Remedial Fields Clarification Desired - LOS

[Tankodactyl]

Recently, the T-Rex, Tambu and I have been playing several individuals online & face-to-face who insist that fields block the LOS of units on the same elevation.

I can't find this stated anywhere in the 4th Edition rules. Maybe I am missing something?

Please consider this example:

In flat, clear terrain, two combat units are 12 hexes apart in clear terrain, and both are on the same elevation level. If there is a large - let's say 5-hexes deep - field in summer wheat in between them, can they observe and fire at each other?

I think they can see each other & therefore engage each other. What do you all think?