How to check the collimation of your binoculars
Many older (and many cheap new) binoculars suffer from decollimation, that is, the optical axes are not aligned exactly parallel. I now check any new binocular with this simple test described by Rafael Chamón Cobos here. All you need is to put the binoculars on a tripod, stand in front of a mirror and follow a simple procedure. Basically you move your head slowly away from the oculars and use "parallel vision", it´s a bit like looking far into the distance or like at one of those "Magic 3D" books where you try to see 3D shapes. As you move about 20 centimeters away from the oculars and try to use "parallel vision" the exit pupils with the objective reflections will look small but merge into one single objective image - IF the binos are properly collimated. If you slowly get closer again to the oculars and manage to stay in parallel vision you should see something like this - and it´s quite a strange effect:
A trinocular! And behind it a two headed guy is watching you.
This means proper collimation. But if you see something like this, the bino is out of collimation:
And you got a problem. Check with the service technician of your trust.
The whole procedure is easier with binos that have long eye relief, with the Nikon 8x32 SE and the Swarovision 8x32 the images just merged instantly, with the Habicht 8x30 and the Zeiss West 8x30 it took me a bit more effort. I had two binoculars with extremely sharp center performance, but which caused a lot of eyestrain and this test was very helpful in proving the decollimation. A great binocular with proper collimation should be like looking through an open window...