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What determines minimum focus distance in binoculars?

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#1 revans

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Posted 02 March 2024 - 03:53 PM

How can I judge what the minimum focusing distance of a pair of binoculars is going to be?  I think it must be dependent on focal length but often the binoculars' focal length isn't provided.  How is minimum focus distance determined?

 

Rick



#2 aeajr

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Posted 02 March 2024 - 04:05 PM

This is a specification that is usually provided by the manufacturer.   If there is a calculation, I am not aware of it. 

 

If you have an old pair with no specs, just try to focus on something that is fairly close, then back away until you get a clear focus.  A brick wall works well for this. 

 

I have binoculars that have a minimum focus of 6.5 feet.  These are my bird watching binos.  On the other extreme, my10x-22x zoom binos have a minimum focus of around 50 feet which is fine for astronomy but not good for bird watching.


Edited by aeajr, 02 March 2024 - 04:08 PM.

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#3 SMark

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Posted 02 March 2024 - 04:26 PM

If I need a really close focus, I just take off my glasses. I'm just so near-sighted. So for some of us, the equation is even a bit longer... 


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#4 revans

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Posted 02 March 2024 - 04:35 PM

Near focus capability doesn't seem to be discussed very often in binocular descriptions.   It is nice to have an astronomical binocular that also is capable of focusing across a room, but it doesn't happen very often and I was wondering if there was a way to search for binoculars with this parameter in mind, in addition to aperture and FOV and optical quality and exit pupil and build quality and weight and magnification which are the usual considerations (besides price).

 

Rick


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#5 sevenofnine

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Posted 02 March 2024 - 04:57 PM

Every new binocular that I've ever purchased has the Minimum Focus Distance listed in the specifications. Oberwerk has it listed in their Binocular Comparison Chart. borg.gif

 

https://oberwerk.com...mparison-chart/.

 

The Pentax Papilio II would not be good for night sky viewing but the close focus is astounding...0.5m/1.6ft. shocked.gif


Edited by sevenofnine, 02 March 2024 - 05:26 PM.

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#6 ButterFly

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Posted 02 March 2024 - 05:22 PM

For porros, the focus is usually accomplished by moving the eyepieces.  How far out the eyepieces move determines how closely one can focus with them.  For roofs, the focusing element is nearly always internal (although one can also move those eyepieces to focus as well).  There may or may not be more space to move the element when it's internal.  For eyepiece moving types, the material has to be strong enough to maintain the eyepieces' positions.

 

As mentioned above, when I take my glasses off, I can focus closer than with them on.  To focus where I was looking with my glasses on, with my glasses off, I would need to move the eyepieces closer to the objective.  The stated manufacturer specs are for corrected vision.

 

For the eyepiece moving variety, it's the same calculation as with a telescope, using the thin lens approximation twice.  The focal length of the objective and the object distance determine the focal position.  Then, the object distance to the focal plane and the eyepiece's focal length determines the collimated focus position (at a distance infinitely far away).  For internal types, you simply don't know how much the element can move, so you have to rely on the specs.


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#7 drt3d

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Posted 02 March 2024 - 05:48 PM

It depends on the design of the binoculars. There is no formula.

 

You can either find it in the specs of the binoculars, or, if you already have the binoculars, just measure it.

 

George

 

PS. I you are into photography, this is like asking what is the minimum focusing distance of a lens. It is not a fixed value that can be calculated based on the focal length, etc., of the lens. It depends on the design of the lens.


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#8 rblackadar

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Posted 02 March 2024 - 06:01 PM

To answer your question, going by the lens equation, the theoretical minimum distance between object and focal plane is four times the focal length of your objective, i.e. exactly 2f on either side. But that would require you to have a focusing mechanism that increases your binoculars' total length by a factor of two, obviously not practical. It's a design choice, based mechanical / ease of use considerations, exactly where on that continuum to set the limit on outward travel of the eyepiece assembly.


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#9 revans

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Posted 02 March 2024 - 06:10 PM

Every new binocular that I've ever purchased has the Minimum Focus Distance listed in the specifications. Oberwerk has it listed in their Binocular Comparison Chart. borg.gif

 

https://oberwerk.com...mparison-chart/.

 

The Pentax Papilio II would not be good for night sky viewing but the close focus is astounding...0.5m/1.6ft. shocked.gif

From the chart, it looks like achieving closer near focus is inversely proportional to aperture.

 

Rick



#10 aeajr

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Posted 02 March 2024 - 06:13 PM

Near focus capability doesn't seem to be discussed very often in binocular descriptions.   It is nice to have an astronomical binocular that also is capable of focusing across a room, but it doesn't happen very often and I was wondering if there was a way to search for binoculars with this parameter in mind, in addition to aperture and FOV and optical quality and exit pupil and build quality and weight and magnification which are the usual considerations (besides price).

 

Rick

First, what do YOU mean by close focus?  15 feet?  10 Feet?  5 Feet?

 

I would not expect close focus to be needed for sports.  If you are using binoculars, likely the subject is 20 feet away or more. 

 

 

Look for binoculars that are recommended for bird watching.   Bird watching binoculars will typically have a close focus under 10 feet.  Typical sizes used by birders are 8X30 to 10X50.

 

My birder binos are 8X32 and 10X40, both with close focus under 10 feet. 

 

Why do birdwatchers need close focus?

Includes a chart that provides the close focus on a selection of binoculars.

https://www.allabout...-birdwatching/#

 

 

Zoom binoculars often have quite long close focus specifications. 

 

You will note that roof prism binoculars are the preferred design among birders due to their more compact design and lighter weight. 


Edited by aeajr, 02 March 2024 - 06:24 PM.


#11 TOMDEY

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Posted 02 March 2024 - 06:14 PM

To answer your question, going by the lens equation, the theoretical minimum distance between object and focal plane is four times the focal length of your objective, i.e. exactly 2f on either side. But that would require you to have a focusing mechanism that increases your binoculars' total length by a factor of two, obviously not practical. It's a design choice, based mechanical / ease of use considerations, exactly where on that continuum to set the limit on outward travel of the eyepiece assembly.

Although true... that 1st order observation otherwise has no significance in the context of focusing a binocular. Indeed the image would be massively aberrated at 1:1 conjugates. Each bino is different. For Birding and (especially) Insects --- they design and build to accept close target range.    Tom


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#12 revans

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Posted 02 March 2024 - 06:17 PM

First, what do YOU mean by close focus?  15 feet?  10 Feet?  5 Feet?

 

 

Look for binoculars that are recommended for bird watching.   Bird watching binoculars will typically have a close focus under 10 feet.  Typical sizes used by birders are 8X30 to 10X50.

 

Zoom binoculars often have quite long close focus specifications. 

By close focus, I mean being able to use them in an average room, so maybe 10 to 15 feet... so that you could read a laptop screen on the other side of the room.  My 7x35s will do this as will my 8x21s, but my 10x50s and anything larger won't. 

 

Rick



#13 TOMDEY

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Posted 02 March 2024 - 06:17 PM

Afterthought! --- Questar actually provided custom scopes for close work --- called them ~Remote Microscopy~ We used those at work to look inside our satellites for dust motes, typically a few meters from the lens.    Tom



#14 Mark9473

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Posted 02 March 2024 - 06:32 PM

By close focus, I mean being able to use them in an average room, so maybe 10 to 15 feet... so that you could read a laptop screen on the other side of the room.  My 7x35s will do this as will my 8x21s, but my 10x50s and anything larger won't. 

 

Rick

There are several things that have an effect. In porro prism binoculars, focusing is generally done by moving the eyepieces in or out, and the larger the binoculars the longer focal length they will typically have. There's only so much movement they can build into the eyepiece bridge, and its effect decreases with focal length.

Additionally, manufacturers put some limit on the amount of FOV overlap mismatch they consider reasonable at close focus - otherwise they'd likely get a lot of returns for "defective" binoculars. In porros of increasing size, that limit is reached at progressively higher distances.

Roof prism binoculars are at an advantage because they have better overlapping FOV's a close distance (the objectives aren't as far apart), and with increasing size there's more room in them for the internal focusing lens to move.


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#15 DeanD

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Posted 02 March 2024 - 07:11 PM

...

Additionally, manufacturers put some limit on the amount of FOV overlap mismatch they consider reasonable at close focus - otherwise they'd likely get a lot of returns for "defective" binoculars. In porros of increasing size, that limit is reached at progressively higher distances.

Roof prism binoculars are at an advantage because they have better overlapping FOV's a close distance (the objectives aren't as far apart), and with increasing size there's more room in them for the internal focusing lens to move.

... and that is why the Pentax Papilio II's, which use close focus as a selling point, are reverse porros: bringing their objective lenses as close together as possible (but I suspect at the expense of the traditional "3D" views that normal porros can give)



#16 John Russell

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Posted 02 March 2024 - 08:25 PM

It is the closest object distance at which the focal planes of objective and eyepiece can be brought to coincidence.

 

Edit, and just to elaborate: The closer an object, the further the distance of its virtual image behind the objectives.

On most Porro binoculars and older roof prism binoculars such as the Zeiss Dialyts focus is achieved by moving the eyepieces away from the objectives.

If the focal planes of objectives and eyepieces are brought to coincidence then the virtual image would be at infinity.

The near -sighted and those of us with some accommodation can focus closer than that, so manufacturers' specifications can usually be bettered in practice.

On most modern waterproof roof prism binoculars the distance between objectives and eyepieces is fixed, so close focus is achieved by shortening the focal length of the objectives with an internal focussing lens.

This can be either +ve (converging), and is shifted towards the other objective elements for close focus, or -ve. (diverging) and is shifted away from the other objective elements.

There are, of course, mechanical constraints to both focussing systems.

 

John


Edited by John Russell, 03 March 2024 - 07:10 AM.

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#17 LoveWillSteerTheStars

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Posted 05 March 2024 - 01:31 AM

To answer your question, going by the lens equation, the theoretical minimum distance between object and focal plane is four times the focal length of your objective, i.e. exactly 2f on either side. But that would require you to have a focusing mechanism that increases your binoculars' total length by a factor of two, obviously not practical. It's a design choice, based mechanical / ease of use considerations, exactly where on that continuum to set the limit on outward travel of the eyepiece assembly.

And binoculars are about f4, right?  But how do you determine the units - meters, feet, parsecs?



#18 Mark9473

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Posted 05 March 2024 - 07:35 AM

He said focal length, so choose your units for that and the result will be in the same units.



#19 LoveWillSteerTheStars

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Posted 06 March 2024 - 02:19 AM

He said focal length, so choose your units for that and the result will be in the same units.

Binoculars are about f4 in what units?  Or is all this distance hypothetical, nothing needs to squeeze into the distanced between the optics in the frame?



#20 ihf

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Posted 06 March 2024 - 04:25 AM

Newton's form of the thin lens equation (scroll down)

 

xo*xi = f^2

 

with xo approximately being the minimum focus distance and xi being approximately the maximum eyepiece travel.

The eyepiece travel xi on a series of binoculars is about constant (say APM MS ED), but the focal length for a 50mm aperture bino goes up from say 150mm to about 370mm for a 100mm aperture bino (or some of the longer 100-120mm binos to f=500..600mm).

 

This simply means near focus distance roughly grows with the square of the focal length.

 

So, yes, large binos have much longer minimum focus. And long (f/5) binos have extra long distance minimum focus if travel wasn't increased a lot compared to small handheld ones, possibly (600mm/150mm)^2 = 4^2 = 16 times further distant minimum focus. Going the other direction down to small apertures like birding binos or especially the Papilio, the f^2 partially explains their near focus advantage.

 

leneq4.gif

 

 

 

IMO another good argument (in addition to the more difficult mounting) to not purchase one of the long f-ratio straight binos, especially if one is not normal sighted and interested in using them without glasses.


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#21 Rich V.

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Posted 06 March 2024 - 01:06 PM

IMO another good argument (in addition to the more difficult mounting) to not purchase one of the long f-ratio straight binos, especially if one is not normal sighted and interested in using them without glasses.

FWIW, the minimum focus of my f/6 22x70s is around 150', as an example...


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#22 LoveWillSteerTheStars

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Posted 06 March 2024 - 09:32 PM

Newton's form of the thin lens equation (scroll down)

 

xo*xi = f^2

 

with xo approximately being the minimum focus distance and xi being approximately the maximum eyepiece travel.

The eyepiece travel xi on a series of binoculars is about constant (say APM MS ED), but the focal length for a 50mm aperture bino goes up from say 150mm to about 370mm for a 100mm aperture bino (or some of the longer 100-120mm binos to f=500..600mm).

 

This simply means near focus distance roughly grows with the square of the focal length.

 

So, yes, large binos have much longer minimum focus. And long (f/5) binos have extra long distance minimum focus if travel wasn't increased a lot compared to small handheld ones, possibly (600mm/150mm)^2 = 4^2 = 16 times further distant minimum focus. Going the other direction down to small apertures like birding binos or especially the Papilio, the f^2 partially explains their near focus advantage.

 

leneq4.gif

 

 

 

IMO another good argument (in addition to the more difficult mounting) to not purchase one of the long f-ratio straight binos, especially if one is not normal sighted and interested in using them without glasses.

binoculars have an maximum eyepiece movement of about 1cm on a typical CF pair.

viewing target at 500m = 50,000 cm

sq rt of (50,000cm x 1cm) = 223.6 cm focal length = 2.235 m ?  when nearest focus

 

Far focus with CF closed down to 1mm away from the prism cover

sq rt of (50,000cm x 0.1cm) = 70.71 cm so 7.071 m ?   when close to infinity focus (infinity is zero).

 

Actually I guess I need to include the fixed distance from the Obj to infinity focused eyepiece, which is about what ? Any nearer focus adds up to 1cm. Do you measure the actual light path thru the prism distances ?  When you do this, does it come out to about f4 for everything from near to infinity?



#23 ihf

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Posted 06 March 2024 - 11:44 PM

I don't understand what you are trying to do. You made up some numbers for xi and xo and are trying to solve for f? I suppose you got a reasonable made up number for that. I suppose if you would actually measure the real values instead of guessing the computed focal length would match the original f better. But as for binoculars one basically multiplies infinity (xo) with zero (xi) solving for f is numerically unstable if focussed far away. Which means if you want to solve for f try to focus as close as possible and try to measure both xi (eyepiece travel from infinity focus to closest focus) and xo (closest focus from front lens) as precisely as you can (calipers and tape or laser). There might be better methods though.



#24 LoveWillSteerTheStars

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Posted 07 March 2024 - 02:34 AM

I don't understand what you are trying to do.

I need to include the fixed distance from the Obj to infinity focused eyepiece, which is about what ?  Do you measure the actual light path thru the prism distances

 

People say that binoculars have about an f4 lens, so I want the numbers to come out to about that.
 



#25 Jon Isaacs

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Posted 07 March 2024 - 05:55 AM

I need to include the fixed distance from the Obj to infinity focused eyepiece, which is about what ?  Do you measure the actual light path thru the prism distances

 

People say that binoculars have about an f4 lens, so I want the numbers to come out to about that.
 

 

It depends on the focal length, not the focal ratio.

 

As others have said, the close focus distance of a pair of binoculars is limited by practical considerations rather than theoretical considerations. 

 

In general, a good pair of 8x42 or10x42  roof prism binoculars will provide a close focus of around 6 feet. I use my 10x42s to get a good look at a laser collimator beam compared to the center marker on the primary mirror during collimation.  It is one of the few astronomically related uses for close focus binoculars. In the photo, the mirror is about 8 feet from the binoculars.. 

 

IMG_03092021_091814_(1024_x_700_pixel).jpg

 

Jon




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