This is why the display needs to create a distorted image to display normal to the eye.
Hence lens properties will be more key to FoV vs lens diameter. Thus Flinnt is right on size of tge lens is not the key factor for FoV. But may need a larger lens to fill ones eyes with the given FoV.
For example Alegant(sp?) Diid 60° FoV with their media projection headset. But with different small optics could likely expand the FoV.
If we try to not divert from the case in hand… if your using a bigger display then the lens apparent diameter must meet a minimal size. No matter how you distort the image you can fake the directionally of the light. It’s the apparent size of the lens vs the lens eye 
that important not the size itself
maybe with complex interfering optic we could manage to create a wave pattern meetings this requirement but with simple optic. Best example is a contact lens vs glasses big vs small FOV even if the contact less have a smaller diameter … It’s a simple case of vignetting
My point was all thing being equal it will be really difficult for valve to achieve an 135 FOV with the index headset if they use a 50mm lens like the vive and the OD+. Pimax is using then equivalent of a 60 mm (issh) lens in the small FOV Option
You could move they eyes closer to the headset like @jojon suggested for the micro lens but this would be detrimental to glasses wearers and more prone to fogging…
Really not simple to make a high end headset!!!
Eye projection. That way don’t need to watch a reflection on lenses. 
Interesting discussion on Reddit:
Well we can be sure of one thing the fov will not be big enough to suffer from popping or it would have been fixed already
This is the Vive Pro also set at the same 90%: https://i.imgur.com/tscuY3m.jpg
Seems similar. Man it would be a HUGE bummer if the res was just Vive Pro resolution.
Indeed though with the multi lens might made sde min. Hopefully if it is the same it is at least an rgb.
Unfortunately it is not possible to derive the panel resolution from the SteamVR settings page, because it just shows the recommended render target resolution. The SteamVR (actually the OpenVR stack) does not even know the physical panel resolution internally. The only information it has from the headset driver is again the recommended resolution which the driver prefers to get from an app, to be able to render the image in a optimal way.
It could just happen that the Index driver was set to recommend the exactly same res as Vive Pro for mimicking the Vive Pro performance requirements, but internally rendering it at the much higher res displays of the Index.
It could also be a fake screenshot.
I would not read too much into it, we will know in two months anyway. 
Oh it’s always about physics…until the mathematician points out physics is maths.
The optical solution could also produce less PPD disparity across the FOV, which should have a significant effect on the chosen default render target.
Kind of interesting we could conceivably have Indexes (…or “indices”? -It’s a proper noun, so probably not. :9), way before our 8k/5k:s get fitted with their full arsenal of accessories… :7
Yes people talk about ‘Valve time’ but we’ll be fully operational in Index a long time before pimax can be used as intended. We won’t even have a choice of face foams or a lens holder let alone lighthouses, controllers or a strap to actually hold it in place.
Watching that ship sail.
15 june release it seems https://www.roadtovr.com/valve-index-vr-headset-leaked-steam-page-release-date/
It’s going to be a long 2.5 months …
In my research Valve Index have 50 mm symmetrical lenses like Vive and also same FOV. About FOV it’s not competitor Pimax 5k+ at all (sadly for VR market).
P.S. People dont like this, but is true
Well in no way it will beat or even meet Pimax wide FoV. But that doesn’t mean it’s not a competitor. If this thing features indeed 2k * 2k and is well built with software that works well, good lenses without distortion or godray’s, at a competing price, then it’s bye bye for pimax in my opinion.
If it’s ‘just’ Vive Pro resolution then I kind of agree with your statement. But somehow I can’t imagine that, I really believe Valve wants to propel the VR market forward, with innovative hardware. Besides, the leaks showed they were testing with 2k * 2k and we do know that both BOE and JDI are about ready with their panels.
With rendering resolution 2016x2240 its 1440x1600 display. Or you know other display with 9:10 aspect ratio?
Identical lenses size and form. I think is just improved lenses for 3.5 inch display and identical FOV.
I kind of feel the same way. My motivation for Pimax was not the FOV originally, but the resolution as in OG Vive everything felt only as an approximation, but it could not really convey the detail (being it a text, a texture or a shape). Plus, being a glasses wearer, my effective FOV is anyway gimped already.
What will really matter is if Valve could deliver the full package without all the issues Pimax has (and which cannot be fixed without redesigning the headset from the ground). If they do, I guess the only thing which could prevent me from getting one will be the price.
As I wrote above, there is no direct relation between the recommended render target resolution displayed in the SteamVR settings and the physical panel resolutioin. We simply do not know.
Of course, but here 2 thing now: 1. It’s OLED, 2. aspect ratio 9:10. 3. Identical lenses form for 3.5 inch display.
