Optics: Parallax vs Focus

Status
Not open for further replies.
If you wear corrective lens it could add to the problem.

Many times when looking through the scope you are not looking through your corrective lens in the optimal spot/area. I am looking through (right eyed) the upper left portion of my right lens. Corrective lens can be made to help this problem.

Leman Optical in AZ will know how to do this. Don't forget to add the bifocals so you can see the marks on your scope;-)=

When using the adjustable objective I will use a tiny dot of yellow paint applied with a toothpick to mark 200, 300, 400 and 500 meters.
 
Not uncommon

Dan you are right about old eyes! I have an Astigmatism and it makes me crazy with cross hairs cause the horizontal cross hair is double! I don't have that with my NightForce with the double dot. And I find it harder to get my scopes were I want them.

Joe Salt

Astigmatic corrections were common in the fitting of corrective lenses to the ocular of scopes. When the eye piece was set to get the vertical crosshair in focus the horizontal would be out and vice- versa. When the astigmatism axis was at 45 degrees the shooter would get this wedge of blur to deal with. Some shooters who were very one eye dominant would need prism to force them to place their eye in the centre of the eyepiece.
Andy.
 
Is this an issue if the shooter is wearing corrective lenses?

Boyd, as I understand it, yes. I have astigmatism. The correction in the eyeglass lenses works only when my head is perfectly straight -- if you tilt your head, the eyeball rolls some to try to keep the eye "perpendicular" (?) to the ground. Of course, the glasses tilt along with the head, so the astigmatism correction in the lens is no longer correct.

Same problem reading in bed, for those of you who read whilst laying on your side....

Edit:

Where I can afford the weight, I have the scope mounted off to the left, so I can keep my head straight up & down when shooting.
 
Last edited:
Makes sense. I have a slight astigmatism in my shooting eye (left)but it does not seem to give me any problem. I wear corrective lenses when I shoot, and have high rings, which I like because of what it does for comb clearance and head position. One thing that has helped my image quality, is the +2 diopter lens that I have screwed into the eyepiece of my B&L 4200 series 36X. I trade off a little eye relief, and get some magnification, and the image quality is very good. Fellows who shoot them have told me the magnification with the lens on appears to be about like that of a Leupold 45X. The 37mm threads just happen to work in this scope. Friends have done the same thing with Leupold 36xs, with adapters, and report liking the results. Some, who have more eyepiece forward adjustment room have used higher diopters, and deal with the additional eye relief loss by setting their scopes up so that the full field of view is not used. (black around the edges)
 
Adapters

Makes sense. I have a slight astigmatism in my shooting eye (left)but it does not seem to give me any problem. I wear corrective lenses when I shoot, and have high rings, which I like because of what it does for comb clearance and head position. One thing that has helped my image quality, is the +2 diopter lens that I have screwed into the eyepiece of my B&L 4200 series 36X. I trade off a little eye relief, and get some magnification, and the image quality is very good. Fellows who shoot them have told me the magnification with the lens on appears to be about like that of a Leupold 45X. The 37mm threads just happen to work in this scope. Friends have done the same thing with Leupold 36xs, with adapters, and report liking the results. Some, who have more eyepiece forward adjustment room have used higher diopters, and deal with the additional eye relief loss by setting their scopes up so that the full field of view is not used. (black around the edges)

I made up quite a few eye piece adapters for shooters who didn't need corrective lenses just to increase the power of the scope back when the Leopould 36x was about as high as you could get. The trade off was eye relief and the reticle focusing had to be wound in quite a but. Something like those bullseye boosters except these threaded on. If you are myopic and already had the eye piece wound quite a lot you could run out of adjustment. That's when the more complex compound systems had to be made up.

With astigmatism's you had to turn the second ring a bit like you do with a polarizing filter to get the axis of the astigmatic lens correct if you tilt your head when shooting. Something you can't do with spectacles. Small astigmatism's won't be affected much but from 2 diopters on up you will.
Andy.
 
I made a WAG and bought a Tiffen new old stock set of closeup lenses (37mm) off of Ebay. I got lucky. The whole set cost around $20. I am nearsighted and what you said about the eyepiece being wound up is exactly my situation. Because of that, I don't have room on my scopes for more than +2, and I really don't want to give away more eye relief. What I have seems perfect.
 
A while back a lot of us were making up or having made up binoculars (some still are) for long range hunting by mounting two spotting scopes together (interpupilary). A lot of them had to be colaminated by the individual because a lot of us don't have our eyeballs lining up perfect together in our skull. If a technician lined both scopes up w/ a collaminator headaches and less than good viewing resulted.
 
I'm a bit near sighted and don't even wear glasses while at the computer but without them at 100 yards I couldn't discern a four inch bulls eye from a 12 inch one. My one problem with the Weaver T36 is adjusting the eye piece since those cross hairs are so fine. I can go from one end to the other in adjusting and can't tell much difference.
 
Are you wearing your glasses when you try to adjust the scope? As a point of reference, 36X Weaver cross hairs are not considered particularly fine. They are fine enough, but not nearly as fine as some. My B&L cross hairs are the same thickness, and I like them just as they are. Also, on focusing your cross hairs, if you are staring at the reticule for more than an actual second or so while trying different adjustments, you are doing it wrong (IMO) This also applies to looking at anything at target distance that is at all in focus while you try to focus the eyepiece. These are very common mistakes. Often, the only way to get it done is to have someone put their hand between the shooter's eye and the scope and remove it momentarily.
 
Last edited:
Yep, wearing glasses and have the rifle pointed toward a blue sky. I try not to stare at them more than a couple seconds, move my head away or close the eye and then look back, turn the adjustment some more and do the same. I did make one mistake you mentioned-looking at a distant white wall that was in focus.
 
Try shortening your looks to the absolute minimum that you can manage and still have a memory of the image. I work from the memory, of the look comparing it to that of the previous adjustment. If you study the reticule while you make this comparison, your eye will accommodate, which in this case, you do not want. Once you get somewhere close, you can fine tune by making small eyepiece adjustments, refocusing on the target with the front or side focus, and then checking for parallax. When you get to the point to where the target is its sharpest and there is no parallax, the eyepiece has to be focused on the reticule. For this, you look at a target, and can stare as long as you like, just pay close attention to the differing amounts of parallax, and be sure to refocus on the target each time.
 
Last edited:
Good advise and explanation. I'll be more observant next time at the range. It's a good scope and no problem seeing those tiny holes at 36x with one exception. I was bore sighting it this past week and started at 50 yards. After all looked good I fired two shots and didn't see a thing. I thought I was entirely off paper. Turns out after I decided to walk down and check it out I was dead center of that 100 yard target I put up but since the bull is black, I couldn't see the holes well enough. With a standard 100 yard BR target you can see the holes great...but, not against black.
 
Well, chaps, I am worn out reading all the responses. Funny, after I posted and there was a day or so of silence, I said wow, this forum has really gone downhill! ;-)

I should add now that I do find an improvement when I wear the prescription for computer screen distance (not common reading glasses) rather than my distance prescription. What that does is make it easier for my eyes to focus on the reticle. My eyes focus fine with no correction at arm's length, so I don't' need "reading glasses" but my arms keep getting longer, and for close up detail work I now use the OptiVisor headband magnifiers. And this is probably why I now wrestle with the second focal plane on these scopes.

Also, while I didn't mention it, I do use the "head movement behind the scope" to adjust the objective for zero parallax (i.e. no apparent movement of reticle.) And when I do this, the target is slightly blurry. I have never paid attention to the yardage markings on a dial, I make my own tick marks once I establish parallax-free for certain yardages. (It would suit me if they left the markings off completely, or just indicated which direction was infinity.)

So, this has now come down to adjusting the ocular as best I can using Boyd's approach. If I cannot get it perfect, I will chalk it up to my eyes / prescription and let it go at that. Target slightly off focus I can live with, as long as parallax error is minimized. I am a varmint shooter first, and a paper puncher only for accuracy load development, not a benchrest competitor - yet.

Thanks all for the great discussion.

Brian
 
Try shortening your looks to the absolute minimum that you can manage and still have a memory of the image. I work from the memory, of the look comparing it to that of the previous adjustment. If you study the reticule while you make this comparison, your eye will accommodate, which in this case, you do not want. Once you get somewhere close, you can fine tune by making small eyepiece adjustments, refocusing on the target with the front or side focus, and then checking for parallax. When you get to the point to where the target is its sharpest and there is no parallax, the eyepiece has to be focused on the reticule. For this, you look at a target, and can stare as long as you like, just pay close attention to the differing amounts of parallax, and be sure to refocus on the target each time.

It cannot be stressed strongly enough how critical this is, more than about one second is too much. This is very difficult with very high grade optics such as the March scopes, compounded by the fact that slight adjustments have big impact and if not done a few times and correctly result in bad parallax and avoidable eye strain.
 
Boyd

A couple of years ago I got a female Korean optometrist rather frustrated as the glasses I had made from her prescription didn't work. We went through three prescriptions before I finally understood what I should be doing when she would say "is three better of four". I was doing exactly what you said. staring at the image too long. When I finally got that through my thick skull I got a good pair of glasses.
 
An ophthalmiologist once told me

My last port of call in optics was working in a lab that made contact lenses and lens implants. I worked closely with ophthalmologists bringing the lab up to iso 9002 standards. I learned that we don't see with our eyes. We see with our brains the eye is just an information collector.

The brain is a computer so powerful it is aware of its own existence> so it has no trouble interpolating. That is making up for information that is missing - which is why time lapse photography works - or corrupted like a bit out of focus. So when determining whether something is out of focus it should be done in short bursts before the interpolation process can be carried out.
Andy.
 
On a somewhat related matter, a friend who is a High Master shooting service rifle, once told me that if you stare at a sight picture too long (He was referring to iron signts.) That the image can get "burned into your retina" so that the actual sight picture and what you think that you are seeing will look the same, when they are not. We both knew that the burning in part was just a way to describe the effect. I also discussed the same thing with a national level pistol silhouette shooter, back in the day, and he told me essentially the same thing. The first fellow told me to get around this, I should look off in the distance for a bit and then come back to my sights, correct and then make the shot. I did a very rough test using an '03 Springfield with a receiver sight, shooting with a rest, from a bench. The results backed up what I had been told to an amazing degree. The difference was easy to see. I wonder if this could also happen with a scope.
 
Latent image effect

I discussed this with one of the Dr's many years ago. If you close your eyes so there is no information coming in then open and close your eyes quickly - which I might add is about one fiftieth of a second which is about as fast as a human can do that - you will see a residual image after you have closed your eyes called a latent image. Staring at an unaltered image for a long period can overwhelm your brain because the latent image becomes quite strong and your brain doesn't notice small changes that occur.

Advertisers exploited this effect and were banned from sliding just one frame of a product they wanted to sell into the movie at the picture theatre because you didn't notice it but it created a subliminal thought process to develop. Once I have the cross hairs locked onto the aiming point I usually just look at the flags before I pull the trigger. Occasionally going back to check on mirage if it's running.

Just so you can see the effect of flag changes more dramatically try blinking your eyes as fast as you can whilst looking at them. You will see them appear to jump from one orientation to the next as changes come through. Not enough information coming in between blinks for your brain to interpolate. It will highlight just how twitchy things really are.
Andy.
 
The last scope I adjusted for parallax was the Lyman Super for 60'. It took me hours because I let the rifle w/ scope stay solid on the 60' target and kept checking every 15 minutes or so and readjusting and rechecking until I got it. Some can do it much faster but I can't.
 
Status
Not open for further replies.
Back
Top