Focus Overtravel (compatibility for the near-sighted) (1 Viewer)

[John Russell]

A recent discussion on another thread prompted me to try and measure the amount of focus overtravel in dioptres in various bins.
This is the amount by which the focus wheel turns beyond the infinity setting for the normally-sighted or those wearing glasses.
If the focus overtravel is limited, the severely near-sighted may not be able to reach infinity focus without glasses.

I should be grateful if anyone could provide a theoretical basis for equating focus settings to dioptres, but my initial thinking that a range for the naked eye (e.g. 5 m to infinity is 0,2 dioptres) could be multiplied by the binocular's magnification was obviously way off the mark.

However, I received a suggestion that I could simulate near-sightedness by stacking reading glasses (and adding their dioptre values) behind the eyepiece. I am somewhat far-sighted and needed my glasses for a precise infinity setting and could not accumulate sufficient dioptres for practical measurement. A hand magnifier of the type often used as a reading aid seemed to be a convenient alternative. Common values for the nominal magnification are 1,5x (6 dioptres) or 2x (8 dioptres). I determined the focal length of mine by projecting the image of a lamp on to a white sheet of paper. Mine came to focus at 17cm and allowing for lamp distance of 4 m would give a focal length of approximately 16,3 cm, i.e. 6 dioptres (the dioptre value is determined by dividing 100 by the focal length in cm).

My bins were set up on a tripod under the night sky and the infinity focus position was determined on a star using corrected vision. The positions for focus with the magnifier behind the eyepiece and for end of focus travel were also noted. I found it easier to estimate focus wheel rotation in minutes (as on a clock dial, not arcminutes!) rather than degrees and it was then possible to extrapolate the focus overtravel from the measured values. These were as follows, though I would not vouch for better than +/- 20% accuracy.

Kowa Genesis 8x33 -6,5 dioptres
Swarovski SLC 7x42 -8,5 dioptres
Meopta Meostar 7x42 -9,5 dioptres
Swarovski EL SV 10x42 -17 dioptres!!!

It would be interesting if others could add to the database.

John

 

[OPTIC_NUT]

Nice experiments!

 

[paul2013]

That sounds interesting, John. In my experience few binoculars have more than 5-6 diopters, though. My prescription in -5.5 (both eyes) and many of the binoculars I've tried in the past couple of years do not allow me to focus to infinity without my glasses. My Zen Ray 10x42 ZRS and Hawthorne 7x42 do, as well as my little Kowas 6x30 (just barely), but recent Olympus, Vanguards and other Leupolds don't.

 

[OPTIC_NUT]

There might be a problem getting stacked reading glass lenses to add up.
Nesting curvatures can lose some power. If you place the curve-out to curve-out
face-to-face you end up with more of the expected additional power and also less
distortion, as with a doublet loupe.

 

[kabsetz]

John,

This is interesting and valuable, but I also suspect that your estimated focus overtravel figures may be too high. Swarovski specifies 8 diopters for the 10x42 EL SV, and they are usually pretty good about data like this.

Kimmo

 

[John Russell]

John,

This is interesting and valuable, but I also suspect that your estimated focus overtravel figures may be too high. Swarovski specifies 8 diopters for the 10x42 EL SV, and they are usually pretty good about data like this.

Kimmo

Kimmo,

I repeated the measurements on the 10x42 SV in daylight today using the antennae on a building approximately 2 km distant to set infinity focus (using corrected vision, of course). The 6 dioptre hand magnifier required only a third of the available focus overtravel, which would give a value of 18 dioptres.
If the method is valid I think my measurements are more or less correct.

You drew my attention to Swarovski's quoted figure of 8 dioptres and they also quote a dioptre correction (right barrel) of +/-5 dioptres. Most bins have an index scale for the dioptre correction but the SV is unusual in having a numerical scale of +/-5. However, it extends to approximately +/-9 dioptres. It is unlikely that anyone would require this amount of dioptre correction or a focus overtravel of 18 dioptres, and these figures would look a little absurd in the specifications, so I think Swarovski are being very conservative with their published values.

Binocular manufacturers try to rationalize components within a binocular family and it looks as though most recent designs commonize objectives and prisms and use shorter focal length eyepieces for the higher magnifications.

I have taken some arbitrary figures for the sake of calculation and have assumed that both 42 mm SVs have objective focal lengths of 170 mm. The 8,5x would then have an eye piece focal length of 20 mm and the 10x one of 17 mm. If one assumed a focus overtravel of 10 dioptres for the 8,5x version, i.e. an apparent image distance of 100 mm instead of infinity, then the objective would have to form an image a mere 13,7 mm from the eyepiece. This would be 3,3 mm inside the focal plane of the eyepiece.

Now making a rather big assumption that both variants achieve infinity focus with the same setting of the focussing element, the focus overtravel would also place the objective image 3,3 mm inside the focal plane of the 17 mm 10x eyepiece, i.e. at 13,7 mm. This would place the apparent image as seen through the eyepiece at 70 mm distance, which is a focus overtravel of 14 dioptres. So I think we can assume that effective focus overtravel increases with magnification within a binocular family.

John

 

[kabsetz]

John,

Focus overtravel definitely increases with magnification, that I have seen pretty consistently with sibling models of different magnification. Swarovski also specifies larger overtravel for their 10x than their 8x models (7 and 5 diopters for the EL SV 32, 8 and 6 dpt for EL SV 42, 8 and 5 for SLC 56. Only for SLC 42 do they specify the same overtravel).

I suspect there may be something going a bit amiss with the 6 dpt hand magnifier method. If the diopter adjustment ranges and overtravel really would be as large as your results indicate, extremely few birders would have trouble with insufficient adjustment range.

I have a friend who does not use glasses while viewing, and who has myopia of about 5.5 dpt in one eye. He used to own the old 8.5x42 EL, and as his accommodation lessened with age, he became unable to use the binocular because he ran out of focus travel.

One method you may find useful for estimating focus overtravel and diopter adjustment range is seeing how many diffraction rings of defocus you can get beyond infinity with stars.

Kimmo

 

[MartinJ]

Hello, John


As nearsighted, I have a lot problems finding binoculars that I can use without glasses.

In addition, most manufacturers do not provide information about different models focus travel, so your initiative to create a database seems very positive.

I think your method is very interesting, and essentially correct. I performed the same test with my binoculars with a slight modification.

To avoid disputes over hand magnifier´s diopter I have used macrophotography close-up filters for my test. They have different diopter´s graduation predefined.

I have found that testing with glasses and close-up lenses I can nail the focus would need without glasses.

My contributions to the database:

Opticron 8x42 HRWP: 3,5-4 diopters Focus Overtravel.(This topic was already seen in Allbinos review).

Minox BV 8x42: 10-11 diopters focus overtravel.

Opticron Oregon 4 8x32: I have not thoroughly tested, but has a significant overtravel. I can focus at infinity wihtout glasses and -6 diopters close-up filter, so is -13 and theres more overtravel beyond this point.


I have yet to improve the focus wheel displacement measurement method, in order to get more precise measurements.

 

[OPTIC_NUT]

If you are nearsighted, this may be of interest to you:

I have ordered a batch of -5.0D (-200mm FL) lenses of 27mm diameter
that I can use as "ocular contact lenses", lightly sticking them over the oculars.
Most newer binoculars have a wide flat zone past the glass. 3 little dots
of flexible craft glue should work.

That will put all binoculars in range for me. I am at -4 D so 95% of binoculars
are in range but a few are not.

It's hard to find that value in a smaller diameter but I got some.
They haven't arrived yet but I'll let you know the results when they do.
Sticking -0.75 D lenses over the front would have the same effect.

 

[John Russell]

If you are nearsighted, this may be of interest to you:

I have ordered a batch of -5.0D (-20mm FL) lenses of 27mm diameter
that I can use as "ocular contact lenses", lightly sticking them over the oculars.

You need negative lenses to correct myopia, positive lenses to simulate it.
5 dioptres is a focal length of 20 cm or 200 mm.

John

 

[pompadour]

John, thanks for the thread. Its title says what I wanted to in the other but didn't because I thought "overtravel" /"overrun" etc. might be obscure to some.

Actually, for myself personally O_Nut provides a most useful answer there. It's more-or-less also here above when he says "... I am at -4 so 95% of binoculars are in range ..." I am presently betw. - 4.25 and 4.5 d R and about -4.0 d L and my short sght is lessening. Seems in a few months I should be there, too.

Kimmo, what is the source of the figure -8 d for the overrun in Swarovski? Where can one get such data for other makes? Thanks.

 

[OPTIC_NUT]

You need negative lenses to correct myopia, positive lenses to simulate it.
5 dioptres is a focal length of 20 cm or 200 mm.

John

Argh...sorry,. I corrected that. 200mm is -5D, yes. Hard to find.

He said he was nearsighted. I am nearsighted too. I want to put lenses on the eyepieces
that are close to my prescription. I am at -4.0D, Those are -5D. Close enough.

His tests involved using + lenses, I think because they are available, and jiggering the system
range. If he has positive correction, of course, the binocular add-ons would be positive.

 

[kabsetz]

Pomp,

I got the data simply from Swarovski's own website. In Technical data for each of their binoculars, they have the spec: Diopter correction at (infinity). Example of the page for the 32 mm EL SV's here.

http://aa.swarovskioptik.com/nature/el-32-c21010102/el-8x32-green-t5006129

As for other makers, I don't know of any that publish this data.

Kimmo

 

[John Russell]

what is the source of the figure -8 d for the overrun in Swarovski? Where can one get such data for other makes?

Pomp,

I don't want to hit Swarovski, because their products are among the very best on the market and I have three of them. However, although they are not disposed to exaggeration, their specifications are not always reliable.
The technical data that came with my Swaro scope quoted FoVs directly converted from metres into feet e.g. 42 m/138 ft (42 m @ 1000 m is the same as 42 yds (126 ft) @ 1000 yds), and a 2012 catalogue showed the transmission of their scopes as 86% with the 25-50x zoom eyepiece and 82% with the 20-60x and 30x W eyepieces, which is obviously implausible.
A Zeiss Conquest I once owned weighed, without accessories, 10% more than in the specs, so I think some of this data is about as reliable as ECE fuel consumption figures!

If you can't try before you buy then this thread is an attempt to provide a reproducible measuring method. There are still some doubts about its validity, but I think we'll eventually get there.

John

 

[pompadour]

Kimmo, don't know how I could miss those there - sorry! Wondering if they're very new - added in the last few days - and if so whether due to the fuss here in Bf!

John, thanks for the caveat - I tend to be dazzled and credulous. Hope you'll soon get your method perfect - i.e. as good as one can without special eqpmt. etc. And then there's that small matter of getting all the world's bins...

 

[OPTIC_NUT]

I'm back from testing the -5.0D "binocular contacts".
The view is well in range at infinity, and very clear.
It does push out may near focus, though. Maybe the near focus is about
that for most people but I've gotten used to nearer focusing.
They came off easily. Small drops of vinyl craft glue on bakelite.

 

[Surveyor]

John,

After our discussion I made a couple of measurements to illustrate the comments about the exit pupil and focal points.

The images show an 8d lens inside the exit pupil, 7d just barely inside the exit pupil, 6d and 5d outside.

Magnification was used to limit my eye error and accommodation to less than 0.2d.

The other images show the cheap lens set I use and the far more expensive dioptometer options. Also the micrometer slide I use for focal length changes.

 

[John Russell]

Here is the post I had been looking for http://www.birdforum.net/showpost.php?p=2810259&postcount=114, which enables one to relate dioptre change to focus travel. Ron also links to an older post showing focus speed for several bins. The binocular dioptre change is free space dioptre x magnification^2.

I reran the tests on the Swarovski 10x42 EL SV, this time taping a millimetre scale to the focussing wheel (97 mm circumference), which enabled a more accurate measurement of focus wheel rotation. Swarovski's close focus specification of 1,5 m was confirmed, measured from the objective plane, and 197 mm (731°) of rotation was required to reach infinity focus. Focus overtravel was 46 mm (171°).

Ron mentioned that he has yet to find a binocular with non-linear focussing, but I nevertheless confirmed the linearity of the Swaro by setting it up at precisely 3 m using a plumb line from the objective plane to the floor. This should give exactly half the dioptre change (1/3, 0,333 dioptres or 33,3 binocular dioptres) to infinity as the close focus value (1/1,5, 0,667 dioptres or 66,7 binocular dioptres). 3 m to infinity reqired 99 mm (367°) focus travel, almost exactly half.

The 10x42 SV has a focus speed of 11°/ dioptre and a focus overtravel of 15,5 dioptres. Allowing for the inaccuracies of my measurements of focus rotation in post #1, the results correlate quite well with those using a hand magnifier.

John

 

[kabsetz]

John,

Good of you to re-do the tests. Since the diopter adjustment has to work both ways, I assume your measured focus overtravel must be the sum of intended overtravel and intended diopter adjustment range. The specs are 8 dpt of overtravel and +-5 dpt of left-right adjustment for the 10x42 SV, which would mean that when the diopter correction is set at zero, there should be 13 dpt of overtravel. So, actually, the 15,5 does not sound as much more than the spec than I initially thought.

Kimmo

 

[John Russell]

John,

Good of you to re-do the tests. Since the diopter adjustment has to work both ways, I assume your measured focus overtravel must be the sum of intended overtravel and intended diopter adjustment range. The specs are 8 dpt of overtravel and +-5 dpt of left-right adjustment for the 10x42 SV, which would mean that when the diopter correction is set at zero, there should be 13 dpt of overtravel. So, actually, the 15,5 does not sound as much more than the spec than I initially thought.

Kimmo

Kimmo,

The EL SVs have -ve focussing elements, which can be observed from the objective end.
At the extreme of focus overtravel there was no further movement towards the right objective when applying -ve dioptre correction, though rearward movement could be oserved when applying +ve dioptre correction. Nothing broke either, so perhaps this could be explained by the infamous preloading .
Consequently the 15,5 dioptres of focus overtravel is, as you say, also the limit for the right barrel.

I mentioned above that the dioptre correction is scaled +/-5 but that approximately +/-9 of travel is available. I was able to verify this by regaining focus on the right barrel with the 6 dioptre hand magnifier at an interpolated dioptre correction setting of -6.

John