TargetTalk

Is anyone using a rear sight mounted lens instead of shooting glasses? If yes, did your prescription change?



The lens in my shooting glasses is +0.75 over my street glasses prescription (0.25 for the extra vertex distance from my pupil to the lens and then 0.50 to focus on my front sight) and this works for me when looking through the sights but I have no peripheral vision. Using my normal glasses would allow me to monitor the wind flags and even see my feet when standing! Yes, I really am that nearsighted.

I'm wondering if my normal glasses, paired with a +0.50 lens attached to my rear sight, would give me the same visual clarity as my current shooting glasses?

Jason

Jason, I am using the setup you show in the picture, though I just use my correction with no added diopter. I too, am very near sighted and without my glasses cannot see flags, targets, feet or anything else. But for me, the trade off is worth it. I had lots of problems with glasses and some problems with contacts.

There is no vision utopia for people like us. Realize you are always trading off one thing for another and then you will come to terms with it.

Strong near sightedness brings two issues to the table that need to be dealt with:

1. Strong anything means vertex distance needs to be considered. As of about 3 diopter correction your lens will add/subtract 0.25 diopters for every inch you move it closer/further from your eye, so going from glasses to this setup will affect required lens strength.

2. For strong NEGATIVE values, you also have the issue that increasing the vertex distance will shrink the image, so everything gets small, and you lose resolution.

If you are strong nearsighted, consider getting throwaway contacts that are simply +0.50 stronger than your prescription. THis eliminates both concerns. The lenses are only about $1 each, so wear one lens in your shooting eye on match days.

ShootingSight wrote:Strong near sightedness brings two issues to the table that need to be dealt with:

1. Strong anything means vertex distance needs to be considered. As of about 3 diopter correction your lens will add/subtract 0.25 diopters for every inch you move it closer/further from your eye, so going from glasses to this setup will affect required lens strength.

2. For strong NEGATIVE values, you also have the issue that increasing the vertex distance will shrink the image, so everything gets small, and you lose resolution.

If you are strong nearsighted, consider getting throwaway contacts that are simply +0.50 stronger than your prescription. THis eliminates both concerns. The lenses are only about $1 each, so wear one lens in your shooting eye on match days.

Art,
Want to make sure that I understand. Nearsighted: Currently have a .5 diopter in 30mm front sight. Use your lens test kit to get lens made for knob lock frames. Have considered the set up above. Understand vertex distance. So, if I understand correctly, changing to this setup would not only change the current prescription via lens test kit but also, and here's the question, reduce the front sight image? Correct or Incorrect?

I'm ok with the knob lock as have a homemade eye blinder but still have room to see downrange. Have never liked attaching a blinder to the rear sight - nothing wrong with it - just a personal preference.

Thanks Much.

Not sure I follow - 0.50 diopter in the front sight will magnify the target image, but that is not relevant to this discussion.

If your Knobloch lens is strong, ie over -3.00 diopters, the difference between mounting in your Knobloch frames and mounting on the rear sight could change the value you need in the lens.

In terms of blinders, be sure to use a translucent blinder, not opaque.

I understand all the impacts of vertex and distance -- my street glasses are -9.75R and -10L -- everything is fuzz without a lens in front of either eye.

If I'm still going to need a +0.50 lens to focus on my front sight (as a baseline), do I need to keep the extra +0.25 that I currently have to account for the extra vertex distance to the lens in my shooting glasses?

Jason

I've not dealt with a prescription that strong, so there are some subtelties that are beyond me. However, what I can say is that any prescription is calculated such that it will take your eye's relaxed focal point and put it at infinity. For a rifle, you want to focus at 2 meters, so that means you want to add 0.50 diopters, so you would want a -9.25 instead of a -9.75.

Now the tricky part: I don't know what vertex distance is assumed when they measured you at -9.75. But here are the numbers:
-9.75 with zero vertex distance, ie a contact lens, gives you an effective -9.75 lens.
-9.75 with a 25mm vertex distance is an effective -8.00 diopter
-9.75 with a 50mm vertex is an effective -6.5 diopter

So your lens is changing by 1.5 diopters for every additional inch of distance to your eye. This means that dialing in the correct distance and lens power has a higher possibility to go wrong. The good news is that if you get it close, you can likely dial in the lens distance slightly to suit your needs.

If they assumed zero vertex distance when they measured you, the above numbers are correct, and that is what I believe they do for a contact lens prescription. For glasses I think they assume a 1" vertex, but am not certain about that.