Close focus adaptor question.

[SteveOak]

Can someone explain the optics of a close focus adapter?

I bought one and it works but the glass is of really poor quality so the sight pic is a bit blurry, the light pass through is not so good and it pinholes. I would like to understand the optics of close focus so I can buy a nice piece of glass from Edmunds or somewhere and make my own close focus adapter.

I also would like to be able to have it focus closer than 50 ft, say 15 or 20 ft.

 

[BoydAllen]

What you will be looking at is a fractional positive diopter lens of suitable diameter to match your objective diameter , and some sort of adapter. I have a closeup lens that was designed to go on the front of a camera lens that is +1/3 diopter that lets me focus on a target at about 12.5' from the front of a 36X rifle scope, sharp and parallax free. I had a lot of trouble finding the lens, second hand on the internet. BTW you are unlikely to find this information anywhere else. I came up with it by experimentation. First I tried a +1 and that brought things in too far, about three feet, so I concluded that I needed a fractional diopter lens and looked for that.

 

[SteveOak]

BoydAllen

Exactly what I was looking for. Thanks!!!!

And yes, I have been Googleing extensively and didn't find anything useful. I call Edmund Scientific (Optical). They wouldn't even talk about it because it was for a gun.

The scope is a 25X. I will experiment with some different fractionals.

The thread pitch on the scope, Leupold FX-3, is very fine, much more fine than you would normally find for a camera. I will cobble together something.

 

[SteveOak]

To aggregate what little information I have I will add this;

The diopter measurement of a lens is the reciprocal of the focal length in meters (one divided by the focal length), so 1 diopter = 1 m, 2 diopters = 0.5 m, 3 diopters = 0.33 meters, and so on.

 

[John Russell]

BoydAllen

Exactly what I was looking for. Thanks!!!!

And yes, I have been Googleing extensively and didn't find anything useful. I call Edmund Scientific (Optical). They wouldn't even talk about it because it was for a gun.

The scope is a 25X. I will experiment with some different fractionals.

The thread pitch on the scope, Leupold FX-3, is very fine, much more fine than you would normally find for a camera. I will cobble together something.

Rule #1 - NEVER mention "gun" or firearms related stuff when calling a non-gun business. They will not talk to you, because they don't want to take a chance on seeing their name in the news next week. You want a close up lens so your telescope will let you watch birds out on the feeder.

That being said, decide how close you want it to focus. Remember that even though your scopes focuses from 100yds to 10,000 yds, with a close focus lens, that focus range will collapse to maybe 2 or 3 feet, so pick your distance carefully.

Then, once you have picked your distance, (example, 25 feet), convert it to meters, (=8 meters). Then put a "1" over it, (1/8), and that is the diopter you want.

So, if you want to focus at 38 feet, you want a 1/11.6 diopter lens.

Keep in mind that such lenses will be next to impossible to find.

Back when I was young pup (just after the Civil War) Leupold made a "Close focus" lens for their scopes. They came in the 40mm size, and focuses at about 25 feet. They show up on eBay once in a while.

 

[3035Odell]

Rule #1 - NEVER mention "gun" or firearms related stuff when calling a non-gun business. They will not talk to you, because they don't want to take a chance on seeing their name in the news next week. You want a close up lens so your telescope will let you watch birds out on the feeder.

That being said, decide how close you want it to focus. Remember that even though your scopes focuses from 100yds to 10,000 yds, with a close focus lens, that focus range will collapse to maybe 2 or 3 feet, so pick your distance carefully.

Then, once you have picked your distance, (example, 25 feet), convert it to meters, (=8 meters). Then put a "1" over it, (1/8), and that is the diopter you want.

So, if you want to focus at 38 feet, you want a 1/11.6 diopter lens.

Keep in mind that such lenses will be next to impossible to find.

Back when I was young pup (just after the Civil War) Leupold made a "Close focus" lens for their scopes. They came in the 40mm size, and focuses at about 25 feet. They show up on eBay once in a while.

Interesting!! So my 50' Leopold focus adapter is basically a 40mm 1/16 diopter lens? And if I wanted to use it for dry firing in my basement at 25' I would need to use a 1/8 diopter instead if I were able to find such a thing?
And when you are adjusting your Objective lens are you changing its diopter value or just the focal length?

 

[BoydAllen]

There is a local optician that is not part of an ophthalmologist's or optometrist's office. She has been in business for many years as a fee standing business where you can take your proscription an buy glasses. I mentioned this to her (She has absolutely no problem with firearms.) and she said that the lab she uses could produce a lens to whatever specification I wanted...diopter and diameter. At that point the only issue would be a mount.

 

[JRS]

Check with Leica. They used to produce them in fractional increments from -3 to +3. You can also find step-up and step-down adapters. The rings have metric threads, as does your scope.

 

[Turbulent Turtle]

Yes, you can use any old filter that fits you riflescope, pull out that lens and replace it with your made-to-order lens. The only safety issue would be to make sure the lens is good quality and will not break during recoil. The lens will most probably not be coated, so you will have flares, ghosts and other things in your image, but it's only for very special occasions.

The filter size will most probably be something like Objective lens diameter plus 6mm. But there can be and there are variations.

 

[John Russell]

Interesting!! So my 50' Leopold focus adapter is basically a 40mm 1/16 diopter lens?

Yes

And if I wanted to use it for dry firing in my basement at 25' I would need to use a 1/8 diopter instead if I were able to find such a thing?

Yes - I use two of the Leupold 50 ft adapters.

And when you are adjusting your Objective lens are you changing its diopter value or just the focal length?

Neither... all lenses form an image of a subject @ infinity, at distance equal to the focal length, behind the objective. A 1000mm lens forms an infinite image at 1000mm behind the lens.

When focused at a subject closer than infinity, all lenses forms an image at distance behind the lens, equal to the focal length, plus the the reciprocal of the ratio of the distance to the subject in view, divided by the focal length.


So, a 1000mm lens, looking at a subject that is 100 meters away, forms an image that is 1000mm plus 1/100x1000 millimeters behind the objective. When you focus the lens, you move it forward so that the new image forms on the cross hairs.

You do not change the focal length or diopter value.