Sunday, January 1, 2012

How can I see my Sight Picture better ?

  "Old Eyes and Iron Sights" -- part 5
   Today was a good day. Yes, it was my birthday, but that's not why. It was a good day because I could see the target (dropped 5 in a 40 shot team match at 500 yds). I was using the AR match rifle, and I could see a good black dot to aim at, and the the front sight aperture was black and pretty sharp. That was exactly the opposite of Thursday, when Bill and I went to Ben Avery to shoot in the 1000 yard practice match, where I was shooting a 6BR.  We zeroed at 600yds but I could hardly see the Bullseye at all.  I could adjust my shooting glasses to get the front sight sharp, but no mater how I moved the lens in and out there was just a light shadow of about 1/2 the bullseye, and I ended up using a "frame" hold to some success (9s and 10s).
     I have been alternately shooting my .223 AR match rifle, or my 6BR Anchutz match rifle in mid-range matches. I know that the 6BR is a tack driver because I get high X counts when using a scope in "any" sight matches, but I always get a higher score, and hold elevation better with the AR space gun if both are shot with Iron sights.                               

     So, why is this? The 6BR has a fully adjustable Anchutz 2213 alum stock. The AR space gun is less adjustable with a fixed height cheek rest. Both have good sights -- Phoenix rear, and 30mm Right Sight front on the 6br, PNW rear with a std. 22mm on the Space gun. Both have .3 and .5 diopters available.

Two things that always seem different from one to the other.
     1. I always get a comfortable position with the Space gun. It fits me perfect. No adjustable cheek piece - so what! I get on the gun and My eye is always lined up with the iris, no matter how high I have the sight set.
With the Anchutz 6BR, I seem to be always adjusting the cheek piece, hand stop, and sling. When I get it adjusted just right, I find that the measurements are way different than with the AR--Huh? When I shoot the Anchutz stock with either the .22, the 6BR or the .308 barrels, the measurements are pretty much the same....But not the same as the AR.
     2. I always see the front sight, and target better with the AR. I always struggle with my vision while shooting the 6BR.

So where does all this lead to ---

Thoughts on front sight optics,   shooting glasses, and  stock settings,.

Front Sight Optics:
    I use a .3 diopter in the front sight for Mid Range and a .5 diopter for long range.  I purchased these diopters from the usual sources.  I 'assumed' that they were quality lenses. However, it's certainly possible that the diopters I use are not good quality lenses, and that is the reason why I see the target well with one gun and not with the other.  Each gun has it's own dedicated front sight and diopter. 
   These diopters are simple concave/convex lenses---i.e. both sides are curved towards the front.   They could be plano/convex with the back side flat and only the front side curved and that would work fine also. These lens are not hard or complicated to grind.  They should  be accurate, and precise if  coming from any reputable lens shop.   However, I suspect that many of them are mass produced and coupled with most being plastic lenses, they may or may not be the quality that shooters really need to see the target properly.  Its also possible that these lenses are cut from larger pre-ground lens blanks, and therefore the optical center may not be in the center of  each lens we get.  Grinding one 3" lens and then cutting out a doz smaller 22mm lenses from that blank would be more profitable for the mfg. but only one of the doz would maybe have been cut from the from the optical center of the parent lens.  
      Solution ?
     1.  I intend to take all my diopters to an 'friendly" optical shop, and have them looked at.  The optician can easily measure the grind with a lens diopter guage and tell me if they really were ground to the proper focal length.
  Remember:  from Bill Luth
 "focal Length = 1/diopter . The lenses that we call 0.3 and 0.5 are really 1.3 and 1.5 diopter with corresponding focal lengths of 30" and 26" (about), so you are correct that the focal length and sight radius are directly related. Now should we think about the image formed at  the rear sight or at the front surface of the eyeball?  
 1/1.3 =.7692m .7629 X 39.36 in/m = 30.03 in focal length
 1/1.5 =.6667m .6667 X 39.36 in/m = 26.24 in.
 The diopter values refer to the reciprocal of the focal length in meters
    2 .Since the diopters have a "focal length"  I will do more experimenting on moving my front sight in and out to test the sharpness of the image formed fore and aft of the exact focal plane.  Moving the front sight farther out will, also change the size of the image  our eye examines. Our eyes have a daylight adapted Pupil opening of about 5mm and if the Focal plane is too far out, the image size will be too large for best viewing.
    3.  If there seems to be any problem with the quality of the diopters I have, I will have the optical shop grind me lenses from good optical glass, and make sure that the optical center is in the center of the lens. I may experiment with diopters other than the standard .3 and .5.  Small lenses like this are not expensive, so having "good" ones is not a luxury. This would be the easiest with the 30mm  lens as it uses no lens holder to fit into Stallings Right sight I use on the 6BR. The 22mm have their own lens holder and I'm not sure how easy it will be to change out the lens.

Shooting Glasses
Once we get past 40 years old, our eyes start to slowly fail.  Most of us compensate for this by getting special shooting glasses to help us see better. I use Knobloch shooting glasses and could not compete in matches without them--period!  These specific glasses allow me to slide the lens left and right, raise and lower it, and twist it to a position perpendicular  and directly in line with the axis of my sights, even though my head (on the gun) is not pointing straight toward the target.  These glasses also allow me to move the nose piece in and out to position the lens the proper distance from the eyeball --- This is very important!  The lens must be the same distance from your eye as your regular glasses or the focal length of the lens will not focus the image at the back of your eyeball.   I see many shooters who have their shooting glasses lens way out from the normal position of about 3/4 inch from the eye. No wonder they can't see the target or the front sight!  These (knobloch) shooting glasses also allow the lens to "tilt" in and out a bit, so I can tilt it in a bit to bring the bullseye into focus or tilt it out a bit to bring the front sight into better focus.  That's right-- I can focus these glasses from the front sight to the bull, and find what looks the best on any particular day.    Think about that advantage!  With my bad eyes (7.50 sph. prescription) I need all the help I can get. If you cant see, you can't hit the middle.

Stock Settings  
  My Stock Measurements (when feels good)
                                                        AR-15 match,              Anchutz 6BR
Pull (from trigger)   =                        13.25"                        12.0"   (shorter)
Butt to Hand stop  =                          24.0"                          22.2"   (shorter)
Butt to Rear Iris  =                             11.0                           10.6"    (shorter)
Rear iris behind Trigger  =                 2.6"                            1.6"    (shorter)
Sight axis above Trigger  =                5.2"                            4.5"    (shorter)
Sight Radius(Iris to iris  =                 34.7"                          31.0"    (shorter)
Sling at                                           #3 hole                      #6 hole   (longer)

   It's obvious that I shoot a lower and more stretched out  position when I shoot the AR space gun even though I end up with the sling 3 holes tighter.  This, I cannot understand.  Why would a comfortable position with these two rifles have such different stock settings?  I can only think of one reason -- I can't move the Rear Sight as far back on the Anchutz 6BR as I can on the AR.  I think that therefore I shoot that gun hunched up, and straining a little to reach the short rear sight eye relief  that suits me best. 
    Solution:    Since the AR feels best when shooting, I will now (again) set the Anchutz stock to all the settings I use on the AR, and start over from there. I have a 3/4 in. iris extention for the rear sight to get that back a little farther.
     However, somehow, I just know that when I get done adjusting it for comfortability , those measurements will end up right where they are now.  It's a mystery!

Old Eyes and Iron sights -- Part 1  (First thoughts)    Click Here
                                               Part 2  (Iron sight optics)    Click Here
                                               Part 3  (Techniques to see better)    Click Here
                                               Part 4  (Too much light from spotting scope)    Click Here

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