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Posted: 10/5/2007 6:32:06 AM EDT
According to Hornady parallax is defined as:The condition which exists when the reticle of the a scope does not lie directly on the image plane. *emphasis added The word "error" really jumped out as I read this in the illustrated glossary of the Hornady Reloading Manual. If parallax is indeed an 'error', why would scope producers build a known error into their optics systems? I have a Luep Mk4 and on 10 power parallax is obvious and persistent. It is very easy to 'spray' a group if you do not positively duplicate your cheek weld each time you saddle up. It appears as if parallax on the Mk4 is more pronounced on higher magnification, and I am assuming that the focal plane is shifting further away from the reticle plane. If this is not correct, then is parallax merely exacerbated relatively at all higher power settings? Is the shift linear or logarithmic? In other words, does a 20 power scope have twice as much parallax as a 10 power scope? or does it have exponentially more? Further, when friends shoot my Mk4 SPR they will group just as tight typically as I do, achieve the same degree of precision, yet the POI is typically shifted relative to the POA. In other words where I might be grouping center high, their groups with the same rifle and rounds is grouping left or right or high or low. I also notice my POI shifting from day to day, and I suspect this is more of a parallax issue than anything resulting from an inconsistent day to day welding. I would appreciate any comments on Parallax, as well as any discussion concerning relative Parallax between scopes of various manufacturers. Are their scopes that are parallax free? whose reticles are on the focal plane? Are fixed power scopes parallax free? and is this condition limited to variable power? edit: clarity |
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Regular scopes with fixed parallax always have the distance set that is close to the intended distance that the scope is to be used. A rimfire scope might be 50 yards. A deer scope 100 yards. A long range scope 200 or 300. The closer you are to the correct distance the less the difference in focus of the reticle and image and the less the relative shift if your head is off a lil bit. So it is mostly a non issue unless you try to use a rimfire scope at 10 feet or 300 yards or use a long range scope at 25 yards or a deer scope at 1000 yards or whatever. If a scope has a parallax of 100 yards when you try to focus your eyeball at 100 yards the reticle will be focused. Likewise the things your are looking at through your scope will alaso be in focus at 100 yards at the same time. If you head shifts when both the reticle and view through the scope are both in focusyou dont get reticle drift. Now as far as magnifcation... that will, all things being equal, decrease the depth of field. The depth of field is the relative range where things are in focus. Low powered scopes have a better depth of field and its easy to have things in focus over a larger range. The higher the magnification the smaller the depth of field. Think of a 3X optic. It is pretty easy to see whatever you look at and see it in focus. Now think of an extreme example like a spotting scope or even better a super high powered microscope. If you dont have it PERFECTLY focused what you want to see is going to be a fuzy unseeable mess. You can only see the thickness of a few cells in a microscope for example. Your focus a little off with 240X magnification? You see jack. So with a reduced depth of field the issue needing an adjustbale parallax becomes more important as the range in whach a fixed parallax scope works well is reduced. It is quite easy to focus on the air in front of a target on a warm summer day with a 20X scope and focus on the mirage instead of the target and see a moving blurry mess. That is not such a big deal with lower magnification. Likewise if you want to shoot from close range, say 50 yards out to 1000 yards a fixed parallax scope is going ot be out of its element on the extreemes and produced a sub par image and the reticle vs image focus will be more off except where it is close to the fixed parallax setting. If you tend to shoot in a narrow range of distances with relatively low magnification and the reticle focus is set near your normal shooting distances you dont need adjustable parallax. If you shoot both close and long range and have higher magnification you will almost certainly want adjustable parallax. Fixed vs variable has nothing to do with prallax other than when turned down varibles have a better depth of field and when turned up the depth of field decreases. A fixed scope will have better depth of field if it is low powered and worse if it is high powered. The only magnified scope I know of that is parallax free is the ACOG line of BDC calibrated scopes. Because the reticle is etched and refelcted on a mirror that is slanted diaginally it has a focal adjustment that is ever changing in a diagonal direction. The top being focused closer and the bottom being focused further away. The BDC markings are roughly set with the distance of optimal parallax to be the same as what is listed at each tick on the reticle. So each piece of the reticle is focused at the correct distance. Since the mirror is on a diagonal some reticles that are very wide like the TA01 series will have the bottom and one side focused beyond ingfinity and they will always be a bit blurry especially on one side no matter what range you are looking at. I hope this explains it for you and if you have anymore questions to clarify things feel free to ask. I might not have stated things as clearly as I should. |
| Just wanted to add... Brand and optical quality have nothing to do with depth of field or parallax issues. These are fixed properties like your exit pupil. If you have the most bad ass glass ever produced by man and have a high powered scope with a small objective you will have a small exit pupil. If you have shitty glass from China and a large objective you will have a large exit pupil. If the exit pupil is much smaller than your pupil on the super high end scope (like at dawn or dusk) and as large or larger on the cheapy scope you can get a brighter image from the cheapy scope. It is simple physics. This is why you are accepting a trade off with something like a Night Force 2.5-10X fixed parallax scope. Small objective with 10X power and no adjustbale parallax means you are limited as to the magnification setting in lower light, head position is more critical due to fixed parallax, etc. even though its well built and has great glass. |
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Thank you so much DevL I knew that Arfcom would explain it in simple easy to understand terms. If you had some sort of mini dot etched at the focal plane, it would seem then you could use the dot with the reticle as if using iron sights. If in fact you were 'off center' a tad on your set up, the miniature etched dot would immediately telegraph that you were dealing with some parallax. You could then adjust your weld around a bit to, in essence, be looking down the straw so to speak. Thanks for the info... |
I am sure he means depth of field....the way a camera opened up to say F2.8 has a very shallow depth of field, and is ideal for portrait work to make your subject pop, and a lens closed down to say F22 has a very deep depth of field allowing you to 'acceptably' focus both your foreground, subject and background... SW |
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Depth of field is the range things are in focus. Field of view is the size of your view through the optic. Both decrease with increased magnification, all things being equal. Of course all things are never equal...  Thought I would add another example of depth of field and the range things are in focus. If you have a high powered scope with a fixed fornt sight tower the depth of field is reduced so you wont see the front sight tower as it will blur out. As magnification decreases it will become more and more clear even though the optic is focused at say 100 yards regardless of magnification. The same thing is happeining to a lesser extent out there when you are viewing distant targets. |
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