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Focusing problem

so2010

By so2010
in Discussions - Binoculars

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so2010

so2010

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Hi!

I got Olympus 8x40 binoculars, and while the focus on the right side is razor sharp, I can't get the perfect focus on the left. It's very close to perfect, but not quite there.

Is this normal for a lower end binoculars ($50) and should I just accept it, or return them and hope the next pair will be better?

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Steve Ward

Steve Ward

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Welcome to the forum . . . :laugh:

Try closing your right eye and then focus the left side using the main focus wheel , then leaving the main focuser alone close your left eye and bring the right side into focus using the diopter adjustment on the right hand eyepiece .

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so2010

so2010

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Welcome to the forum . . . :laugh:

Thank you! :smiley:

Try closing your right eye and then focus the left side using the main focus wheel , then leaving the main focuser alone close your left eye and bring the right side into focus using the diopter adjustment on the right hand eyepiece .

Yeah, I'm always focusing that way. :grin:

Another thing I noticed: If I look slighly off the center of FOV, towards the right edge, the focus becomes much better...

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Steve Ward

Steve Ward

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Well that's me out of ideas , my advice would be return them and the buy a pair that you've actually tried out physically , not sure where you're located but I'm sure there's a shop nearby that you can try some before purchase.

Personally I never buy stuff like this without testing hands on , and never on-line but I'm a bit of a Luddite .... :p

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BinocularSky

BinocularSky

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Try closing your right eye and then focus the lef... .
This works, but far more accurate, IMO, is to cap the objective on the side you are not focusing and focus with both eyes open.
Another thing I noticed: If I look slighly off the center of FOV, towards the right edge, the focus becomes much better...
If it is only on one side that the focus is better, it is an optical error and should not be present, even if the binocular was only £50. Send it back. If it's all around the centre that is in focus, it is field curvature and will be present in all budget binoculars.
I've set the correct interpupilary distance and I do see a single circle.
Worth checking that; I have seen people think they have IPD set properly because they are only seeing one circle, but actually, they are only seeing through one tube. Easy way to check this is to keep the binocular still relative to your head, but move so that each tube in turn is occulted by, say, the edge of a door.
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so2010

so2010

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If it is only on one side that the focus is better, it is an optical error and should not be present, even if the binocular was only £50. Send it back. If it's all around the centre that is in focus, it is field curvature and will be present in all budget binoculars.

After closer inspection, it appears that the area all around the center is in better focus than the center itself.

But if I correctly understand field curvature, one can get the centre into the perfect focus, but not at the same time as the off center area. On this binoculars the center is never in perfect focus... At least in left tube.

Worth checking that; I have seen people think they have IPD set properly because they are only seeing one circle, but actually, they are only seeing through one tube. Easy way to check this is to keep the binocular still relative to your head, but move so that each tube in turn is occulted by, say, the edge of a door.

I am looking through both tubes :)

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BinocularSky

BinocularSky

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OK, as long as you are not short sighted, the centre should reach focus (and most binocs have a small amount of "beyond infinity" travel to allow for mild myopia -- in some, e.g. the Helios Stellar 15x70, it is derisory). If it doesn't reach focus with corrected vision, it is faulty and should be returned. If it does reach focus with corrected vision but you have myopia and prefer to observe without spectacles, it is unsuitable for you and should be returned. (If this is the case, some sellers may offer to modify the binocular so that it can reach focus for you depends on the binocular and the seller).

Just curious: Can you focus the right tube to your left eye and vice versa, and if so, did you need any eyepiece dioptre adjustment for the left eye/right tube?

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so2010

so2010

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OK, as long as you are not short sighted, the centre should reach focus (and most binocs have a small amount of "beyond infinity" travel to allow for mild myopia -- in some, e.g. the Helios Stellar 15x70, it is derisory). If it doesn't reach focus with corrected vision, it is faulty and should be returned. If it does reach focus with corrected vision but you have myopia and prefer to observe without spectacles, it is unsuitable for you and should be returned. (If this is the case, some sellers may offer to modify the binocular so that it can reach focus for you depends on the binocular and the seller).

Just curious: Can you focus the right tube to your left eye and vice versa, and if so, did you need any eyepiece dioptre adjustment for the left eye/right tube?

I do have some myopia, but very low, so that shouldn't be the problem.

When focusing the left tube, it's not that is runs out of travel and can't reach the focus, but when focusing it almost snaps to perfect focus, and then goes out of focus as I further move the knob. There's plenty of room on each side of focus.

If I try looking with right eye through the left tube and vice versa, the result is the same. Left tube looking with right eye is slightly out of focus in the center. The right tube with left eye is in perfect focus almost to the very edge. I do need to make slight dioptre/focus adjustments when changing tubes between eyes.

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so2010

so2010

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Obviously unsuitable for you and probably optically faulty. Return it.

On the batphone, so expect typos...

Thank you for your help!

I'll consider returning them. But since I bought them online, and from abroad, it may be a hassle. And the new pair may be even worse :).

Anyway, the lack of focus is very subtle. Strangely, I don't even notice it when stargazing...

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