Focussing: Just Do It! (1 Viewer)

[Binocollector]

Yes, that's what it means. The sharp field of view of human vision is only 0.5 to 1 degree wide depending how strictly you define sharpness. This means, in your typical binoculars with 60-degree apparent field of view, only 1/3600 of the visible area is seen sharply at any given time!!

Very interesting, so I guess my thinking "a larger FoV is always better, even if not perfectly sharp to the edge" is based in fact. And as long as moving the binoculars and not rolling around the eyes in the FoV -- the bluriness of the outer field wouldn't even matter, right?

 

[Omid]

Very interesting, so I guess my thinking "a larger FoV is always better, even if not perfectly sharp to the edge" is based in fact. And as long as moving the binoculars and not rolling around the eyes in the FoV -- the bluriness of the outer field wouldn't even matter, right?

An excessively large field of view will interfere with your perception of horizon. In the natural world, your peripheral vision constantly monitors the position of ground relative to your body. If you look at the world under magnification, your peripheral vision will see (more correctly, sense) a virtual ground whose orientation and location have little to do with the position and orientation of the real ground. As you move the binoculars around, the virtual ground will move and tilt very rapidly in your field of view while the gravitational sensors in your ear do not register such movements. This will cause fear, anxiety and stress not the pleasant "wow feeling" that you might expect. There is no such a thing as an immersive magnified world.

-Omid

 

[Binocollector]

@Omid
Hm, interesting. I never had that effect however. 7x35 with 11° or 8x30 with 10° seems just about perfect to me. But I also never get carsick or seasick. So others might have a different experience with super wide angle binos.

 

[Troubador]

An excessively large field of view will interfere with your perception of horizon. In the natural world, your peripheral vision constantly monitors the position of ground relative to your body. If you look at the world under magnification, your peripheral vision will see (more correctly, sense) a virtual ground whose orientation and location have little to do with the position and orientation of the real ground. As you move the binoculars around, the virtual ground will move and tilt very rapidly in your field of view while the gravitational sensors in your ear do not register such movements. This will cause fear, anxiety and stress not the pleasant "wow feeling" that you might expect. There is no such a thing as an immersive magnified world.

-Omid

But how do you define 'excessive' ?

Lee

 

[Binastro]

Pilots lose all sense of position and orientation.

They have to be taught to fly by instruments.

But if these instruments are faulty the result can be fatal.

I was in the cockpit of a Trident going down from about 20,000 ft into the Thames estuary.
The pilots could not understand why I could not get my bearings, even though I was flying over our holiday home.

It took several minutes until I understood the ground beneath me.

Our senses are very flawed.

B.

 

[Omid]

Hi Lee!

But how do you define 'excessive' ?

Lee

Excessive field of view is when the eye doesn't positively see the field stop as a "frame" surrounding the "magnified view".

@Binastro: Very nice story! It proves my point

 

[Troubador]

Pilots lose all sense of position and orientation.

They have to be taught to fly by instruments.

But if these instruments are faulty the result can be fatal.

I was in the cockpit of a Trident going down from about 20,000 ft into the Thames estuary.
The pilots could not understand why I could not get my bearings, even though I was flying over our holiday home.

It took several minutes until I understood the ground beneath me.

Our senses are very flawed.

B.

I think this happens to some drivers too......

Lee

 

[Charlie Yardbird]

I need a clearer understanding of how eyeglass prescriptions are written, and how that relates to focusing binoculars in the "nearer" and "further away" directions.

When you turn the CF wheel such that the shaft extends, that moves the focus "nearer."
Usually that is CCW.

I find that the diopter knob works the same way: CCW (from a top view) brings the focus "nearer."
I see that the knob is marked "minus" in that direction.

So "more minus" equals "nearer."

I am nearsighted, and my eyeglass prescription "add" is written as minus-something diopters. Let's say -2 diopters for the left eye..

IIRC, opthalmologists and opticians write a prescription in opposite ways. The opthalmologist writes down whether your eyeball is plus or minus. Then the optician writes the opposite, because the glasses need to be the opposite of your eyes.

So to focus binoculars, don't I need to know if my EYES are plus or minus, and adjust the binoculars in the opposite direction in order to get to zero (neutral) ??? Right ???

So what is correct: If I focus with my glasses on, and then take them off, I will need to re-focus in which direction? Its night so I can't see anything further than the minimum focal distance indoors & that doesn't show any difference.

Another item: if I have my glasses on, my vision is at "zero correction needed." At what adjustment are the binoculars at "neutral" -- shaft fully inwards or fully extended or halfway?

Diopters are not linear, so if you start at the nearest focal distance, how much distance does each diopter of "further away" adjustment on the CF wheel add to the focal distance? Until you get to "infinity" or I guess more accurately, the hyperfocal distance ? I don't understand why when I focus on something several hundred yards away, I still need to change the focus to look at things even further away. Is it because my old nearsighted eyes lack accommodation?

How do binoculars work for people who have artificial lenses giving them no accomodation?

My only glasses are progressive, so its hard to experiment with on vs off because the diopters depend on exactly where I look thru them from top to bottom.

 

[Binocollector]

If you are short sighted you need more focus "leeway" at infinity, so in case of external focusing - the ocular needs to be able to travel closer to the body. On some models there is barely enough "wiggle room". Even with my relatively moderate prescription (near sighted) some models just barely work for astronomy without glasses. Fun fact - when switching from observing the moon to the stars, I need to readjust the focus.

 

[Maljunulo]

Fun fact - when switching from observing the moon to the stars, I need to readjust the focus.

Any idea why?

 

[Dyrlege]

The stars are way farther out?

 

[Binocollector]

The stars are way farther out?

Exactly. I only think it's remarkable because in astronomy people like to say, "the stars are at infinity focus" -- the moon obviously isn't. So depending on the bino, I need to refocus, even if only slightly.

 

[Maljunulo]

I never did that with binoculars or telescopes.

I focus on a star, then move to the moon. It works every time, and fiddling the focus never did improve it.

Add me to the “If it’s at infinity, it’s at infinity.“ crowd.

 

[Binocollector]

When I start with the moon, as I did yesterday, I need to refocus. When I switch back to the moon from the stars, I notice that the moon is slightly overfocused. I haven't checked this with every single magnification though. Yesterday I used an 8x20 - not really ideal for stargazing, I just wanted to check alignment and sharpness of my Tasco skeleton bino.
On my 7x50 Fuji, I never fiddle with the focus when using them at night.

 

[Dyrlege]

The moon is literally only light-seconds away, the closest star 4,3 light years, most of them hundreds of light-years to infinity (that's including clusters of stars and galaxies. To the closest star that's a factor of 135 million +. You need to refocus, even though the focus changes like the tail of a hyperbolic curve.

 

[wllmspd]

Anything beyond a few miles is probably essentially infinity, though you can be more precise, for visual it depends on your visual accommodation as well. I was out with a group at the weekend and offered people to look though the scope (not to infinity) and found many people don’t even try to refocus, even when I prompted people about where the focusser was…. Are their eyes as messed as mine, are they being polite???

Peter

 

[Dyrlege]

Anything beyond a few miles is probably essentially infinity, though you can be more precise, for visual it depends on your visual accommodation as well. I was out with a group at the weekend and offered people to look though the scope (not to infinity) and found many people don’t even try to refocus, even when I prompted people about where the focusser was…. Are their eyes as messed as mine, are they being polite???

Peter

It depends. Many people seem to not care about the resolution they get in a "picture" in an optical device, and will accept it. Some of the birdwatchers on this forum do not belong to that group. I totally agree that the difference in focus between the moon and the stars is minimal, which was the point mentioning the hyperbolic curve, but these will (want to) notice the "problem", as will I. It is but a detail, and does not detract from the image of a star.

 

[Binastro]

Our eyes have CA, so the colour of the Moon and stars, although seemingly the same are often slightly different.

So some eyes may need very small adjustment.

In addition, the Moon is an extended object and the stars essentially points.

The actual different in focus to the Moon or stars is essentially zero.
Some astro scopes and lasers can focus on the 90km layer above us, but I doubt that the largest scope will detect the Moon star difference, although I may be wrong.

Focusing on a point may give a slightly different result to the markings on the Moon with binoculars.

I don't have to change focus from stars to the Moon, but often the two sides of a binocular are slightly different as stars can be critical, especially if ones glasses are not quite up to date or if one is tired.

Incidentally, on one of the specially made 20x60 USSR binoculars made for a dozen astronomers that were hand made to high tolerances I always saw the Moon in full 3D even though it obviously isn't.
I don't know the reason.

Red stars have a different focus to white stars.

This is particularly obvious on astro photos even with good scopes or lenses.

As to all the other questions above, including focus in the dark, then just focus first with some available light.

As I think the thread says, just do it.

After long periods of observation a small change in focus is helpful.

Regards,
B.

 

[Binocollector]

I suspect brightness might be a factor. I definitely need to check this again with other binos. Haven't been doing much stargazing lately. One reason was almost constant clouds during the winter time.

 

[tenex]

So to focus binoculars, don't I need to know if my EYES are plus or minus, and adjust the binoculars in the opposite direction in order to get to zero (neutral) ???

So what is correct: If I focus with my glasses on, and then take them off, I will need to re-focus in which direction?

There is no "neutral" on a binocular, so you don't need to know anything about your eyes, just use the focuser. It's not there to correct your vision, but to help your eyes cope with the magnification, which requires 8 or 10x as much accommodation as unaided vision. This is the answer to most of your questions.

I don't understand why when I focus on something several hundred yards away, I still need to change the focus to look at things even further away. Is it because my old nearsighted eyes lack accommodation?

Most everyone needs to do that (except children?), once again because the magnification of binoculars requires more accommodation.