hello, i am a newbie in security system.
i facing one problem is which type of lens should i use to view 400 meters front.
Any solution of this?

not enough info.....need distance from and if you want to view that entire 400? There are many lense calculators out there..........do a web search or a search on the forum.

400 is a lot to view if........maybe your asking to view an object (person) at 400?