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  #1  
Old 03-24-2010, 10:48 AM
GBertolet GBertolet is offline
 
Join Date: Feb 2010
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Default diopter aperture sights

I have a HW target airgun. It came with the HW match aperture sight and various front sight inserts. I am in my mid 50's and I have a vision problem getting a relatively sharp definition of where the top of the post front sight ends, and the bottom of the bull begins. They all run together causing elevation stringing of shots. I have seen many various types of rear eyepieces on a shooting supply website, having adjustable apertures with polarizing lenses and optics etc, with a wide range of costs. What do these eyepieces do, and would they help my situation? They are not cheap and I don't just want to purchase one without knowing. I know it is an optical impossibility to make both the front sight and target sharply in focus at the same time, but can the bull be made more in focus using these sights? I have been told that Anschutz, FBW, Walther and HW sights all are interchangable, as all having the same size threads for eyepieces. Is this true?
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  #2  
Old 03-24-2010, 10:52 PM
ShootingSight ShootingSight is offline
 
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It is not an impossibility for the target and the front sight to be in reasonable focus at the same time, though it takes exact optics. What is needed is for your focal point to be in the right place, and to have sufficient depth of field.

Getting your focus to the right point is done by your eye's lens accomodating to shift focal point, or by adding a lens which shifts focal point. As you get older, you eye loses its accomodation capabilities, so you have to strain the eye muscle more and more to focus at the right spot. In your case, it is likely that your are trying to focus at the strain limit of your eye muscle, and as the muscle tires from repeated supreme exertion, it can't always hold focus to the same spot. This causes varying amounts of blur on the front sight, causing the front post to appear slightly bigger or smaller as the width of the blur line changes, causing vertical stringing.

From an optical physics perspective, you want your eye to focus at the hyperfocal distance of the front sight, which is at 2x the distance from your eye to the front sight. Assume the front sight is 25" from your eye, you want to focus at 50".

To figure out the lens strength you want to shift you there, take 50", convert it to meters 50" = 1.27 meters, then invert the meters figure: 1/1.27 = 0.79 diopters. Always round down to the nearest lens strength, in this case +0.75. If you don't wear distance glasses, this is the theoretical lens power you need. If you do wear glasses, simply add +0.75 to your distance prescription. Note, theoretical values are a good starting point, but don't account for an individual eye, so you might be one step higher or lower.

Next, you want as small an aperture as possible, without going so small the image dims. This small aperture will, just like in photography, give your eye a greater depth of field, so near and far objects are both in focus. You have seen photos of people close to the camera, and mountains in the background, where both are in good focus, despite the fact that it is theoretically impossible. This is done by setting the camera to take a picture with a small lens aperture to give good depth of field. Your rear sight does the same for your eye. The smaller the aperture, the bigger your depth of field, and the better the focus on both the sight and the target.

Net, I recommend the right lens and a small aperture.

How to do it is a question of convenience and money. My favorite is the adjustable diopter rear sight. This has a focussing lens in it, so you can twist the ring, and get the desired balance of focus btween the front sight and the target. A cheaper but less convenient option is to figure out the right lens strength, and just put that one fixed lens in the rear sight or in shooting glasses.

Next step is an adjustable rear aperture. Again, you can get cheaper fixed rear apertures, but as light goes up/down on the range, the adjustability is a nice feature.

I do not recommend colored, or polarized lenses. They rob you of light - a polarized lens blocks up to 70% of transmitted light. Colors block various amounts, depending on the color, but upwards of 30% is likely. In extreme sunlight, this might be acceptable, but if I were willing to give up 30% light, I'd rather make my aperture 30% smaller, and get the improved depth of field.

As to getting both in focus, it depends on your eye. There is a spacing of the photoreceptors in the human eye, and once the blur in an image is smaller than about a receptor diameter, it apperars in perfect focus to your brain. Further focus improvement do not help. Thus, there is a theoretical aperture size where both the front sight and target will be at the detection threshold of focus, and both will be practically in perfect focus. In reality, the image will dim unacceptably before you can get to this point, but the closer you get by getting the right lens power and the right aperture, the better the junction between the post and the target will be.

A different option is to move to a front sight ring, versus a post. While this won't fix your focus issues, at least the front sight and target will have the same amount of blur all the way around, so the blur on one side offsets the blur on the other, and you can still center the target in the sight. This will eliminate your vertical stringing.

Art

Last edited by ShootingSight; 03-24-2010 at 10:58 PM.
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  #3  
Old 03-26-2010, 03:12 PM
GBertolet GBertolet is offline
 
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Art, can you recommend a particular eyepiece that has the adjustable diopter and aperture that will fit the HW sight?
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  #4  
Old 03-26-2010, 09:10 PM
ShootingSight ShootingSight is offline
 
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Sorry, I am not familiar with HW, or with the specifics of match sights, so I cannot give a specific recco. However, give Bob Jones a call at bjonessights.com, he knows this stuff better than I do, and he usually has a few used ones kicking around. They are not cheap. Even used are upwards of $100 for the adjustable diopter.
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  #5  
Old 03-30-2010, 05:29 PM
dave tengdin dave tengdin is offline
 
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Can the adjustable diopter rear sights stand up to the recoil of a spring air rifle, such as the FWB 300s?

I bought a Gehmann from Champ Choice (part # 268) with the polarizer and colored filters. (The polarizer and filters were a waste of money.) then I read that high power rifles can blow these things up, (and springer arir rifles are even HARDER on scopes and stuff than high poer rifle) so I pulled it and put it on my 40x instead, and just went with the plain old adjustable iris model on the 300s. I seriously doubt a 13 pound rifle generates enough recoil in a .22 to do any damage!
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  #6  
Old 03-31-2010, 10:39 PM
GBertolet GBertolet is offline
 
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Champion's Choice was where I was looking also, Dave. You raised a good question. Would an spring airgun wreck an optical eyepiece? My eyes are about shot for metalic sights. I need something. Same vision problem for M1 and AR15 also. Growing old is a bummer!
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  #7  
Old 04-01-2010, 08:03 AM
ShootingSight ShootingSight is offline
 
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GBertolet,

Nothing wrong with metallic sights that a lens insert wouldn't fix for you. As you get older, most people get presbiopia, and have trouble focussing close, so they need reading glasses. Same is true for shooting, you need a little help focussing close. Difference is that reading is done at 16-18", so reading glasses (which typically start at 1.25 diopters) will move your focal point in really close. To shoot, you don't want nearly that much power.

The solution you want is to either get dedicated shooting glasses, or a lens insert that goes in the hood of the M-1/AR-15. The correct power will be around a +0.5 diopter for the M-1, and a +0.75 diopter for the AR (assumes you don't have any distance vision issues).

Art
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  #8  
Old 04-02-2010, 12:14 PM
GBertolet GBertolet is offline
 
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Thank you, Art, my vision problem is exactly as you described.
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  #9  
Old 04-06-2010, 06:54 PM
GBertolet GBertolet is offline
 
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I just checked the aperture size on my HW. I measured with a pin gauge, it is .062. My metric math is a little rusty, but I think that makes it about 2.4mm.
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  #10  
Old 04-06-2010, 09:51 PM
ShootingSight ShootingSight is offline
 
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Lens inserts fit in the hoods of an aperture - typically a 5/16" diameter lens for an AR hood, and a 3/8 diameter for an M-1 NM hood. Target rifles are all bets off, as they typically don't have hoods. Here, yo would need shooting glasses.
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