My wife, who is a DVM showed me a little blurb of an article in a trade journal (the article was actually referenced to a real journal) that was talking about vision screening of working dogs. It the study, they took a small number of young FT dogs and let them mark naturally (The marks were about 150 yards... nothing terrible), and then they installed contact lenses with 1.5 (need glasses) and 3.0 (you have terrible vision) diopters and let them run marks again.
The dogs (all labradors) were timed and judged by field trial judges. The handlers and judges did not know which treatment was applied to which dog. Differences in time and FT judge score were significant across all groups.
I just have my own dogs and have no idea how to screen for vision short of running long marks, I'm curious to know how professional trainers screen for visual acuity as marking by itself is more complex than that. Also, as our dogs age, how do we account for changes in their vision. Just like anything important, there are a ton of Genetic x Environmental influences here.
I thought this was a interesting topic... But not interesting enough to stop me from going Pheasant hunting tomorrow.