There’s a very short answer to this question and it should not be surprising to any good photographer.
- Both filters and flash should never be detectable in any photo!
It’s an obvious answer isn’t it? I’ll write it again: Nobody should know that you used a filter or flash in any of your photos.
With filters, the main things to avoid are
- color casts from using a colored filter,
- horizontal lines from using a split ND filter,
- over-polarization in skies (weirdly dark blue) from using a circular polarizing filter.
With flash, some things to avoid are
- weird light in a subject’s eyes,
- harsh artificial highlights introduced by the flash being too bright or at the wrong angle,
- artificial shadows caused by too much flash or the wrong angle.
Sometimes I need to be reminded of these things, and while I rarely if ever use flash in any of my photos, I do use filters. And I do mess up occasionally by allowing myself to “see the filter” within my final images. This is never o.k. and I always regret it later. So, keep these simple rules in mind and don’t let your filters or flash intrude on your photos.