Custom Eyepiece Lenses
As someone who also suffers from the same condition as yourself, I fully appreciate the issues it can throw up and I've done a lot of research into the very same area that you ask about.
I have to say that at the moment there aren't any really successful solutions out there, however I'm hoping to solve this after a few more discussions with someone in the optics trade here in the UK as it's a pretty common problem that I believe should be addressed by the manufacturers - after all Nikon are one of the leaders in optics so it would be pretty bad if they can't provide some help in this field.
Currently the main problems that I've come across are as follows:
1. Any lens that corrects for astigmatism would need to be mounted in a rotating mount so that when the camera is rotated from landscape to portrait orientation the lens can be rotated to match. If this is not done then the view could be even worse than an uncorrected one as the lens would be focussing light on the wrong plane altogether.
2. Few of the lens makers I've spoken to appear to be to confident in making a lens small enough to fit the eyepiece of our cameras whilst effectively carrying out the correction function to an accurate level.
I'm in talks with an engineer about the mount and someone who makes lenses for one of the major optical care chains here in the UK and I'm hopeful that we may see a solution in the near future. I'd be happy to keep you informed of any developments if you'd like to drop me an email (my address is in my profile).
For the time being I just get along with a pair of normal high grade spectacles and on my D700 I have a DK17-M eyepiece with a nice DK-19 eyecup to stop any scratching and cut down the light. This combo works really well for me with my normal specs but I'd still like to get rid of them if at all possible. Let me know if you'd like me to keep you informed of progress...
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