017 - 09:59 PM

We can only get so worked up over the in-focus range, since in astronomy......we are all

   looking at optical infinity  ;-)

 

For extremely stable mechanics, extra field edge quality,

   easy maintenance, and the ability to tweek the focus now and then,

   I do love independent focus.   Sure, it plays havoc when you follow around 

   a birdy flitting from 25ft to 75ft in the daytime, but...I have 6x30s and 7x35s for that..

    Seems silly using 10x50s for that.

Or want to share the views with another observer or three...

 

:grin:

 

- Jim




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