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tube type dot POA distortion with eye position
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Author:  GerardSamija [ Wed Nov 20, 2013 8:50 am ]
Post subject:  tube type dot POA distortion with eye position

I've been using a tube-type VISM green dot/laser combo for a couple of weeks, with my Brocock Atomic let into a Boyd's Blaster rifle stock I skeletonized to lighten it somewhat. Fun trying out a compact 'rifle' with this setup, but I'll probably go back to using the Atomic as a pistol eventually. Just more comfortable with pistol aiming. Meantime however I'm using the green dot, and have found that point of aim and point of impact are quite often markedly different. Surprising, considering how accurate the pistol is with open sights or the Burris FastFire III red dot. Then I realised that the problem was cheek placement on the stock relative to the sight. If the green dot is not perfectly centred, POA drifts this way and that like a boat on a rough ocean.

So yesterday I made a black delrin washer to snap into the black delrin cup I'd previously turned to fit onto the VISM's rear lens. I'd made the cup and one like it for the front end to keep fingers off the glass mostly. But it made for a convenient place to snap in a thing with a smaller hole to keep my eye centred. The hole is 1/4", so still plenty big to see the whole front view out the tube, but small enough that finding centre is relatively easy without thinking about it. Accuracy jumped immediately, problem solved. Making such a cup with smaller hole in one piece of delrin should be easy enough for anyone used to lathe work. A slight press-fit and it stays in place on the sight tube nicely. Just putting this out there for anyone using a tube-type dot sight and finding themselves wondering why shots might be going wide now and then. Of course it could easily be because I'm a rifle noob too... haven't shot one since I was a kid, and with open sights there's no such worry - the sights are either aligned or they're not, same as with most of my pistol shooting. With a dot sight it's not so simple, it seems.

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Author:  blarg [ Wed Nov 20, 2013 12:41 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

By the looks of the second image there is some magnification at play.
This may be a parallax issue, but looks like your workaround is effective and simple to implement.
Nicely done.

Author:  GerardSamija [ Wed Nov 20, 2013 12:49 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

Parallax, that's it I guess, thanks. Sorry but I'm new to dot sights, seen the word tossed around but wasn't sure it applied. The dot's apparent position shifts by over 10cm at 10 metres when moving my eye from the left extreme edge to the right edge. By putting a 6mm hole in the plastic disc to prevent my inattention from letting my eye wander more than 3mm from true centre I've limited the error potential if I'm sloppy with eye placement to about 1cm at 10 metres by appearances, and more importantly given myself a close reference circle with which to judge centre. The big lens with a clear view gave little sense of centredness so I could easily have been 5 or more millimetres off centre without realising it.

Oh, and no, there is zero magnification with this VISM dot/laser sight. I was just showing two pictures for the clear image of the plastic work, and a clear shot of the actual size of the green dot in the one with the blurred plastic parts.

Author:  blarg [ Wed Nov 20, 2013 3:03 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

Pretty simple explanation of Parallax:
http://www.youtube.com/watch?v=6ziKTDIMCig
As they note in the video, even fixed magnification scopes are set for parallax at specific ranges.
I suggested there was magnification because the view through the reticle did not show grain of table as one would expect at zero magnification.

Author:  GerardSamija [ Wed Nov 20, 2013 6:41 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

I'm not sure what you mean about grain in the table. I did photograph the tubular dot sight with a doublebass belly in the background, and that part is sometimes called a table... but the spruce grain isn't really very clear for such a low resolution phone camera and 3 feet away, in rather glaring halogen lighting.

I watched the video, thanks. Not sure exactly how a reticle relates to a reflected dot but I suppose there are similarities. If anything I suspect the dot bouncing off the half-mirrored front lens is going to have a lot more parallax error than a crosshair or mildot scope where the reticle is actually in the line of sight, not relayed from a slanting mirror. In any case there's no focus to be had with this unit. Next best thing seems to be restricting eye movement relative to the dot.

Author:  blarg [ Wed Nov 20, 2013 7:26 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

I like your solution.
It is simple and it works, and that holds elegance.

I ran into parallax issues on a scope when I did not yet know what it meant and your solution would have worked better than adding a cheek piece to ensure eye alignment, or even figuring out I could adjust as the factory did - eventually.

Author:  rrdstarr [ Wed Nov 20, 2013 8:58 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

Dude! You amaze me everyday in what it takes to make a person a better shot!

I imagine it is sort of like a fixed iris on shooting glasses to look through?

Cheers Buddy!

Author:  Edmonton<500 [ Wed Nov 20, 2013 9:07 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

blarg wrote:
Pretty simple explanation of Parallax:
http://www.youtube.com/watch?v=6ziKTDIMCig
As they note in the video, even fixed magnification scopes are set for parallax at specific ranges.
I suggested there was magnification because the view through the reticle did not show grain of table as one would expect at zero magnification.


Excellent reference. Thank you
Anyone having questions related to "what scope should I buy" should watch this.

Author:  57scott [ Wed Nov 20, 2013 9:24 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

Excellent solution! I'm going to try it on my red dot when I get a chance...

Author:  GerardSamija [ Wed Nov 20, 2013 9:29 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

rrdstarr wrote:
Dude! You amaze me everyday in what it takes to make a person a better shot!

I imagine it is sort of like a fixed iris on shooting glasses to look through?

Cheers Buddy!


You can shoot circles around me Rick, come on, your air pistol scores are right up there with mine and I'm a rifle noob.

The aperture is much larger than I use for 10m AP, but that's because my eye is much further from it when using the Atomic as a carbine. The shooting iris I dial down to between 1.2 and 2mm depending on the lighting and it's just over 10mm from my eye. Such a small aperture wouldn't work with this dot sight of course. But 6mm looks just fine. Here's a leaf showing two shots in a row from 17 metres, 25 degrees above, with the lower shot taken after correcting the dot downwards one click. The iris definitely helps.

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Author:  rrdstarr [ Wed Nov 20, 2013 9:39 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

GerardSamija wrote:
rrdstarr wrote:
Dude! You amaze me everyday in what it takes to make a person a better shot!

I imagine it is sort of like a fixed iris on shooting glasses to look through?

Cheers Buddy!


You can shoot circles around me Rick, come on, your air pistol scores are right up there with mine and I'm a rifle noob.

The aperture is much larger than I use for 10m AP, but that's because my eye is much further from it when using the Atomic as a carbine. The shooting iris I dial down to between 1.2 and 2mm depending on the lighting and it's just over 10mm from my eye. Such a small aperture wouldn't work with this dot sight of course. But 6mm looks just fine. Here's a leaf showing two shots in a row from 17 metres, 25 degrees above, with the lower shot taken after correcting the dot downwards one click. The iris definitely helps.

Attachment:
maple.jpg


You'll whip my butt with your Pardini vs my 46M.
I am a better rifleman then you. Left for work an hour and a half early to shoot the big rifle. Freezing a$$ cold at -4 and a 15k SW wind(in my face) i hit my small 35 gallon oil drum 5 of 5 times at 1800m.

Author:  mike87gt [ Wed Nov 20, 2013 9:51 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

This is a cool idea that I have to try out!!

Author:  GerardSamija [ Wed Nov 20, 2013 11:13 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

rrdstarr wrote:
You'll whip my butt with your Pardini vs my 46M.
I am a better rifleman then you. Left for work an hour and a half early to shoot the big rifle. Freezing a$$ cold at -4 and a 15k SW wind(in my face) i hit my small 35 gallon oil drum 5 of 5 times at 1800m.


Equipment has a limited capacity for winning competitions. A score of over 580 has been shot in international competition with a Baikal 46m! My own 46m still impresses me with its accuracy. I just find the weight, even reduced to about 1150 grams with a lot of metal work, is slightly on the heavy side compared to my 1000 gram Pardini. The difference is primarily in how long I can hold before letting a shot go... which is a bad habit anyway.

So a 35 gallon drum at over a mile. That's what, a 1mm margin of error at 10 metres? Less? Dang, that's some fine shootin' dude! What's the 'big rifle'?

Author:  rrdstarr [ Wed Nov 20, 2013 11:17 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

GerardSamija wrote:
rrdstarr wrote:
You'll whip my butt with your Pardini vs my 46M.
I am a better rifleman then you. Left for work an hour and a half early to shoot the big rifle. Freezing a$$ cold at -4 and a 15k SW wind(in my face) i hit my small 35 gallon oil drum 5 of 5 times at 1800m.


Equipment has a limited capacity for winning competitions. A score of over 580 has been shot in international competition with a Baikal 46m! My own 46m still impresses me with its accuracy. I just find the weight, even reduced to about 1150 grams with a lot of metal work, is slightly on the heavy side compared to my 1000 gram Pardini. The difference is primarily in how long I can hold before letting a shot go... which is a bad habit anyway.

So a 35 gallon drum at over a mile. That's what, a 1mm margin of error at 10 metres? Less? Dang, that's some fine shootin' dude! What's the 'big rifle'?


Barrett M82 .50BMG

Author:  GerardSamija [ Wed Nov 20, 2013 11:23 pm ]
Post subject:  Re: tube type dot POA distortion with eye position

You have that thing!? I somehow developed the notion that the Barrett was a historical reference, not a current element of your collection. d.a.m.n. Bet the scope is wicked too.

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