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PostPosted: Fri Jun 09, 2017 4:06 pm 
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Joined: Sun Mar 07, 2010 7:45 am
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Location: Hampshire
(Sorry about the rather inept title.)

I am very long sighted, especially with advancing years, so I have a lens (non-magnifying) that clips over the end of my Gehmann rearsight and puts the focal length some way ahead of the foresight on a standard Anschutz 1800 series barrel. The effect is to improve quite appreciably the sharpness and clarity of what I see of the foresight.

However, I have been told that a fixed lens like this, as distinct from a lens in special spectacles that would move with my head and eye, is likely to increase the risk of misalignment as I look through the rearsight, which I keep at the usual 1mm. Is this actually true? Would I be more likely to have my eye correctly aligned if I dispensed with the additional lens and accepted a certain loss of sharp focus on the foresight?

I realise of course that misalignment could also come from inconsistent position of the head on the cheek piece, but there are times when shots go wrong even when I believe that my cheek weld is perfectly o.k. and when I have not made any obvious mistake in, for example, the triggering or follow through. In other words, I have had no reason to call the shots out and I am left to assume that my eye was misaligned.

If I got rid of the lens, could I compensate satisfactorily by sticking an extension tube on the barrel to project the foresight further away from my eye or would the tube have to be impracticably long to achieve the same result as the lens currently does? I am not thinking of a tuner, merely an extension of the barrel.

Apologies if these questions are either stupid or impossible to answer. Perhaps I should add that my shooting eye unfortunately is somewhat worse optically than my non-shooting eye, so it needs all the help it can get.


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PostPosted: Fri Jun 09, 2017 4:25 pm 
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Location: A new global Great Britain
Rearsight mounted lens is the ideal solution in my opinion.
Dont know why you say non magnifying as correcting long sight requires a plus dioptre ie magnifying lens. All legal now on rearsight.

PS if you are long sighted your point of focus is beyond infinity and thats a darn long barrel.


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PostPosted: Sat Jun 10, 2017 2:45 am 
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Joined: Mon Jan 03, 2011 11:33 am
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Location: Taunton, Somerset
Roger,

Yes, you could swap the lens holder you have for shooting spec frames. There the lens moves with your head, so it's still centred even when your eye isn't centred behind the sights. But wouldn't it be cheaper to get the cheek piece sorted? Your Gemini has plenty of adjustment, and the wood is easily modified. Providing the lens holder is stable and centres the lens, why change? Having an experienced shooter look you over to check the head placement issue doesn't have another root cause would be sensible. Have you made any changes to your position since Sam Huish?

I would not recommend replacing the lens with an extension tube. As Peter noted, it's not enough on its own. However a tube can make for a more natural focal length WITH a rearsight lens. Adding a tube may require a new lens, to ensure your focus is still sufficiently far ahead. My optician who is well known in shooting circles did not make extra correction for my 6in tube, but wold for a 12in tube. Obviously the usual caveats of fitting a tube apply: balance, barrel harmonics, increased surface area for wind buffeting.


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PostPosted: Sat Jun 10, 2017 4:31 am 
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Location: Hampshire
TenMetrePeter wrote:
Rearsight mounted lens is the ideal solution in my opinion.
Dont know why you say non magnifying as correcting long sight requires a plus dioptre ie magnifying lens. All legal now on rearsight.

PS if you are long sighted your point of focus is beyond infinity and thats a darn long barrel.


TMP,

Thank you very much for your contribution. I especially like your point about infinity and 'a darn long barrel'. I have my magnifying lens on the foresight, the one on the rearsight is simply for focal length.

Roger


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PostPosted: Sat Jun 10, 2017 5:06 am 
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Location: Hampshire
Tim S wrote:
Roger,

Yes, you could swap the lens holder you have for shooting spec frames. There the lens moves with your head, so it's still centred even when your eye isn't centred behind the sights. But wouldn't it be cheaper to get the cheek piece sorted? Your Gemini has plenty of adjustment, and the wood is easily modified. Providing the lens holder is stable and centres the lens, why change? Having an experienced shooter look you over to check the head placement issue doesn't have another root cause would be sensible. Have you made any changes to your position since Sam Huish?

I would not recommend replacing the lens with an extension tube. As Peter noted, it's not enough on its own. However a tube can make for a more natural focal length WITH a rearsight lens. Adding a tube may require a new lens, to ensure your focus is still sufficiently far ahead. My optician who is well known in shooting circles did not make extra correction for my 6in tube, but wold for a 12in tube. Obviously the usual caveats of fitting a tube apply: balance, barrel harmonics, increased surface area for wind buffeting.


Tim,

Thank you very much for all your points, perhaps especially the advice not to bother with an extension tube for this problem. My position is pretty much as Sam left it after his visit, so I am thinking more and more that I need to look into fitting a different adjustable cheek piece. I don't like the idea of carving the Gemini laminate wood because I know from my attempts to make a cheek piece for my alternative stock for my BSA Mk 5 that it is very, very difficult to get it just right in wood. The Anschutz cheek piece looks good and has had some favourable comments, but it might be quite a job to marry it up with the Gemini frame.

I have recently joined another local rifle club - our usual hired range being out of action this summer - and there are some pretty good shooters there, so I will ask them to take a look at my head position. One of them said to me last week, entirely spontaneously and without my raising this question, 'It's at the rearsight that all the problems arise'.

Roger


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PostPosted: Sat Jun 10, 2017 7:23 am 
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I recommend the lens attached to the rear sight as a superior solution to attaching it to spectacles.

You introduce spherical and astigmatic distortions when you look through a lens off-center, or non-perpendicular. Attaching it to the rear sight guarantees that no matter what position you shoot, your vision will be more or less perpendicular and centered to the lens. In spectacles, even fully adjustable ones, it is easy to get the lens off center or crooked, plus you need to re-adjust for every position.

From a pure optical position, if your head is misaligned, having the error between your eye and the lens, or between the lens and the sight likely makes no difference.

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PostPosted: Sat Jun 10, 2017 9:48 am 
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Joined: Mon Jan 03, 2011 11:33 am
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Location: Taunton, Somerset
Roger,

I think you can get close enough with wood. If you don't want to carve the original, I'm sure you can find another piece of wood that's the right size. Given the cost of a MEC or Tec-Hro cheekpiece, it's worth a shot making your own.


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PostPosted: Sat Jun 10, 2017 10:23 am 
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Joined: Sun Sep 04, 2016 10:59 am
Posts: 469
Location: A new global Great Britain
Cumbrian wrote:
TenMetrePeter wrote:
Rearsight mounted lens is the ideal solution in my opinion.
Dont know why you say non magnifying as correcting long sight requires a plus dioptre ie magnifying lens. All legal now on rearsight.

PS if you are long sighted your point of focus is beyond infinity and thats a darn long barrel.


TMP,

Thank you very much for your contribution. I especially like your point about infinity and 'a darn long barrel'. I have my magnifying lens on the foresight, the one on the rearsight is simply for focal length.

Roger


The one on the foresight may not be legal (eagle eye?) depending on if you shoot to ISSF rules.

Focal length correction for long sight will have a + dioptre value. That is magnifying by most definitions.

I wear spectacles because as a pellet popper I can't see the pellets to load them! With lens fixed on the rearsight I need half round readers to see the pellets.


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PostPosted: Sat Jun 10, 2017 1:32 pm 
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Joined: Sun Mar 07, 2010 7:45 am
Posts: 225
Location: Hampshire
ShootingSight,

Thank you very much, that's good to know.

Tim,

Right, o.k., I'll have a go at making a platform/shelf cheek piece, rather as we discussed some while ago, if I remember corrrectly. The idea being that the rounder Gemini is maybe a bit too comfortable, perhaps a shade 'slippy' in both dimensions, and that a sharper rest would fix the cheek better.

TMP,

Yes, what I have would be illegal with the ISSF but I occupy too lowly a position as a shooter to worry about it (like many others, I am inclined to believe). I am fairly confident that I conform to my UK national (NSRA) regulations, which, thank goodness, are not so rigid as the ISSF ones.

Roger


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PostPosted: Sat Jun 10, 2017 3:05 pm 
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Joined: Mon Jan 03, 2011 11:33 am
Posts: 1279
Location: Taunton, Somerset
Roger,

This might seem obvious, but have you tried chamois leather to prevent your face slipping?

Yes a foresight lens is legal under NSRA rules when paired with a single fixed lens in the rearsight or spectacles. An eagle eye is not permitted with an adjustable dioptre.


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PostPosted: Mon Jun 12, 2017 1:15 pm 
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Joined: Sun Mar 07, 2010 7:45 am
Posts: 225
Location: Hampshire
Tim,

Sorry for the slow reply.

No, I have yet to use a leather facing but I will try to find a suitable piece and give it a go.

Glad to have your confirmation that my two different lenses are NSRA legal, as I thought.

Roger


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