What scopes are being used for field target?
There are a wide variety of scopes available today for airguns.
Here is a list of some of the most common ones used today:
Leupold Competition 35x, 45mm
Tasco Custom Shop 8-40x Rifle Scope
BSA 10-50x, 60mm
Nikko Stirling 10-50x, 60mm
Bushnell Trophy 6-18x, 40mm
Simmons 6-18x , 40mm
Leupold 6.5-20x, 40mm EFR
Simmons 6.5-20x, 40mm .44 Mag
Bushnell Elite 4200 6-24x, 40mm / Bushnell Elite 4200 8-32x, 40mm
Swift 6-24x, 50mm / Swift 8-32x, 50mm
Burris 8-32x, 44mm R/A
Nightforce 36x, 56mm Field Target
This is not a complete listing, but it gives you the general idea. In general, a scope for all but Hunter division will work better with at least 25X power and at least 40mm objective diameter. Focusing is essential both for eliminating parallax and for range finding.