Physical sweet spot distance not matching ipd reading. A cause for eyestrain?

Pimax 8K Series
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I measured the sweet spot distance between the lenses while also setting the pimax to the smallest IPD I can do (60mm software reading) and it comes out to be about 70mm measured with tape.

Usually the sweet spot is supposed to be the dead center of the fresnel lenses from what I have researched. But it seem that since the pimax lenses are at a slight angle it sort of morphs it to the top left of center a tad on left lens, and top right of center on right lens.

I measured that distance between the new sweet spots again and it also physically measured something like 72mm on my flexible tape measure. Im not an expert in how this should be reallyā€¦ but it seems to me that the sweet spot distance should match what the software is reporting?

Could someone please try this test with the dead center of their vive or rift fresnel lenses and see if physical measurement matches the software reading? I dont have a rift and vive anymore to measure it.

Im also wondering why adjusting the ipd seems to offets the image and even mess with the convergence to some extent (which doesnt seem to be the case on the rift and vive)

Ive always felt something felt slightly off with the ipd adjustment, a subtle eye strain that my brain just adjusts to after a few min on any pimax I try,
(think it might be binocular overlap and one eye always gets a blurry object while the other is clear) i have 66ipd, just trying to get to the bottom of it as saw another poster report this too.

Here is his post https://community.openmr.ai/t/5k-causing-significant-eye-strain/11958

Edit: sorry i am half asleep and sort of rambling. Hope this all makes sense, gn

Edit2: Also wanted to point out I really love the headset! I feel maybe these are some really minor things that can actually be improved in software.

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So, Iā€™ve also been having issues with clarity. I went back to my Vive and put it on the smallest setting (60.9mm) and then measured between the center of the lenses where the rings converge. It measures about 60mm. Going to try the same on the 5k.

I think youā€™ve hit the nail on the head. I set the 5k to 60mm IPD and measured between the center of the lense where the rings converge (as the crow flies) and it is indeed 70mm.

Would somebody from Pimax care to comment on this!? That means for someone with a 62mm IPD they will never even get close to the correct position, PLUS it squeezes the nose at the current 70mm even too much to wear.

I donā€™t think itā€™s unreasonable to say that Pimax have produced a headset thatā€™s not possible to even get a clear picture unless your IPD > 70 and you have a small nose. I would say they have a duty to provide replacement lenses where the center of the lense comes in at LEAST 5mm on each side, preferably 10mm so itā€™s also wearable for larger nose users. New lenses would also likely need a software setting since the software IPD would need to change based on which lenses are being used if the true center has changed.

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If you try with one eye is the picture best focused when your eye is dead on the center of the fresnel part of the lens?

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As far as I can tell that it true. Unfortunately I canā€™t get both sides perfectly clear at the same time though.

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Well, yes, exactly. The optical axes face a bit out to the sides, perpendicular to the screens, so thatā€™s where the sharpest image will be, instead of straight ahead - perfect for walleyed users - the rest of us will look through the lenses a little diagonally across. :stuck_out_tongue:

The IPD should still be set correctly, though - straight in front of you, because that determines where the image is positioned on the screens, and makes for the midway ā€œequal tradeoffā€ point between left- and right eye focal point.

It is indeed irritating to have one eye clear and the other blurry, but that is an unfortunate effect of the design; The Rift CV1 suffers the same a little bit (it doesnā€™t have canted lenses/screens, but it does have asymmetric eye FOVs ā€“ some (most, actually, by the looks of it) Rift users actually perceive the having at least one eye in focus as the device having a larger clear area than it actually does).

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Just to make sure I understand correctly - you changed the IPD to show 60mm inside the headset and then measured the distance of the Fresnel lenses centres, right?

I was experimenting with the IPD settings recently because I started to feel a bit weird in my prolonged ED sessions (+4 hours) and started to feel also an eyestrain. I cannot however say that it was a problem of Pimax as do not recall any session with my Vive so long (I could not really enjoy ED in Vive), but I noticed that need to rotate the IPD adjusting rocker before it starts showing a change inside the HMD.

Now, considering this and the difference between Vive and Pimax are built I can imagine that it might be possible to introduce a disparity in Pimax. IIRC Vive moves both lenses and the panels at the same time (as they are probably glued together). Pimax, on the other hand, has the panels fixed and only moves the lenses, which means that Vive calibration does not really suffer if the actual readings from the ā€œIPD sensorā€ (whatever it is what reports the IPD settings back to the software) is off. You simply adjust it to match your comfort zone and you are done and do not really care if the reading it shows is off. You may experience the shift in the virtual space as the scene will be rendered at a different IPD and thererfore look bigger/smaller, but it should not lead to the eyestrain, because the lenses will allways align with the images on the panels.

With Pimax there is another point of failure. Imagine that the ā€œIPD sensorsā€ reports different IPD than physically matches the lenses position. In this case the software which applies the IPD correction (it has to place the images on the panels so they ā€œcenter alignedā€) puts them slightly off.

If we assume that the IPD is reported at the time it is shown inside the HMD there seems to be a quite big leeway where the lenses can already move (and change their physical position), but the software has not not rectified that yet as it did not receive anything from the headset (yet).

Calibrating such a system presents also a technical challenge because clearly the physical position of the lenses with the same reading may actually differ when you are coming at it from below or above and difference correspond exactly to the ā€œleeway distanceā€.

All this to say, I can imagine there are many things which may go wrong and given the technical prowess Pimax showed so far, I would fear it may have already.

What I think could help, if Pimax exposes the calibration loop to the user, so we could eventually reset the IPD distance reported by the headset and thus reposition the image on the panels relative to the lenses (without actually moving the panels), but this would be also a good tool to screw up oneā€™s Pimax even more, so I would also understand if they do not.

Just for the record, I found out when trying to find the IPD where I can see the same object sharp by both eyes (using jump navigation markers in ED for that) that I would need to put IPD to much lower value (e.g. <65 mm) compared to my actual IPD (~ 72 mm measured by an optician). But it did not feel right.

So I eventually settled on ~ 69,5 mm (reported by the HMD), when I tried to measure the physical distance of the centres of the lenses, it seems to be around 70 mm, which seems to be consistent with the IPD readings.

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My IPD 66, I use 59.9

https://community.openmr.ai/t/how-i-calibrate-my-ipd-setting/12352

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Thanks for the confirmation with your vive. Yes thats exactly what im talking about and you can never quite get it in the exact sweet spot. I would actually prefer the lowest setting myself but the artificial software adjustment changes things too much to be comfortable. This all feels like a guessing game when setting IPD imo. I have 66 ipd btw.

Warning, another wall of text and ramble ahead which I tend to do:

A side affect of the lower ipd setting i have noticed is the image convergence or stereo shrinks too much then and then it really starts hurting my eyes even though im actually closer to the physical sweet spot.

Something I noticed which may also be contributing to eye strain for some, might be hard to follow me here but: I tried holding the pimax 1 ft away from my face for a very small 10 degree view of a far away 3d teacup inside the headset. I then look at my real life window which was about 6 feet alway and the blinds on the window. When i set the lowest IPD the teacup (distance was 15 unity cubes away, which is 15 meters) it would be on the same 3d plane as the blinds and not feel far away at all. Where as if I set the largest IPD setting the teacup would appear much farther away behind the real life blinds, but everything up close in in VR would feel sort of larger in scale.

Basically this is a side thing iā€™ve noticed and it might be normal behavior but havent noticed this effect on the rift and vive when adjusting the IPD. Wondering if it might also be contributing to some eye strain in addition to the sweet spot not physically measuring at what the software ipd reporting.

So my recommendation to pimax would be to create a test pitool beta. Create a tickbox in pitool to allow locking the image separation in place at a specific software ipd setting and be able to move the physical lenses without the image moving. (But still updates the game so scale wont be wonky like in the oculus DK2 days were) Then a reset button for all if this) We could provide feedback on it.

I have found out from another poster above that the screens inside the pimax donā€™t physically move like the rift and vive do when you adjust the ipd so they had to make up for it in software. I feel something could be going subtly wrong here.

But that still doesnt fix the fact that on the smallest hardware ipd setting it measures 70mm (between sweet spots)

With this only a small number of people could actually reach the optimal sweet spot. The average is closer to 63-64 i have read.

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@risa2000 thanks for the info. Yes after reading that it seems there can be many software ā€œpoints of failureā€ without a physically moving screen assembly as you describe. That is my concern. That a subtle mistake was made somewhere on the software side.

There are times where I maybe wouldnā€™t even describe the effect as eye strain but more like my brain feels something is slightly off with the image compared to rift and vive. I only see the effect for a couple minutes each day usually when I first put it on. Hard to describe.

Im leaning more towards each eye might always seeing a blurry object, binocular overlap issues. No matter what IPD I set. But sometimes I have doubts because I am never quite sure if IPD adjustment is helping the feeling.

I notice you said the physical sweet spot distance almost matched the software for you (at 69.5)? Could you try to measure the smallest IPD setting. Another poster also comfirmed that he measure about 70mm physically when the hmd is set to smallest setting.

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@frc yes using my left eye as an example. overall sweet spot is dead center if I look at the lens straight onā€¦ that is with my eye straight on to the screen.

But If I view with just left eye again and just put my face as normal it sort of shifts to up and to the left a few mm (no longer is the center) because viewing the lens is at a slight angle (as it normally is) hard to explain.

I can never get both eyes into this perfect spot (i have 66 ipd)

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Iā€™m still thinking about this but maybe in the case of non axial panels configuration the IPD MUST be measured using the intersection of your eyes pupil and the imaginary line emerging perpendicular to center of the lens. This form a triangle is space and the measure IPD at the lens surface would be greater the one at your eyes ( less then 70mm.)

So your saying your not able to have a focus and sharp image on both eyes even at the lowest IPD setting?

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Could be, it would be nice to get confirmation on that. I have no idea if this is the case with this layout.

Yes at lowest ipd its the best I can get it. But the software is adjusting the convergence or stereo so much at that ipd that 3d effect feels extremely close and compressed.

I think the subtle discomfort could be somewhat alleviated for me also if i could manually lock the background sofware image separation so it doesnt move when I do the physical ipd slider, but maybe not.

Does anyone know if there like an .ini edit or hack where I can play with these values?

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If you are comfortable with hexediting executablesā€¦

https://community.openmr.ai/t/pimax-distortion-editing/11493/3

Maybe: ā€œseparationā€ could be involvedā€¦ There is: ā€œlens_separation_to_ipd_rateā€, but this should, to my reasoning, be a factor on the IPD reading (from the potentiometer), to account for the screen/lens_shuttle cant, so it would produce relative values.

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Interesting discussion ! As I mentioned elsewhere, I have been struggling to get a clear view too, irrespective of the IPD setting. Itā€˜s not a complete train wreck, I do see objects sharp-ish. Itā€˜s just this strange sensation that I canā€˜t adjust immediatelly, my eyes retry to get a proper view, donā€˜t entirely manage and there is a slight feeling of eye-strain. Not really bad, but I havenā€˜t played with the 5K+ for longer periods as a result because I feel there just is something that needs to be fixed before I would want to use it as my regular VR headset.

I had my IPD measured today, it is 64,0. Actually the same value my Rift had established, and which I started off with on the 5K+, but then tried other IPD settings through the whole range and ended up at 60, which was feeling less off - but not quite there yet either. That would confirm your theory.

But on the other hand it just seems to be the few of us while the vast majority of users doesnā€˜t seem to have the same issue. And with all the critical minds/eyes around here itā€˜s highly unlikely that all others just are to polite to mention it while their eyes are bleeding after every sessionā€¦ so I am not sure our issue is of such fundamental nature as it then would have to apply to all users, not just a few ?

The other aspect letting me consider the different options is the fact that I canā€˜t recall this issue from the demo in Gouda. Admittedly, I was a bit tired that day and may have been so focussed on the comparisons of the SDE et al, so it could have been a case of ignoring it thinking that I needed to get my IPD properly measured anyway, but I simply cannot recall the weird sensation I get now. As said, I am not entirely sure about the Gouda thing, but it is another caution sign to me.

One of the other thought I had is that it may have to do with the shorter focal plain of the 5K+. In another thread a fellow backer said it was approx. 1-2 m as opposed to the 3 m of the Rift. I am far-sighted since a few years, and indeed canā€˜t read properly anymore, not even with a stretched arm. So 1 m might indeed be an issue for me, while the Riftā€˜s 3 m are fine. My glasses donā€˜t fit well under the 5K+ so I couldnā€˜t really properly test them. I ordered a smaller pair of glasses today which should fit better, engineered to work at 1 m distance. Will take a week but I am hopeful that that may contribute to less strain.

The canted displays may well be a reason for the differences between the IPD and the center of the fresnel rings.

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Your right eye is dominant then
Hold your thumb up at arms length over a far off object in the room and line it up with both eyes whilst focusing on your thumb , the hold your thumb there and close each eye and you will find that your right eye is on the object but your left is way out.
Same must be true in a headset
maybe some are confusing the correct positioning of the image
should not effect the focus though

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Another possibility is that most peopleā€™s heads are not quite symmetrical. IPD measurements assume that the left and right eyes are equidistant from the center of your nose. Thatā€™s not generally the case, but itā€™s usually close. If you have a face which is less symmetric, you might only be able to get one eye (or the other) in the correct position to align with the lens.

That might explain why some people have problems, yet others do not.

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Make sure the lens hasnā€™t popped out a bit. I remember seeing a post about that happening to someones headset. You can just push it back into place I thinkā€¦

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I just got my 5k+ yesterday and Iā€™m experiencing eye strain as well. I donā€™t know my exact IPD, but using a ruler a mirror it should be around 63-64 mm. Setting that on the 5k+ results in pain that seems to feel better when I reduce the IPD to the minimum 59.9, but itā€™s still quite painful.

Things arenā€™t really blurry, but my eyes just HURT. The pain goes away the moment I take off the headset. I put on a VR cover extra thick face cover that my friend uses for the Vive and that seems to have helped a tiny bit. I get a bit less FOV and Iā€™m seeing what appears to be god rays but my eyes feel a little less strained.

I hope there is some kind of software solution that we can create. Currently I canā€™t use it for more than 30 minutes due to the pain.

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As I said in this other post, there are two differents IPDs to adjust.

https://community.openmr.ai/t/5k-causing-significant-eye-strain/11958/43?u=yen