•

# Near out-of-focus point, program to determine

155 replies to this topic

### #1 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 18 May 2021 - 07:19 PM

I thought this should have its own thread..

The  question is:

---If I focus my binoculars at infinity

---How close can I then make a target before it gets noticeably blurry?

from a C program using the basic lens equation...

For various binocular powers / apertures:

nearfoc 6 30  \   Nearpoint:      14.60 meters       47.89 feet

nearfoc 7 35  \   Nearpoint:      19.80 meters       64.94 feet

nearfoc 7 50  \   Nearpoint:      28.30 meters       92.82 feet

nearfoc 10 50 \  Nearpoint:      40.20 meters      131.86 feet

*Most binoculars have a prime focal length of ~4 times the objective diameter.

*The eyepice focal length is    Prime-focal / power

*Perfect focus at infinity is where the FL of the objective and eyepiece meet

* A standard of the prime focal plane  being 1/20th of FLep from a perfect meeting (Fo + Fep)

triggers the out-of-focus standard. (for sharper standards I would just tighten that down).

* This is checking out pretty well with a 'Yardage Pro' laser rangefinder for distances..

So, to confirm observations, the 'near-point'  does indeed move out as power increases.

Why does the 7x50 have a farther near-point than the 7x35?  Because the F4 rule makes the 7x50 longer..

...the longer the binocular Fl, the more error as you get closer.

But why did I choose to set infinity and then look for the near out of focus line?

Why not pick the special camera mean distance to start?   (te hyperfocal distance)

1)  It doesn't shift the near point much

2)  It's far easier to set your binocs for "far away' (practically infinity)

than for the hyperfocal distance...you would need a rangefinder on the side.

So it's practicality that makes me set my 6x30s to infinity and then troll the yard.

I found the secret of why they work so much closer than 7x50s:  the 7x50s simply have an extra-long objective FL.

Now my 10-yr itch is scratched.

Edited by MartinPond, 19 May 2021 - 07:31 AM.

• DeanD, j.gardavsky and MrZoomZoom2017 like this

### #2 TOMDEY

TOMDEY

Cosmos

• Posts: 9,717
• Joined: 10 Feb 2014
• Loc: Springwater, NY

Posted 18 May 2021 - 08:46 PM

Mmmm... yes, as far as it goes. The depth of focus is bases on allowing the geometric impulse response size to match the ideal diffraction impulse response size (Airy Core) more or less... with some agreed close to unity fudge-factor thrown in. All well and good.

For visual-use binoculars this fails to take into account one's personal dioptric accommodation. So it applies to geriatrics (like most of us here) but young people will be able to see even closer, based on their age --- although they will indeed ~have to strain~ a bit to do it.    Tom

Closely-related... Optometrists have a heck of a time refracting kid's eyes, because they are so profoundly accommodating that they can focus on almost anything. It's only when their eyes are so severely near of far sighted that it becomes problematic. So the child may be straining, but still able to read the Snellen chart just long enough to pass the test. This happened to my sister in grade school. The teachers even accuse her of faking difficulty reading blackboard... and she went without sorely-needed glasses for years before a qualified optometrist discovered her severe myopia.

• ECP M42 likes this

### #3 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 18 May 2021 - 10:31 PM

Accomodation   can help with this....just a little.

It can never solve the range, only nudge things a little closer.

What is a healthy 5 diopter shift compared to a 55-diopter eyepiece?  Not much. Not enough.

Moves the near-point in just a littlle.

Airy core?  That is a tiny fraction of the cirle of confusion caused by the objective image

being  1-2 mm out from where you want to be.  That's a very different ballpark.

It's also a totally different  thing than the Airy limit.  Diffraction, not refraction.

Take a kid's eyes and tell me they can eliminate all that mismatch.

It's not going to happen.  It's not a magic spell.

An experiment: remove the eyepiece....now, can the child focus without it?

It's the simple focusing of two lens groups.

Tossing around furniture that is made of more minor effects does not change the 500-lb gorilla.

Range is based on focus.  1/S1 + 1/s2 = 1/f .

Edited by MartinPond, 18 May 2021 - 10:37 PM.

### #4 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 20 May 2021 - 05:41 AM

Two more examples, for extreme in-focus-range (Depth Of Focus):

D:\all_prog\tcc\tcc-0.9.26-win64-bin\tcc>nearfoc 5 12
Nearpoint:       4.90 meters       16.07 feet

D:\all_prog\tcc\tcc-0.9.26-win64-bin\tcc>nearfoc 5 10
Nearpoint:       4.10 meters       13.45 feet

The first is for a 5x12 low-vision aid.

Focusing it at infinity, the view will look sharp down to 16 feet.

The second is for the Zeiss   5x10 T*   MiniQuick.

Focusing it at infinity, the view will look sharp down to 13.45 feet.

In case you were wondering "what use is the Zeiss 5x10 MiniQuick?"

No squirrel, sparrow, grand-daughter, or shop window can escape your attention.

Close-up, no art piece eludes you.

5x10 achieves your missing accomodation, too.

Edited by MartinPond, 20 May 2021 - 05:44 AM.

### #5 John Russell

John Russell

Lift Off

• Posts: 19
• Joined: 30 Jul 2009
• Loc: Cologne, Germany

Posted 20 May 2021 - 11:02 AM

Depth of field is solely a function of magnification. It has nothing to do with objective diameter or focal ratio.

A telescope or binocular, as opposed to a camera lens, is an afocal device.

The image of a viewed object is formed by the objective and the eyepiece is racked in or out to place this image in its focal plane. Rays from the image emerge parallel from the eyepiece for the normally sighted and for the eye are effectively at infinity.

Point sources in front of or behind the viewed object will form circles of confusion but their apparent size is only a function of the relationship of objective and eyepiece focal lengths, i.e. magnification.

Doubling the magnificaion involves halving the eyepiece focal length. The circle of confusion appears twice the diameter and four times the area.

Depth of field is inversely prportional to the square of the magnification.

Of course, accommodation or even field curvature can affect the perceived DoF and an exit pupil smaller than the eye's pupil would effectively increase the eye's DoF but that, at least for terrestrial use, is an undesirable condition.

John

• Rich V., Pinac and oldmanrick like this

### #6 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 20 May 2021 - 12:13 PM

A telescopic system is only afocal

when you focus at a faraway object for a perfect relaxed-eye

virtual image (at infinity).

When then look at a closer object without refocusing, it is  in point of fact,

and by definition,  NOT afocal anymore....far from it.

In binoculars:

----a larger diameter causes them to use a longer focal length (binoculars are designed at F4..this means the FL increases)

----a longer focal length necessarily means a more displaced image IF YOU DO NOT REFOCUS FOR A CLOSER TARGET

So, the range that the image stays within a certain focus sharpness is definitely reduced.

Observations bear that out for me.

"A telescope or binocular, as opposed to a camera lens, is an afocal device."

...It is an afocal device only when you focus on a target at infinity to produce

a virtual image at infinity.    As you say, "at infinity".

Please see the true definition of "afocal".

If you DO NOT REFOCUS and look at a closer target, it is no longer an afocal system...at all.

I refer specifically to the range that appears in-focus without refocusing.

Does the circle of confusion increase with power?

Sort of, but at the very small and gentile dimensions of  aberrations and diffraction.

If it has to do with being out of focus.....the circle of confusion expands radically

according to how far off your  primary image plane is from the eyepiece's focal plane.

Magnified point sources are complely lost in the soup

when you are 5% away from you eyepiece's focal plane.   Look.

I can repeat the results in a measurable way  on a telescope:

1)  I mount a 2" 40mm eyepiece in my 80x400

2)  I focus on a scene 300 yards away

3) I swing to look at a tree trunk 45 feet away

4) It is terribly blurry

5) To regain focus  I have to move my marked Crayford focuser 2.8--->3.7 cm.

--->the focal plane of the object has in fact moved by 0.9 cm

6)  I repeat 1 through 5  with the objective masked to 40mm

....and the travel needed to focus is the same,  0.9 cm.

7) I repeat with a 20mm eyepiece, for twice the power

....the image when first swung to the near target is even blurrier

....it takes 1.1 cm travel to find a sharp image again.

Please note that the difference in eyepiece focal lengths is :  20mm , or 0.2cm.

8)  I repeat with the 20mm eyepiece in an 80x720 telescope .

....it takes a whopping 3.8cm, or 38mm , to refocus

Did it depend on power?          yes, but mainly by the focuser travel difference per eyepiece

Did it depend on focal length?  yes, whoppingly, yes!

Focus long, swing to short, and look....just look, please.

Accommodation means very little compared to a 20mm eyepiece

that is 9mm off from perfect focus.  Acommodation is in the wrong ballpark to fix that.

Try the experiment with and without glasses: it means almost nothing.

Tested 3 more times, proven 3 more times:

---> Objective Focal length is in fact the main factor limiting in-focus range.

For binoculars, the objective diameter has effects but

only because the object focal length  is, by design, ~4 times the diameter.

And, once again,  a telescope or binocular system is ONLY afocal when you

focused with a relaxed corrected eye on a distant object.

It means very little when you do not retouch the focus and swing to a near object..

.that is now a very, very non-afocal situation.

Defocusing is, in fact, due to the system becoming non-afocal.

Edited by MartinPond, 20 May 2021 - 12:26 PM.

• j.gardavsky likes this

### #7 John Russell

John Russell

Lift Off

• Posts: 19
• Joined: 30 Jul 2009
• Loc: Cologne, Germany

Posted 20 May 2021 - 02:42 PM

Is there confusion here between depth of focus and depth of field?

The longer the focal length the greater the depth of focus because of the shallower light cone.

Depth of field though is only dependant on magnification.

Try it if you can attain the same magnification with two instruments of differing focal ratio.

John

### #8 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 20 May 2021 - 04:11 PM

Is there confusion here between depth of focus and depth of field?

The longer the focal length the greater the depth of focus because of the shallower light cone.

Depth of field though is only dependant on magnification.

Try it if you can attain the same magnification with two instruments of differing focal ratio.

John

The range of field that is in focus when you focus on infinity.

is definitely not dependent on power only.  It is, as proven,

tied most to main focal length.

I actually did test with 2 instruments of differing focal ratio.

(same FL)

IT makes barely any difference at all.

Edited by MartinPond, 20 May 2021 - 04:11 PM.

Messenger

• Posts: 433
• Joined: 31 Mar 2004
• Loc: Greensboro, NC

Posted 20 May 2021 - 06:19 PM

Martin,

What was your pupil dilation when you conducted these tests?

### #10 TOMDEY

TOMDEY

Cosmos

• Posts: 9,717
• Joined: 10 Feb 2014
• Loc: Springwater, NY

Posted 20 May 2021 - 06:57 PM

Accomodation   can help with this....just a little.

It can never solve the range, only nudge things a little closer.

What is a healthy 5 diopter shift compared to a 55-diopter eyepiece?  Not much. Not enough.

Moves the near-point in just a littlle.

Airy core?  That is a tiny fraction of the cirle of confusion caused by the objective image

being  1-2 mm out from where you want to be.  That's a very different ballpark.

It's also a totally different  thing than the Airy limit.  Diffraction, not refraction.

Take a kid's eyes and tell me they can eliminate all that mismatch.

It's not going to happen.  It's not a magic spell.

An experiment: remove the eyepiece....now, can the child focus without it?

It's the simple focusing of two lens groups.

Tossing around furniture that is made of more minor effects does not change the 500-lb gorilla.

Range is based on focus.  1/S1 + 1/s2 = 1/f .

Well Martin --- you were batting 1000 until you visualized that curve ball. Yes indeed, a child can easily focus without the eyepiece, provided he is a few inches or more back from the image and refocuses his eye:

Range to image     accommodation        age able

40 inches                    1.0 diopter             60 years

20                                2.0                        53

13                                3.0                        47

10                                4.0                        44

8.0                               5.0                        42

6.7                               6.0                        39

5.7                               7.0                        36

5.0                               8.0                        32

4.4                               9.0                        28

4.0                             10.0                        24

3.6                             11.0                        20

3.3                             12.0                        16

3.1                             13.0                        12

2.9                             14.0                          8

I did some casual rounding of the numbers, but you can see the trend. And I also experimented with that back when I was... eight years old! I had noticed that I could easily focus "razor-sharp" on the plain vanilla peep-sight on dad's rifles and then the down-range target almost instantaneously, albeit not simultaneously. So my shooting was more accurate than dad's unless he used the cheater telescopic sight. I just figured his eyes were bad because he was old. And my judgment turned out to be ~spot-on~    Tom

PS: Try using a telescope with the eyepiece pulled entirely out. You will see the star field just fine, but the AFOV is miniscule, roughly defined by the F# feeding the image.    Tom

### #11 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 20 May 2021 - 07:46 PM

"

PS: Try using a telescope with the eyepiece pulled entirely out. You will see the star field just fine, but the AFOV is miniscule, roughly defined by the F# feeding the image.    Tom

"

At this point, Perry Mason says:

"objection... your honor, the question is irrelevant ans immaterial"

Because for you, and for the child, the tiny field and the performance is unacceptable.

The child, with no eyepiece, would produce vey little gain with only 14 diopters.

Field: as you say, tiny.

Power: as per you numbers, not worth it

Exotic conditions make this a red herring.

The lack of an eyepiece makes it not even a telescope at all.

I can stand behind my scope without an eyepiece and see a tiny upside down image.

That's useless, and has a totally different mechanism.

Shame.

Curve ball? You threw a boomerang and were hit by your own evidence.

### #12 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 20 May 2021 - 08:29 PM

Is there confusion here between depth of focus and depth of field?

The longer the focal length the greater the depth of focus because of the shallower light cone.

Depth of field though is only dependant on magnification.

Try it if you can attain the same magnification with two instruments of differing focal ratio.

John

I did a whole series, with masking.

I will repeat results, since this thread is the right place for it.

It was   looking through a 7x50 barrel,

with the

the 50mm objective  (F4)

Note that all magnifications were 7x ... the same binoculars, but masked.

I could barely notice a difference, maybe a slightly in-focus limit at F12.

I was actually shocked, since I had believed the aperture mattered a lot before that.

It was during daylight, overcast, forest.

The 12mm F12 mask looked pretty dim....but...not much effect.

### #13 ECP M42

ECP M42

Ranger 4

• Posts: 323
• Joined: 28 Apr 2021
• Loc: central Europe 45°N

Posted 20 May 2021 - 08:54 PM

Depth of field is solely a function of magnification. It has nothing to do with ... focal ratio.

And the reason would be?

### #14 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 20 May 2021 - 10:05 PM

Martin,

What was your pupil dilation when you conducted these tests?

Are you stating there is no legitimacy until I measure my pupils?

Perhaps you have actual data about exactly how it matters, as opposed to

simply blurring someone else's arguments and observations.

I see others doing tests and referring to articles ..... earning the case.

The lighing conditions I use are the ones that matter., because DOF does not matter at night.

Here's the thing :

I have conducted experiments in overcast daylight and dusk.

The depth over which the images look sharp enough is a daylight matter, isn't it?

At night, depth of focus has no relevance....all the stars and planets and Moon

are effectively at infinity.

My viewing conditions and pupils are relevant to the times when focal range matters.

Do my pupils open up at night?  It doesn't matter when looking at the sky, does it?

Edited by MartinPond, 20 May 2021 - 10:28 PM.

### #15 John Russell

John Russell

Lift Off

• Posts: 19
• Joined: 30 Jul 2009
• Loc: Cologne, Germany

Posted 21 May 2021 - 06:25 AM

I did a whole series, with masking.

I will repeat results, since this thread is the right place for it.

It was   looking through a 7x50 barrel,

with the

the 50mm objective  (F4)

Note that all magnifications were 7x ... the same binoculars, but masked.

I could barely notice a difference, maybe a slightly in-focus limit at F12.

I was actually shocked, since I had believed the aperture mattered a lot before that.

It was during daylight, overcast, forest.

The 12mm F12 mask looked pretty dim....but...not much effect.

If you view through an 8x56 with f/4 objective in daylight it effectively becomes f/14 because the 2 mm eye pupil stops down the binocular's exit pupil and you are then only using the central 16 mm of the objectives.

My suggestion for a comparison was more like this: a 15x56 binocular (f/4) vs a Takahashi FOA-60 (f/8,8) with 35 mm eyepiece, also 15x  magnification.

The depths of field would be identical.

John

Messenger

• Posts: 433
• Joined: 31 Mar 2004
• Loc: Greensboro, NC

Posted 21 May 2021 - 06:42 AM

Are you stating there is no legitimacy until I measure my pupils?

Perhaps you have actual data about exactly how it matters, as opposed to

simply blurring someone else's arguments and observations.

I see others doing tests and referring to articles ..... earning the case.

The lighing conditions I use are the ones that matter., because DOF does not matter at night.

Here's the thing :

I have conducted experiments in overcast daylight and dusk.

The depth over which the images look sharp enough is a daylight matter, isn't it?

At night, depth of focus has no relevance....all the stars and planets and Moon

are effectively at infinity.

My viewing conditions and pupils are relevant to the times when focal range matters.

Do my pupils open up at night?  It doesn't matter when looking at the sky, does it?

As John has pointed out, in this test your eye's pupil acted as a stop that effectively equalized the apertures of all your masks (except possibly the 12mm mask) down to some unknown smaller aperture size. I made a series of photos of a defocussed artificial star through several binoculars that demonstrates how this works. I'll post them later today.

Edited by Henry Link, 21 May 2021 - 06:53 AM.

### #17 Echolight

Echolight

Vanguard

• Posts: 2,327
• Joined: 01 May 2020
• Loc: Texas

Posted 21 May 2021 - 07:11 AM

Maybe I don't always have my binoculars focused exactly right, but it seems that to go from getting the finest detail on the Moon, to getting the smallest point of light on a star, I have to slightly change focus.

I should probably reassess the situation again. But it seems for astronomy I am more often fiddling with fine focus, than I do when say in the daylight, for general scanning, I focus for 400 yards and go between 400 and 100 yards when not much if any refocusing is required. But.... if I find something of great interest at say 150 yards, then I will often attempt to retune the focus to match that exactly.

So I guess, while the accommodation for a fairly sharp view over the distance is there, it seems I am always searching for some more perfect view with tiny adjustments in focusing. Although I rarely use extremely short binoculars with aperture smaller than 42mm.

Edited by Echolight, 21 May 2021 - 07:12 AM.

### #18 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 21 May 2021 - 08:04 AM

As John has pointed out, in this test your eye's pupil acted as a stop that effectively equalized the apertures of all your masks (except possibly the 12mm mask) down to some unknown smaller aperture size. I made a series of photos of a defocussed artificial star through several binoculars that demonstrates how this works. I'll post them later today.

If this is true, it is true for all normal daylight use.

Which is the only time the in-focus depth matters.

Question:  what is the entrance pupil for your photos?

### #19 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 21 May 2021 - 08:47 AM

One small step at a time, maybe,

without confounding, less significant,

2nd-order effects.....

All invidual lenses (of not-to-severe strength)

form an image at the location indicated by

what is called   "the lens equation".:

If the distance to the object, "o" , is very high (practically infinity),

the image forms at distance "f" , that is, at the focal length

of the objective in this case.

Q1:  Yes or No?

To focus at infinity, we move the eyepiece to where its focal plane

coincides/meets  with the image location (at "i").

Q2: Yes or No?  (is is the way we focus on a star?)

...by the way, at this fleeting moment, with the object and

final eyepiece image at infinity, the system is "afocal"...

Now, let us say the focal length "f" is 200mm,

and,  let the distance "o'" be 20000mm (20 meters, nearby).

Where will the image be formed?

The lens formula says that   1/f - 1/o = 1/i

Q3: Yes or no?

So,  1/200 - 1/20000 =  .005 - .00005  = 0.00495

and thus, i =   202.02 mm (which is 1/0.00495)

(NOTE: the distance to the image has moved out to 202mm, from 200mm)

Q4: Yes or No?

Now, because we have focused with the eyepiece image plane at 200mm,

and not re-focused, the image plane of the objective has pushed 2mm into

the eyepiece's 20mm focal length.

Q5:  is this true.... Yes or No

Does anyone  disagree with the image locations in this, post #19.

Edited by MartinPond, 21 May 2021 - 11:14 AM.

• ECP M42 likes this

### #20 Echolight

Echolight

Vanguard

• Posts: 2,327
• Joined: 01 May 2020
• Loc: Texas

Posted 21 May 2021 - 09:31 AM

So "F" appears to be like a bank shot with a smidge of English.

### #21 ECP M42

ECP M42

Ranger 4

• Posts: 323
• Joined: 28 Apr 2021
• Loc: central Europe 45°N

Posted 21 May 2021 - 10:00 AM

So I guess, while the accommodation for a fairly sharp view over the distance is there, it seems I am always searching for some more perfect view with tiny adjustments in focusing.

This also happens to me, especially with the 10x, but I think it happens a little to everyone.

The search for a sharper focus on a object of interest is normal: our eyes continue to refocus every time we shift our gaze on the objects we observe.

A higher DOF should help to avoid refocusing various objects with the hand, every time, and to use only the accommodation capacity of the eye.
Since the deep field of the DOF depends on various factors, including the distance of the object, it is always possible to find the best position of the focus, to better serve that area of interest (eg: from 20 to 200ft).

The magnification of the binoculars should be chosen based on the distances of the objects and based on the DOF that we will see with our eyes (the eye is also a factor).

With all these factors at play, each of us may prefer binoculars that are different from the others.

Edited by ECP M42, 21 May 2021 - 10:01 AM.

### #22 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 21 May 2021 - 11:06 AM

So "F" appears to be like a bank shot with a smidge of English.

here:

What does the letter on the right look like?

It could be worse, though.

So far, no one elese seems to recognize the lens equation at all.

The mid-term optics 101 test score might be: F

Edited by MartinPond, 21 May 2021 - 11:33 AM.

• j.gardavsky and MrZoomZoom2017 like this

### #23 Echolight

Echolight

Vanguard

• Posts: 2,327
• Joined: 01 May 2020
• Loc: Texas

Posted 21 May 2021 - 11:34 AM

Is this an ad-hominem attack ?

Or.....do you not know what the letter means?

Gimme a second. I have to look that up. But I don't think it was an attack of any kind. Just my potentially inaccurate interpretation of the sketch. I often hear of light rays bending... like Beckham? But I tend to think of them as bouncing, with maybe either the ray or possibly even the surface being softer and absorbing part of the impact.

Anyway. My apologies if I have offended. My intent was not to project spit wads at your chalk board.

I guess I was not looking at what "F" actually was. But instead the path of the light traveling through the doublet objective..

Carry on.

Edited by Echolight, 21 May 2021 - 12:00 PM.

### #24 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 21 May 2021 - 12:00 PM

Sorry...more miffed at those who do not

respond to the first chapter of optics primer, I guess.

Might as well go on from post #19, I suppose.

So, because of the lens equation we know that our infinity-focused

binocular/telescope has had the objective's image plane

shoved 2mm past the 20mm eyepiece's focal plane, towards the EP.

So what happens?

Whether you trim the opening to F5 , F8 , F12 , or F20,

you have an eyepiece looking at an image stuffed towards it rather severely.

If you hold your eye where you did before, there is just profound blurring.

You can focus though:  you have to move your eye about 6+ inches back,

and what you see is a tiny portion of a truncated view.

As the lens equation predicts for the eyepiece.

All this happens when I set the opening for F25!

The de-focusing caused by 1/o + 1/i = 1/f  is way beyond eyes, irises, masks.

It dwarfs their effects.

The power does have an effect, but it is 2nd place behind the focal length.

This is because shifting to a closer object pushes the prime focus 2mm into a 20mm EP fl,

And at half the power it is pushing prime focus 2mm into a 40mm EP fl....not so bad.

Edited by MartinPond, 21 May 2021 - 12:06 PM.

• j.gardavsky likes this

### #25 MartinPond

MartinPond

Fly Me to the Moon

• topic starter
• Posts: 6,241
• Joined: 16 Sep 2014

Posted 21 May 2021 - 12:46 PM

Ah....I was able to get the apertue to make focus happen!

With a 25mm EP on my 80x400 short scope,

---I focused sharply on a tree trunk 65ft away

(makes it less challenging than an infinity start)

---I swung the scope to a tree 45 ft away..

It was blurry, of course, and the scope fully-open was F5.

So I tried a        40mm/F10 mask:   didn't look any clearer.

Then I tried the 27mm//F15 mask:  might be a little better....can't tell.

And then,     a   10mm//F40 mask: boom....sharp!

So, if I reduce the challenge (less range) and radically reduce the apertur/raise the F-ratio,

I can improve the focal range.   Doesn't seem very usable, though...so dim.

-------------------------

So, I might as well change the power, correct?

I have the results for EPfl 25 / or 16x above.

So, let's go to an EPfl of 10mm, for 40x  power:

----focused at 65 ft, swung it so 45 ft target ....same way

----F5 and F15 were still fuzzy

----it still took F40 to clean up the image..

------------------------------

And for the sake of exhaustive research:

---focus at 65ft

---evaluate view at 45 ft

6x30     65ft-sharp     45ft-sharp    (180mm FL)

7x50     65ft-sharp     45ft-slightly out of focus  (200mm FL)

10x50      65ft-sharp    45ft--a little more out of focus

------------------------

So, power doesn't affect the range noticeably at the same FL...

More power that comes along by design due to a longer FL does affect range....

Aperture/F-ratio, with no length change...

does improve in-focus range, but only with severely-reduced aperture / increased F-ratio

At least that's how it tests out...

I suspect there is a trap where longer binocular barrels that come with more power

give the impression that more power is by itself the one source of range reduction.

Edited by MartinPond, 21 May 2021 - 12:54 PM.

• j.gardavsky likes this

## Recent Topics

 Cloudy Nights LLC Cloudy Nights Sponsor: Astronomics