They can definitely startle a deer, though I don't think bad enough for him to not come back. Deer are scared of everything anyway. The flash really shoudn't be any different to them than a flash of lightening or headlights from a passing car in the distance. The sound of the camera going off shouldn't be any different than a squirrel jumping up a tree right beside the deer or a lizard scurrying under the leaves. I use trail cameras alot. Personally, I usually will remove them froman area, a week or two before I start hunting there, just to keep any extra commotion down before I hunt. I've been fortunate enough to take four bucks in the last two years over the age of 4 1/2. Three of them, I found with my Cuddebacks.