IÂ´m installing my PJ on ceiling and projecting from front. My sony manual tells me that the angle of optical axis should be between +3 to +13.6 degrees. Have you tried different angles and how does it affect?
The lower the projection angle the less keystone you will use.
I had my 1251 lowered at the back when ceiling mounted - reducing the projection angle. Initially because my 16:9 screen was too high on the wall. But then I realised that my picture was much better because I was using much less keystone.
You can probably only do this with a 16:9 screen, and you may have to modify the flapping to get even focus - I did. The modified flapping was way better than the stock focus anyway.
Also, the more angle you introduce to the the set up, the more your lenses will be off of their focal plane.
I have a 1271 that I just mothballed and for about 6 months I had it set up so that there was more than the usual angle of incidence.
What I found was is was very difficult to have sharp focus between the top and bottom of the image.
The solution is to install Moneyasses modification. Do a search on the forum for a link to his web site, which has a PDF file for reference.
It's not difficult, but block some time aside to do it. Basically, it involves removing the lenses and the lens spacers and installing allen bolts that will allow you to continuously vary the projection angle of the lenses.
You'll need to do a pretty substantial re-covergence afterward.
Having said that, once I had the focus dialed in, the image was sharper than ever.
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