# Let's talk - scopes and diopters



## Dusty Britches (Feb 10, 2003)

I have been shooting with a 4x Extreme scope (don't know the diopter) for many years now and I'm thinking I need some understanding about some things.

Wikipedia says a diopter "is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (that is, 1/metres). It is thus a unit of reciprocal length. For example, a 3-dioptre lens brings parallel rays of light to focus at 1⁄3 metre."

I'm thinking that simply put in archery terms, it is the distance from the eye to scope lense at which parallel rays of light come to focus. I'm thinking that means, if I was to use a 32mm diopter lense, it needs to be 32 mm from my eye to obtain the best focus potential of the lense.


Is that correct? Once we clear that up, I have some more questions.


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## brtesite (May 24, 2002)

Dusty Britches said:


> I have been shooting with a 4x Extreme scope (don't know the diopter) for many years now and I'm thinking I need some understanding about some things.
> 
> Wikipedia says a diopter "is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (that is, 1/metres). It is thus a unit of reciprocal length. For example, a 3-dioptre lens brings parallel rays of light to focus at 1⁄3 metre."
> 
> ...


not really.the diopter is the focal length of the lens. it is just like a shoe size. A one d lens will focus a light beam one meter from the lens. they pass a light source thru a collimator which passes light in parallel light rays. 
the lens then focuses the light to some point beyond . Measure the distance from the lens to the focus point , do the math, that is the diopter rating. Any light rays coming thru the lens is already bending.
It would be almost imposable to try to get the lens at those distances from your eye. A .5 d has a focal length of 2 meters. 
Any given lens changes magnification as the distance from your eye to the lens varies. 
Look thru a lens at the distance from your eye,. If you like it, buy it


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## Dusty Britches (Feb 10, 2003)

Thanks, Mike.

So, when I look at new scopes diopter comes into play after I determine the distance to my eye and power of magnification I want to acheive. Is that right?

How does this affect the size (diameter) of the lense? Or does it?


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## brtesite (May 24, 2002)

Dusty Britches said:


> Thanks, Mike.
> 
> So, when I look at new scopes diopter comes into play after I determine the distance to my eye and power of magnification I want to acheive. Is that right?
> 
> How does this affect the size (diameter) of the lense? Or does it?




yes Unless you know the diopter rating , power means nothing
diameter has no effect


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## Dusty Britches (Feb 10, 2003)

Ok, thanks.

Diameter - when I use a smaller diameter lense I notice I can see the target (3D) much better than with a larger lense of the same power magnification. In fact, the larger lense tends to produce a blurry sight picture, unless I reduce the peep size so small that I can only see through it if I close my weak, off eye. My dominant eye loses out to the weaker eye with the smaller peep.

Why does the larger lense produce a blurry picture?


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## brtesite (May 24, 2002)

Dusty Britches said:


> Ok, thanks.
> 
> Diameter - when I use a smaller diameter lense I notice I can see the target (3D) much better than with a larger lense of the same power magnification. In fact, the larger lense tends to produce a blurry sight picture, unless I reduce the peep size so small that I can only see through it if I close my weak, off eye. My dominant eye loses out to the weaker eye with the smaller peep.
> 
> Why does the larger lense produce a blurry picture?


the size should not mater. You said that if you look thru a small lens vs. a large one , since they are not the same lens the actual diopter rating of both of them may not be the same. A larger lens may not have been ground very well to the edges of the lens.


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## Dusty Britches (Feb 10, 2003)

That's interesting, Mike. So I should not even have to change my peep?

With the smaller diameter lens, I can also remove the lens when it rains and shoot the same without needing to adjust anything (my sights are dead on). With the larger lens, if I do that, I might as well pack up and go home. The shots will drop to the ground at 30 yards on a 40 yard target.


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## brtesite (May 24, 2002)

Dusty Britches said:


> That's interesting, Mike. So I should not even have to change my peep?
> 
> With the smaller diameter lens, I can also remove the lens when it rains and shoot the same without needing to adjust anything (my sights are dead on). With the larger lens, if I do that, I might as well pack up and go home. The shots will drop to the ground at 30 yards on a 40 yard target.


Now you have put something else into the game.
If you are dead on with the lens in, then take it out & drop low, the lens is a prism. To prove it , just rotate the lens 90 -180 degrees & see where the impact point is. if that is so, it is a bad grind. However it isn't bad if you don't take the lens out.


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## Dusty Britches (Feb 10, 2003)

You've brightened my world, Mike.

However, I'm not sure I follow your last statement.

If I take the lense out, rotate it 90 or 180 degrees and the point of impact changes, it is a bad grind on the lens.

The lens is not bad if I don't take it out? Oh, are you saying as long as I keep the lens in that it really doesn't matter if the grind is bad?


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## brtesite (May 24, 2002)

Dusty Britches said:


> You've brightened my world, Mike.
> 
> However, I'm not sure I follow your last statement.
> 
> ...


there is an optical center as well as a physical center of the lens. They may or not be the same . They should be on a well made lens. 
It doesn't mater if you are not removing the lens. 
In your case by taking it out , You have changed the whole sighting system.


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