Here are some examples of filtered and non-filtered perceptions:
"She could see him in the moonlight."(Filtering)
"He stood in the moonlight." (Non-filtered alternative)
"He heard a shot. (Filtering) . . .
"A shot rang out." (Non-filtered alternative)
To spot filtering, look for noun-verb combinations like the following:
- she felt, she saw, she smelled, she heard, she tasted, she knew, and
- "could-forms" like she could feel, she could see, she could smell, she could hear, she could tast, etc.
Sometimes you want a filtered effect; sometimes not. Whether you do or not depends on how solidly you've established the viewpoint character in the scene. If the character viewpoint is clear to the reader, then the non-filtered version is best because it keeps the reader immersed in the viewpoint character's point-of-view, and it feels more "immediate". However, if the character viewpoint needs to be "established" yet, the filtered version may be better because it helps do that.
In general, the unfiltered version is typically the more intense way to involve the reader. As long as the reader doesn't become confused as to who is experiencing the sensations, unfiltered perceptions are usually best.