Optics Tested:
Introduction:
I recently purchased an EOTech EXPS3 with a flip-down magnifier because I thought it would have much less parallax induced POA error than my red dot sights. Two other motivations for trying the EXPS3 are that with holographic sights the center dot diameter does not increase in MOA size with magnification and the window size and FOV is larger as compared to most reflex red dot sights.
Parallax error in a CQC weapon is important in my view because shots are often taken under time pressure from highly improvised positions with a non-standard cheek weld or no cheek weld at all. The more forgiving the sight is with respect to where the dot is located in the window, the better. Holographic sights, including EOTech sights, are widely reported to have far less parallax error as compared to reflex red dot sights. Some of online and print sources even claim the EXPS3 sight is "parallax free".
This is the only reference I was able to find that has measured red dot sight parallax error results, and the results say that EOTech holographic sights are not parallax free but have significantly lower parallax error than most reflex red dots:
Parallax Induced POA Error Measurement Procedure:
The first thing I did when the EOTech arrived was to unbox and place the sight on a stable aiming platform with approximately 9.5" eye relief for an apples to apples comparison with (1) a Trijicon Patrol MRO (MRO-C-2200018) and (2) a Holosun 530-RD (HE530G-RD). The test procedure for each sight was to center the dot in the middle of the window while aiming it at a grid target with close grid spacing, then move my eye side to side by the same distance for each sight (+/- 0.5”) and up and down by the same distance for each sight (+/- 0.35”) and then record the dot movement in inches on the grid. The target was set up at 7 yards which was far enough away to get enough dot movement for a reasonably accurate estimation of POA shift on the grid, but not so far away so that the grid lines were hard to discern at 1x. The smaller vertical eye movement as compared to the horizontal eye movement was to account for the smaller vertical window dimension of the EOTech as compared to the horizontal window dimension. Although the Trijicon and Holosun are round which would allow the same vertical and horizontal eye movement, the vertical eye movement was limited for all three sights in order to maintain a consistent comparison. This test procedure is not very rigorous, but the results allow for a reasonable relative “ballpark comparison” of the three sights. The results are definitely “good enough” to determine whether the EOTech parallax error is clearly superior or about the same as the reflex sights.
The bottom line - The EOTech has pretty much the same parallax error as compared to the reflex red dots, which was somewhat surprising to learn.
Another interesting finding was that the Trijicon has the smallest horizontal POA error when the eye is moved in the horizontal direction, and the smallest vertical POA error when the eye is moved in the vertical direction, but there is a translation of horizontal eye movement to vertical POA error and a similar translation of vertical eye movement to horizontal POA error. I have noticed this in the field when using the MRO but this is the first time I measured it and it was surprising to find that the horizontal to vertical and vertical to horizontal translation errors were larger than the vertical to vertical and horizontal to horizontal errors.
Measured results in inches of error for parallax induced POA error at 7 yards distance from the front of the sight to the target:
While the above error measured in inches are small for a CQC engagement at 7 yards, when the errors are converted to MOA they are substantial for 100 to 300 yard engagement distances.
Measurement results in MOA for parallax induced POA error:
Another interesting finding was that although the Trijicon sight exhibited horizontal/vertical translation errors in POA, the total POA error for a given amount of eye movement seemed to be a little less than the total error for the same amount of eye movement for the EOTech and Holosun sights. This makes me wonder if the Trijicon engineers designed a reflective surface shape to minimize total error at the expense of some translation error, or if this just happens to be how the design of the Trijicon reflective surface shape works. It is an individual shooter decision as to whether the translation in POA error from horizontal to vertical and vice versa is acceptable in exchange for a slightly smaller total POA error.
Take aways:
I have been using the Trijicon Patrol MRO for about 10 years and although the same model number is still a popular option that is readily available, optic manufacturers sometimes update designs without advertising the fact so newer Patrol MROs might have internal design updates that could change the parallax error performance, I don’t know about this. The EOTech EXPS3 is brand new and I picked up the Holosun 530-RD about 6 months ago, so these should be the latest designs.
- EOTech EXPS3 holographic weapon sight
- Trijicon Patrol MRO reflex red dot weapon sight
- Holosun 530-RD reflex red dot weapon sight
- Contrary to widely held belief, the EOTech EXPS3 holographic sight has about the same parallax induced POA error as compared to reflex red dots Trijicon Patrol MRO and Holosun 530-RD, which was surprising to learn.
- The parallax induced POA error is between 3 MOA to 5 MOA for the EOTech and the two reflex red dots tested.
- To see the measurement results and learn how the measurements were made, read on.
Introduction:
I recently purchased an EOTech EXPS3 with a flip-down magnifier because I thought it would have much less parallax induced POA error than my red dot sights. Two other motivations for trying the EXPS3 are that with holographic sights the center dot diameter does not increase in MOA size with magnification and the window size and FOV is larger as compared to most reflex red dot sights.
Parallax error in a CQC weapon is important in my view because shots are often taken under time pressure from highly improvised positions with a non-standard cheek weld or no cheek weld at all. The more forgiving the sight is with respect to where the dot is located in the window, the better. Holographic sights, including EOTech sights, are widely reported to have far less parallax error as compared to reflex red dot sights. Some of online and print sources even claim the EXPS3 sight is "parallax free".
This is the only reference I was able to find that has measured red dot sight parallax error results, and the results say that EOTech holographic sights are not parallax free but have significantly lower parallax error than most reflex red dots:
Comparative Study of Red Dot Sight Parallax | Green Eye Tactical
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Parallax Induced POA Error Measurement Procedure:
The first thing I did when the EOTech arrived was to unbox and place the sight on a stable aiming platform with approximately 9.5" eye relief for an apples to apples comparison with (1) a Trijicon Patrol MRO (MRO-C-2200018) and (2) a Holosun 530-RD (HE530G-RD). The test procedure for each sight was to center the dot in the middle of the window while aiming it at a grid target with close grid spacing, then move my eye side to side by the same distance for each sight (+/- 0.5”) and up and down by the same distance for each sight (+/- 0.35”) and then record the dot movement in inches on the grid. The target was set up at 7 yards which was far enough away to get enough dot movement for a reasonably accurate estimation of POA shift on the grid, but not so far away so that the grid lines were hard to discern at 1x. The smaller vertical eye movement as compared to the horizontal eye movement was to account for the smaller vertical window dimension of the EOTech as compared to the horizontal window dimension. Although the Trijicon and Holosun are round which would allow the same vertical and horizontal eye movement, the vertical eye movement was limited for all three sights in order to maintain a consistent comparison. This test procedure is not very rigorous, but the results allow for a reasonable relative “ballpark comparison” of the three sights. The results are definitely “good enough” to determine whether the EOTech parallax error is clearly superior or about the same as the reflex sights.
The bottom line - The EOTech has pretty much the same parallax error as compared to the reflex red dots, which was somewhat surprising to learn.
Another interesting finding was that the Trijicon has the smallest horizontal POA error when the eye is moved in the horizontal direction, and the smallest vertical POA error when the eye is moved in the vertical direction, but there is a translation of horizontal eye movement to vertical POA error and a similar translation of vertical eye movement to horizontal POA error. I have noticed this in the field when using the MRO but this is the first time I measured it and it was surprising to find that the horizontal to vertical and vertical to horizontal translation errors were larger than the vertical to vertical and horizontal to horizontal errors.
Measured results in inches of error for parallax induced POA error at 7 yards distance from the front of the sight to the target:
- Approximate EOTech EXPS3 POA error induced by +/- 0.5” horizontal eye movement: +/- 0.35” horizontal POA error, negligible if any vertical error.
- Approximate EOTech EXPS3 POA error induced by +/- 0.35” vertical eye movement: +/- 0.2” vertical POA error, negligible if any horizontal POA error.
- Approximate Holosun 530-RD POA error induced by +/- 0.5” horizontal eye movement: +/- 0.35” horizontal POA error, negligible if any vertical POA error
- Approximate Holosun 530-RD POA error induced by +/- 0.35” vertical eye movement: +/- 0.25” vertical POA error, negligible if any horizontal POA error
- Approximate Trijicon Patrol MRO POA error induced by +/- 0.5” horizontal eye movement: +/- 0.15” horizontal POA error, +/- 0.2” vertical POA error
- Approximate Trijicon Patrol MRO POA error induced by +/- 0.35” vertical eye movement: +/- 0.1” vertical POA error, 0.2” horizontal POA error
While the above error measured in inches are small for a CQC engagement at 7 yards, when the errors are converted to MOA they are substantial for 100 to 300 yard engagement distances.
Measurement results in MOA for parallax induced POA error:
- Approximate EOTech EXPS3 MOA error induced by +/- 0.5” horizontal eye movement: +/- 5.0 MOA horizontal POA error, negligible if any vertical POA error
- Approximate EOTech EXPS3 MOA error induced by +/- 0.35” vertical eye movement: +/- 2.9 MOA vertical POA error, negligible if any horizontal POA error
- Approximate Holosun 530-RD MOA error induced by +/- 0.5” horizontal eye movement: +/- 5.0 MOA horizontal POA error, negligible if any vertical POA error
- Approximate Holosun 530-RD MOA error induced by +/- 0.35” vertical eye movement: +/- 3.6 MOA vertical POA error, negligible if any horizontal POA error
- Approximate Trijicon Patrol MRO MOA error induced by +/- 0.5” horizontal eye movement: +/- 2.1 MOA horizontal POA error, +/- 2.9 MOA vertical POA error
- Approximate Trijicon Patrol MRO MOA error induced by +/- 0.35” vertical eye movement: +/- 1.4 MOA vertical POA error, +/- 2.9 MOA horizontal POA error
Another interesting finding was that although the Trijicon sight exhibited horizontal/vertical translation errors in POA, the total POA error for a given amount of eye movement seemed to be a little less than the total error for the same amount of eye movement for the EOTech and Holosun sights. This makes me wonder if the Trijicon engineers designed a reflective surface shape to minimize total error at the expense of some translation error, or if this just happens to be how the design of the Trijicon reflective surface shape works. It is an individual shooter decision as to whether the translation in POA error from horizontal to vertical and vice versa is acceptable in exchange for a slightly smaller total POA error.
Take aways:
- Based on these test findings, the reported EOTech EXPS3 improvement in parallax induced POA error as compared to some of the best red dots is not accurate. The EOTech parallax induced POA error is about the same as a good reflex red dot sight. I suspect, but do not know, that the same may be true for some of the other EOTech sights because this is one of EOTech’s most recently released models so it would be surprising if the parallax induced POA error is worse than the earlier models.
- It is important to state the obvious. The bottom line for practical use of all red dots is that centering the dot in the sight window during both zeroing and shooting longer distances is very important for accuracy.
- When using a flip-down magnifier with either a holographic sight or a reflex red dot sight the window sight picture and FOV is greatly reduced, which naturally forces the shooter to keep the aiming dot more toward the center of the sight window, but it is still important to center the dot. It is also good practice to zero with the magnifier because any POA MOA shift due to the magnifier being on and off will produce a larger error measured in inches or cm at distance under magnification if the sight is zeroed without magnification as compared to the error in inches or cm at close range under 1X power if the sight is zeroed with magnification.
- Even though the parallax errors are about the same, the EOTech EXPS3 and other EOTech models have the other advantages over reflex red dots. These advantages include a large FOV window and the same size aiming dot under magnification. While some red dots have similar FOV window sizes as compared to the large window EOTech models, the reflex sight aiming dots expand under magnification. Therefore, if the shooter is careful to keep the aiming dot in the center of the window the EOTech models can still be more accurate than a reflex red dot at long range under magnification due to the higher resolution of the aiming dot.
- For me though, the main attraction was better parallax error and I'm not willing to trade the extra weight, size, cost and much lower battery life as compared to the excellent large window reflex red dots that are available for what amounts to a smaller, more precise aiming dot when using a flip-down magnifier. The EXPS3 I bought is going back to the seller.
I have been using the Trijicon Patrol MRO for about 10 years and although the same model number is still a popular option that is readily available, optic manufacturers sometimes update designs without advertising the fact so newer Patrol MROs might have internal design updates that could change the parallax error performance, I don’t know about this. The EOTech EXPS3 is brand new and I picked up the Holosun 530-RD about 6 months ago, so these should be the latest designs.
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