I've had my eyes checked fairly recently. I don't get them checked often because my vision is 20/15 and my eyes are healthy overall. I work as a software engineer, so this is one of the first times in a long time I've had trouble with this. I didn't have the same trouble with my 13" MacBook Air. Not sure what the difference is.
When I look at my wifes retina iPad I experience eyestrain alike. My explanation to this is the timelag the device needs to focus a newly displayed picture/page to retina-level.
Due to performance limitations at the beginning a newly opened image/page is always somewhat blurred. It takes some time to display the image at full resolution. During this short period of time my eyes experience loss of focus (compared to the previous picture) and start focussing which they can't because the device is still building up the image. So in practice when viewing photos on a retina-screen my eyes are continiously focussing while in contrast the device is kept at the same distance. It's like an autofocus camera trying to focus on a grey area. The motor keeps on running without result.
Or seesickness due to opposing inputs from eyes and body.
So to me the problem is caused by lack of processingpower or the image-building algorithm, not the display.
That's why I don't like retina displays ... yet.