educate me on far sighted optics

[seamaster]

for minus lens, focal point is 1/diopter

so -2 diopter, the focal point is 1/2 or 1/2 meter (50cm)

Now for +2 lens, where is your focal point?

Let say a hyperopic person +3, to make a shooting lens with add 0.75, just order +3.75 diopter lens?

Educate me on how a farsighted person focus

[Gwhite]

The definition of 1/(focal length) holds for both + & - diopters. Negative lenses don't have a conventional focal length where everything comes together. It's more of a virtual focal point.

There's a good discussion here:

http://www.shootingsight.com/define%20diopter.html

Your analysis for far sightedness is correct. If you require a +3D correction to see well at a distance ("infinity"), then adding +0.75D should bring your focus to about the location of your front sight.

I wear contact lenses, and my right eye is set up for distance vision. I use ~ +1D to see my sights well. Depending on the light & what pistol (sight radius) I'm shooting, I actually like a slightly stronger correction so I can also see my rear sight OK. I increase my depth of field slightly with an aperture.

[Rover]

If you want to shoot, forget all this.

Tell eye Doc what you want to do and have ONE lens made to his prescription. That will also take care of your astigmatism (is your Red Dot round?).

It's better to put this lens in a custom adjustable frame so that you can center it between your eye and the sights.

[seamaster]

Let's say you adjusted your eye to +0.75, your focal distance would be 1.3 meter.

Now let's discuss how a myopic nearsighted person see things, and how a hyperopic farsighted person see things.

Near sighted person, things inside focal point clear, things beyond that point not clear.
Far sighted person, things outside focal point clear, thing inside that point not clear.

You have one guy who is myopic to start, but made relative to +0.75, your focal distance would be 4/3 meter or 1.3 meter. That would be slightly ahead of your front sight (unless if you are 6'5" and have super long arm), you still have some native lens accomodation left (you are not 75 year-old yet), you can bring this hyperfocal point back to your front sight. Front sight clear. Beyond hyperfocal point, ie target, not clear.

You have another guy who is hyperopic farsighted, you made him a relative lens to see +0.75. Focal point would still be 1.3 meter. But he is FARSIGHTED. That means every thing beyond that point is clear, he had NO ability to accomodate toward him. So if his hyperfocal point is beyond his front sight, and he is farsighted to start with, he can not focus on his front sight inside of 1.3m at all.

Are there guys here who are farsighted? I assume your shooting lens would be +1.0 (1meter) or +1.25 (80cm) to see front sight clearly.

But for native farsighted people, how do you make target blurry? By definition you can't, no?

[Silvershooter]

But for native farsighted people, how do you make target blurry? By definition you can't, no?

I had a cataract op on my R eye which I shoot with (but is the non dominant eye). I have a single focus lens implant. The distance prescription for this eye is + 0.75. I have 2 lens for shooting of +1.25 and +1.50 (both with astigmatism correction).
I am baffled why I can see the target and scoring rings clearly on a 10m AP target when there is no accommodation on that eye. Even with the +1.50 lens I find it difficult to bring my focus on to the front sight.

[Gwhite]

But for native farsighted people, how do you make target blurry? By definition you can't, no?

Because of the way my contact lenses are set up, I am effectively farsighted in my right eye. I am 65, and am very nearsighted and have presbyopia. I have "monovision" contacts, where my left eye is set up for reading, and my right eye is set up for distance. With my right eye, I can't focus on anything closer than 4 or 5 feet away. I use ~ 1.0 Diopter to move my focus close to my front sight, and the target is nice & blurry.

It's no different than setting up a lens for a youngster who can still focus over a range of distances. Their natural relaxed focus is at infinity. If they add a +1D lens, their relaxed focus is moved closer to the front sight, and they will not be able to focus clearly on the target.

[seamaster]

I think fundamentally there is a difference between making a person with normal vision/ or myopia into +0.75 vs. making a hyperopia farsighted person into +0.75.

Both will see clearly at +0.75 focal point, but what ELSE they can see clearly is the fundamental difference between them.

Doug, you are a near sighted make to see +0.75, you can see front sight clearly with that set up.

But I would bet if you are a native farsighted made to see +0.75, by definition, you are still farsighted. By definition, Every thing beyond to distance will be clear; Any thing closer, like front sight, would be still unclear.

I am try trying to find out the puzzle to this fact.

Are there any youngster who are natively farsighted, made to see +0.75 in the forum? I am betting you are seeing target super clearly.

[Spencer]

... By definition, Every thing beyond to distance will be clear; Any thing closer, like front sight, would be still unclear...

Nope! Not how it works.
If the focus is adjusted (by the appropriate orrective lens) to the front sight, the depth of field will be same for either myopia or hyperopia - check with an optician.

[seamaster]

Of course if you make your lens to focus on front sight, your front sight will be clear.

But for native hyperopic, TARGET would still be CLEAR, unlike myopic or people with normal vision made +0.75.

I will check with my ophthalmologist friend, and confirm with my optometrist about the validity of my argument.

[Gwhite]

That would imply that your depth of field increases with the additional lens. That is not how it works.

There are three things at work:

1) Natural focus: Where your eye focuses when it is relaxed. For people with normal (good) eyesight, this is at a long distance ("infinity"). If you are farsighted, it is also at distance. If you are nearsighted, it is at a shorter distance, and you can't focus at distance. Adding a lens will move the natural focus closer (+ diopter) or further away (- diopter).

2) Accommodation: This is a measure of how well you can bring your focus closer by using the muscles in your eye. When you get old (50-60), you lose a lot of this (presbyopia). If you are naturally farsighted, even when you are young, you can't bring your focus in very close. If you are nearsighted, you can crank it way in to a couple inches. With presbyopia, you lose ability to pull your focus in to closer distances. Adding + diopter lenses moves the range over which you can focus closer (like reading glasses).

3) Depth of field: this is the range over which things are in focus simultaneously without adjusting your eye's focus. It is affected by the size of your pupil, or an external aperture. The smaller the pupil/aperture, the greater the depth of field. Unfortunately, you also get less light, which can cause eye strain if you try to get too much depth of field without a lot of light. When you are outside on a bright day, your pupil cranks way down, and your depth of field will increase considerably. Indoors or at dusk/night, your pupil opens up and your depth of field will decrease substantially.

If you are farsighted, and have your vision corrected with glasses or contact lenses so you can see well over a normal range of distances, adding additional + diopter correction will move your natural focus closer. You will NOT be able to focus at a large distance anymore.

Several people have tried to explain this. If you don't believe us, go to the nearest drug store and slap on a pair of +2D reading glasses over your regular correction. Then come back & tell us if you can still see at infinity.

[Spencer]

Of course if you make your lens to focus on front sight, your front sight will be clear.

But for native hyperopic, TARGET would still be CLEAR, unlike myopic or people with normal vision made +0.75.

I will check with my ophthalmologist friend, and confirm with my optometrist about the validity of my argument.

PLEASE do check.

[seamaster]

Doug, the three points you stated are all academically correct.

But what puzzle me is this. Let's say a farsighted person, uncorrected by any lens, is able to see front sight clearly, but blurry within, i.e. can not read.


Now, what is he seeing in the distance?

Distance would be very clear for him.


The reason for my question is this.

I noticed that most autistic children, ones with reading disabilities, can not see well in the reading distance. They just don't like to see things in reading distance.

However, this is not pick up until much later in life. In their school years, their eye exams are always 20/20.


But I will discuss with my ophthalmology friend and my optometrist about what those farsighted see, when their eyes are artificially refocused to front sight for shooting purpose, what happens to their target distance vision.

[Gwhite]

Doug, the three points you stated are all academically correct.

But what puzzle me is this. Let's say a farsighted person, uncorrected by any lens, is able to see front sight clearly, but blurry within, i.e. can not read.

Now, what is he seeing in the distance?

Distance would be very clear for him.
<snip>

Distance will NOT be very clear IF he is focused on the front sight. He can re-focus at distance (accommodation), but he can't focus on BOTH unless he has a very large depth of field, which has very little to do with his natural or corrected focusing ability. No matter what your correction or vision, you can't focus at two widely separated distances SIMULTANEOUSLY. You can increase the depth of field with an aperture, but unless you have a LOT of light and a very small aperture, no one can focus clearly on both the front sight and the target at the same time.

Indoors at 10 meters, I have a +1.25D lens and an adjustable aperture. I can set my aperture so I can see my front sight very clearly, and my rear sight is also in reasonably good focus. The target is blurry, even if I try to focus on it. If I crank my aperture down as far as it will go (~ 1mm), I can see both the target and the front sight fairly clearly. However, the light reaching my eye is way too dim to comfortably shoot for more than 10 or 20 shots.