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PostPosted: Thu Jul 06, 2017 11:19 pm 
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Location: Melbourne, Australia
Hi,

Just looking for some recommendation on getting eyes tested regards to shooting. I shoot with my non-dominant eye, but this is beside the point here. I notice sometimes the target (smallbore prone) is a bit fuzzy, now granted this may be the intent with the aperture being sharp, but anyway, the point here is that I was looking through my spotting scope with my non-shooting (dominant) eye watching someone else’s target and the image was clear and sharp. Then I switched eyes to use my shooting eye through the spotting scope, and there was a noticeable lack of clarity. This wasn’t a focus issue, as I realise two eyes are different and focus for one may not match the other, there was just a lack of clarity compared to the other eye. This gets me thinking that maybe my shooting eye needs some help, so when I go to see an optometrist what from a shooting perspective do I need to ask/tell to ensure I get the right outcome?


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PostPosted: Fri Jul 07, 2017 1:48 am 
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Modena,

I don't think it's rare that your eyesite are not the same, that's what my optician tells me. I have a slightly different prescription between my right and left eyes.

If you do not have lens now, then you may well benefit from one. If your optician isn't a rifle shooter, expanding you want to focus at the hyper focal distance of your foresight. Typically for a rifle (no/short extension tube) this is about 2 metres out. Placing the focal point ahead of the foresight still gives a sharp foresight, but the target is a little clearer.

This equates to a lens of +.50 on top of any distance correction and astigmatism. My first lens was just +.50, as I didn't need glasses for anything else. The lens put my focus where I wanted without thinking,and reduced eye fatigue.


Last edited by Tim S on Fri Jul 07, 2017 6:17 am, edited 1 time in total.

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PostPosted: Fri Jul 07, 2017 1:57 am 
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A good scan or viewing by a skilled optometrist with your pupils dilated will show up most issues. The main ones that affect sharp vision even with the right lens are Cataracts, MD (macular degeneration) and ERM (epiretinal mebrane or "cellophane eye") Also there are entoptic phenomena like floating debris. Along with glaucoma which may have no symptoms, MD is the one that needs early intervention.
Personally my dominant eye has ERM which may or may not get worse but is tricky to treat as all the vitreous content has to be removed and the membrane picked out and could lead to total loss if the op went wrong. If I had described it as "like looking through a cellophane candy wrapper" I may have gotten a faster diagnosis but I was transfixed by MD that my mother had.
My plan is shoot with it until its just too fuzzy then go for a MEC Swap pending surgery. Despite the fuzziness my vision is still 20/20 its just annoying.
Google all of the above. Print off an Amsler Grid and how to use it. Hopefully you just need a lens!

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PostPosted: Fri Jul 07, 2017 6:39 am 
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While all the above may be true, just swapping eyes at the scope might require a re-focusing.


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PostPosted: Fri Jul 07, 2017 8:44 am 
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I'm not a doctor, so I cannot comment on any of the diseases mentioned, but I did study optics and photography, so I fully support the notion of hyperfocal focus, which works out to an add of +0.50 for most match rifles, or +0.75 for pistols or shorter carbines like the AR-15. THese values are added to any distance prescription you have for the eye.

You also want to avoid shooting with progressive (no-line) lenses. If you want a bifocal, get the old fashioned type.

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PostPosted: Fri Jul 07, 2017 9:33 am 
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I am no doctor either but when you go on a journey you become more aware of the wider possibilities. The reason I described my personal experience is because using an aperture of around 1mm the use of lenses has less impact than on open pistol sights. The physics of optics fascinates me too and you will know that if the aperture is small enough the depth of focus will be almost infinite. I can use no lens or my distance lens and it makes no difference. However when you have a defect in the eye the smaller apertures make things worse. More obscure studies show that entoptic phenomena are made more of a nuisance the smaller the aperture used. To quote one article, with a small aperture iris the shooter can often see his own cataracts.
Bottom line is annual eye exams.

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PostPosted: Sat Jul 08, 2017 8:12 am 
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I would love to see that article about the small apertures and cateracts. This has been a hypothesis of mine for the appearance of 'dust balls' in rear apertures. If you can find the link, I would appreciate it.

Small apertures do exaggerate defects in the cornea/lens. Normally, the image formed on your retina is overlay all the images coming from all the different parts of your lens. So the left part of the cornea sees the image from a slightly different angle than the right side of your lens. The top of your lens sees the image from a slightly higher perspective than the bottom of your lens, and so on. Each of these parts of your lens projects an image onto your retina, and they all superimpose and create the composite image that your brain sees.

Two things about this:

1. Since the left image and the right image are not exactly the same, they don't perfectly line up, and there is some blur. What happens when you use a small aperture is that you block out the extreme left and the extreme right parts of your lens from contributing to the composite image, and the differences between the slightly-left and slightly-right perspectives is not as great, so the images align better and there is less blur. This is why a small aperture helps you focus. As stated, if your aperture is small enough (pinhole cameras), only one perspecive can reach the retina (or the film in the camera), and there is no blur. Obviously, the smaller the opening, the less light gets through, so the trade-off for focus is loss of brightness.

2. Since the image on your retina is a composite of the images that come through all points of your lens, if there is a micro defect in your lens, that small area around the defect will be transmitting a blurry or distorted, or totally useless image. However as long as the total area of the defect is small compared to the total area of the lens, the amount of 'good' image will be much greater than the amount of 'bad' image, and the composite formed on the retina will still look fine. However what you do by using a small aperture is exactly that you cut down on the area of the lens that is helping build the image. If the defect is located in the small area that you are using, it is suddenly a much bigger percentage of the total that it was when you were using a big lens. and the effect of the defect becomes much greater.

If I understand, it is not uncommon to have 'micro-cataracts' Dunno if that is a medical term, but they are micro defects in the cornea. Some people have them. I do not know if these are the beginnings of cataracts that will grow to be 'real' cataracts later, or if some people just have them. But the bottom line is that some people see spider webs or dust balls inside a sight aperture, and I think this is the cause. Your ERM would similarly be made worse by this effect.

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PostPosted: Sat Jul 08, 2017 11:20 am 
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I posted the link in a previous thread too.

https://www.ssusa.org/articles/2017/1/2 ... -aperture/

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PostPosted: Wed Aug 09, 2017 7:29 pm 
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Modena, make sure that your lens is straight up and down. Any inward or outward positioning can cause a blurry sight picture.


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PostPosted: Wed Aug 09, 2017 8:11 pm 
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Location: Melbourne, Australia
bugman1955 wrote:
Modena, make sure that your lens is straight up and down. Any inward or outward positioning can cause a blurry sight picture.


I don't currently have a lens.


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