Focal length of magnifying glass for VR?

[dwaring28720]

Hi the standard focal distance seems to be 45mm for vr headsets such as google cardboard. In other words there's a pair of convex lenses in front of your eyes with the screen being 45mm away from the lenses ... I am not sure why you can't use different lenses to reduce the focal distance to the screen whereby making the whole rig shorter and ... more comfortable to wear.

First does anyone know why this is the distance that's being used as the standard? and second can someone dumb down the calculation to work out what would make a shorter focal length such as 25mm instead of 45mm.

I apologise for any grammar or typos I am using the wifes phone and its too small for my fingers

Kind regards for any answers

 

[Andy Resnick]

I haven't played with any of those, but my guess is that the optical system puts the image plane at (negative) infinity: your eyes will be relaxed when viewing the image, which makes using the device less tiring on your eyes. Most eyepieces work that way.