Fault - flat spot on lens - can I swap in a different one?

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howiem
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Fault - flat spot on lens - can I swap in a different one?

Post by howiem »

My rift has turned up (UK) and after a bit of fiddling I've found a problem with one of the lenses; it's got a flattened off spot in the middle. Easily noticeable if you find something near your desk with nice straight lines on it (like ventian blinds) and compare the reflections on the surface of the two lenses.
rift-lens-issue.jpg
Figured I'd ask for suggestions on here, rather than just asking Oculus to send another lens out:

- Are the lenses in the A, B and C eyecups the same? IE can I just swap a good one in from one of the other eyecups? They're glued into the eyecups, so better to ask before hacking :)

Thanks folks :)
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Re: Fault - flat spot on lens - can I swap in a different on

Post by LordJuanlo »

I'm sorry for you mate, you had a terribly bad luck. B and C eyecups are for nearsighted people so I don't think you should mix them with A. But if you are nearsighted you could happily use B or C pair, whatever fits you better. But you will have problems showing your Rift to people with correct vision. If I were you, I would contact Oculus asap.
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Re: Fault - flat spot on lens - can I swap in a different on

Post by 3dcoffee »

Contact Oculus.
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Re: Fault - flat spot on lens - can I swap in a different on

Post by geekmaster »

The lenses in the A/B/C eyecups have different focal lengths, which you can determine by focusing sunlight through them. Be careful to not start a fire with them.

If you swap lenses, the side with more curvature goes toward the inside of the eyecup.
howiem
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Re: Fault - flat spot on lens - can I swap in a different on

Post by howiem »

Contacted Oculus for a replacement, but I dare say it'll be a while, airmail and all that, plus they're probably fairly busy at the mo :)

I'm not convinced the lenses are different focal lengths.

Following Geekmaster's suggestion, I grabbed an A and a C cup and used them to focus an image of the scene outside my window onto the opposite wall. They both produce the clearest focussed image with the lens (not the cup) 4.5cm away.

So surely it's only the height that differs between cups?

After all, my SLR camera lets you adjust the optical viewfinder for different vision by moving, rather than replacing the viewfinder lens.

Geekmaster, you probably know more than me about these things - have you checked the Rift kit's lenses yourself?
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Re: Fault - flat spot on lens - can I swap in a different on

Post by lnrrgb »

my gut feeling is that the lenses are interchangeable - my mind says I am not intelligent enough to prove my gut wrong
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Re: Fault - flat spot on lens - can I swap in a different on

Post by geekmaster »

howiem wrote:... Geekmaster, you probably know more than me about these things - have you checked the Rift kit's lenses yourself?
I did, and the sunlight focused at different distances. The problem with your experiment is that different parts of the image outside your window focus at different distances. Did you focus the same object when measuring the lens distance from your projection surface?

Which side of the lens did you point toward your projection surface?

I just did my measurements again. I took photos this time. To simulate my RiftDK, I used my Doom3 BFG DVD as a simulated RiftDK screen, and I viewed the text on that DVD by holding the RiftDK eyecups in my hand, positioned next to a measuring ruler (fresnel magnifier with ruler marks).

Here is my test setup:
setup.jpg
Here is how I attached the ruler, so that the zero-mark is at the DVD surface:
measure.jpg
Here is the A-cup at my best focal point:
A-cup.jpg
Here is the B-cup at my best focal point:
B-cup.jpg
Here is the C-cup at my best focal point:
C-cup.jpg
It looks to me (taking image rotation into account) that these are my focal distances from target to lens:
A-cup: 42mm
B-cup: 43.5mm
C-cup: 45mm

It makes sense that they are a little different from each other, or else ALL of them would focus the same for me in my RiftDK, and there would be no reason to have multiple lenses or eyecups. The different eyecups and lenses SHOULD focus at different distances.

CONCLUSION: As you can see, the differences are minor but significant, and without careful measurement it could be easy to assume that they all focus at the same distance. However, it only makes sense that they SHOULD be different, or there would be no reason to include multiple sets of eyecups with the RiftDK.

EDIT: To be even more accurate, I need to repeat this while holding my RiftDK eyecups in a calibrated fixture, instead of holding them in my hands. I think I was reasonably accurate though, because I exercises care in my experiment. But still, a lens-holding fixture would be better...
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howiem
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Re: Fault - flat spot on lens - can I swap in a different on

Post by howiem »

Hi Geekmaster,

Well, I tried a similar experiment to you but got different results.

I took the lens off my DSLR (7D), mounted it on a tripod pointed out the window, then used the A and C cups in turn to focus an image onto the CCD. I used a distant tree as an object.

Not easy to measure the lens-CCD distance without hurting my camera, so I took a different tack - I used plastic shims between the inner lip of the camera's lens mount and the Rift's A cup until the tree was bang in focus. Then I tried the C-cup with the same shims (still perfectly in focus) and then with an extra shim (+1mm approx) and with one shim less (-1mm) - both were less focussed.

(The shims rested against the lens in the cup, in each case, not the cup itself, and the throat of the DSLR is large enough that the eyecup itself didn't foul against it. )

My conclusion was that the lenses are similar. So I hacked the lens out of the C-cup and stuck it in the duff A-cup, and it works perfectly. I can find no difference between them - swapped them over a few times to see, but they seem identical. As an ex cameraman I'm pretty focus-sensitive, but this is perfectly sharp. Much sharper than swapping in the B-cups.

I understand your point about "why would they include all these lens/cup pairs", but it seems to me that if you want to provide focal correction, you need to either vary the distance between the lens and the object, or change the focal length of the lens, but not both. Cheapest way to provide configurability meant using the same lenses but in different height stand-offs (the cups), otherwise Palmer would have had to create some sort of twist-to-focus mechanism like a normal camera lens (or, more pertinently, like the viewfinder diopter adjustment on my SLR). Different stand-offs mean the lenses can be identical.

Proof of the pudding for me is that it works :)

Hopefully Oculus will come back with more info. I've asked them about the swappability of the lenses.

One possible explanation for our differing results - our eyecups (and therefore, possibly, the lenses) are different designs. Yours appear to have a subtle bulge at the top - mine do not, they're perfectly conical, with no bulge, instead holding the lenses in with three tiny flanges, and a plastic ring that clips inside. Will take a pic when my SLR's stopped sulking.


**EDIT** - forgot to answer your question - throughout experimenting, I had the flatter side of the lens toward the camera - it's the only way it would fit anyway, as the lenses ended up sitting slightly inside the camera throat. The shims I made sat inside the camera with the lens itself (not the cup) resting on them.
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Re: Fault - flat spot on lens - can I swap in a different on

Post by geekmaster »

howiem wrote:Not easy to measure the lens-CCD distance without hurting my camera, so I took a different tack - I used plastic shims between the inner lip of the camera's lens mount and the Rift's A cup until the tree was bang in focus. Then I tried the C-cup with the same shims (still perfectly in focus) and then with an extra shim (+1mm approx) and with one shim less (-1mm) - both were less focussed.
Although my test results seemed to be repeatable, my differences in focal length were small enough to be within the range of measurement error from hand-holding the lenses, which is why I quickly added my edit saying a calibrated test fixture should be used. Your shims served the purpose of that proposed test fixture, fixing your eyecup and camera at known accurate distances.
howiem wrote:I understand your point about "why would they include all these lens/cup pairs", but it seems to me that if you want to provide focal correction, you need to either vary the distance between the lens and the object, or change the focal length of the lens, but not both. Cheapest way to provide configurability meant using the same lenses but in different height stand-offs (the cups), otherwise Palmer would have had to create some sort of twist-to-focus mechanism like a normal camera lens (or, more pertinently, like the viewfinder diopter adjustment on my SLR). Different stand-offs mean the lenses can be identical.
Realizing that focus is performed by changing the lens distance from the screen, it does make sense that a single lens could be adjusted for focus by using different sets of eyecups, now that you explained it that way. It is nice to have the extra eyecups, but it seems more practical to me to have a single adjustable eyecup, perhaps with a threaded base so it can be screwed in at out of the eyecup holder to adjust the focus. Such an adjustable eyecup could be added later, using the existing design to mount a threaded base in place of an eyecup.
howiem wrote:One possible explanation for our differing results - our eyecups (and therefore, possibly, the lenses) are different designs. Yours appear to have a subtle bulge at the top - mine do not, they're perfectly conical, with no bulge, instead holding the lenses in with three tiny flanges, and a plastic ring that clips inside. Will take a pic when my SLR's stopped sulking.
Even with that eyecup bulge (or lack of a bulge) around the lens, our lenses still might be the same. Or they may have changed, but we would need a side-by-side comparison (or information from Oculus) to know for sure.
howiem wrote:**EDIT** - forgot to answer your question - throughout experimenting, I had the flatter side of the lens toward the camera - it's the only way it would fit anyway, as the lenses ended up sitting slightly inside the camera throat. The shims I made sat inside the camera with the lens itself (not the cup) resting on them.
Normally, we are using these lenses to focus a nearby object. In that case they only work correctly in one direction such that you can focus all of the screen at the same lens distance. When reversed (more-bulged side toward the eye), you get a LOT more tangential distortion (which could provide additional FoV like I use in my lens stack experiments), but you also cannot focus on the entire flat FoV (it wants to focus on a curved surface). Depending on your application, extra FoV may be more important than even focus, but the RiftDK design chose focus over FoV from extra lens distortion.

In your camera experiment, you are focusing on distant objects, and unless your target is a flat wall with grid lines marked on it, you may not see that it is focusing a curved surface (including the things peripheral to your target tree). Because you are using the lenses in a telescopic configuration instead of the microscopic configuration used in the Rift, I am not certain which way the lenses should go.

CONCLUSION: It is difficult to draw a conclusion about the varying results between our tests. On one hand, I hand-held my lenses, while acknowledging at the end that I should use a test fixture for more accurate results, and my different measured distances may be attributable to measurement error because of that. But on the other hand, your test results used the lenses in a telescopic configuration, to image a distant object, while mine used a microscopic configuration just like is used in the Rift Dev Kit. I cannot draw a definitive conclusion from our results. What I would LIKE to see is your "SLR with shims" method used in a microscopic configuration similar to what I used. Then we can be certain of the results (unless somebody with more optical mathematics prowess can confirm that the telescope/microscope difference has no affect on the results).

That said, it is only the end results that matter in this situation, and I am glad that swapping lenses worked out so well for you.

Congratulations!
:D
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Re: Fault - flat spot on lens - can I swap in a different on

Post by MSat »

Interesting. Sounds like Oculus could have used a single lens assembly with threaded cups and maybe click stops to allow for a greater range of focusing options. I don't know if the such an assembly would end up having to be too large to prevent obstruction of the display though.
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Re: Fault - flat spot on lens - can I swap in a different on

Post by geekmaster »

MSat wrote:Interesting. Sounds like Oculus could have used a single lens assembly with threaded cups and maybe click stops to allow for a greater range of focusing options. I don't know if the such an assembly would end up having to be too large to prevent obstruction of the display though.
The height difference between the different eyecups is quite significant, so a single threaded cylinder for the eyecup would probably not work (blocking part of the screen as you suggested). A proposed design I described previously in my custom eyecup thread avoided that problem by using a pair of telescoping cylinders, which could be threaded between them, so that the adjustment does not block the screen at short focal distances (similar to how some binoculars have a threaded lens adjustment for one eye to compensate for eye differences).

While designing a variable focus eyecup, it would be nice to also add variable IPD. The design I suggested in the thread mentioned above uses a replacement eyecup mount with tabs on top and bottom that allow the eyecups to slide left and right, for IPD adjustment (while still blocking dust from getting on the screen).
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Re: Fault - flat spot on lens - can I swap in a different on

Post by MSat »

geekmaster wrote:
MSat wrote:Interesting. Sounds like Oculus could have used a single lens assembly with threaded cups and maybe click stops to allow for a greater range of focusing options. I don't know if the such an assembly would end up having to be too large to prevent obstruction of the display though.
The height difference between the different eyecups is quite significant, so a single threaded cylinder for the eyecup would probably not work (blocking part of the screen as you suggested). A proposed design I described previously in my custom eyecup thread avoided that problem by using a pair of telescoping cylinders, which could be threaded between them, so that the adjustment does not block the screen at short focal distances (similar to how some binoculars have a threaded lens adjustment for one eye to compensate for eye differences).

While designing a variable focus eyecup, it would be nice to also add variable IPD. The design I suggested in the thread mentioned above uses a replacement eyecup mount with tabs on top and bottom that allow the eyecups to slide left and right, for IPD adjustment (while still blocking dust from getting on the screen).
Ah, I must have missed, or forgot it. Wonder what Oculus will do for DK2. We know they're going to have those features for the consumer version.
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Re: Fault - flat spot on lens - can I swap in a different on

Post by Ben »

MSat wrote:Wonder what Oculus will do for DK2. We know they're going to have those features for the consumer version.
Has anything actually been confirmed? I'm sure they have designs, prototypes and a list of things they'd like to have, but I don't think even they know for sure what's going to actually make it.
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Re: Fault - flat spot on lens - can I swap in a different on

Post by gray »

Ben wrote:
MSat wrote:Wonder what Oculus will do for DK2. We know they're going to have those features for the consumer version.
Has anything actually been confirmed? I'm sure they have designs, prototypes and a list of things they'd like to have, but I don't think even they know for sure what's going to actually make it.
I'm guessing they will if it's all possible. They need people to work with the upcoming positional tracking prior to the commercial Rift.
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Re: Fault - flat spot on lens - can I swap in a different on

Post by MSat »

Ben wrote:
MSat wrote:Wonder what Oculus will do for DK2. We know they're going to have those features for the consumer version.
Has anything actually been confirmed? I'm sure they have designs, prototypes and a list of things they'd like to have, but I don't think even they know for sure what's going to actually make it.
Palmer has stated numerous times the importance of having focus and IPD adjustments in the consumer version.

There has been mention of DK2 in at least one magazine, and some slides from a presentation, but Oculus has not formally announced anything.
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Re: Fault - flat spot on lens - can I swap in a different on

Post by Ben »

Ah, okay. I hadn't seen anything beyond the panel resolution being touted as a high priority. Hopefully any adjustments could be detected and reported to the software so the distortion can be updated automatically too.
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