The most difficult thing to diagnose in a horse is visual acuity but it might also be that his vision is changing. He is not that old a pony, I don’t consider a horse old until they act it : ) but his ability to focus quickly or to adjust to changing light intensities could be a factour.
I have one mare that can’t see a thing in hazy light but is fine otherwise (uveitis), another can’t adjust normally from bright to dark light so sees best in haze. Having had older horses as well as rehabs and rescues most of my life, I have grown accustomed to the odd, less obvious details that many won’t think about. I hope I can be of help because of this tendency : )
Of course, this would be associated with his spooking, the bucking, I still wonder, is tweak-related.
Sure there's right and wrong but mostly there's just a whole lotta different.