The smoke ring is most definitely visible, blurry or not!
Question - how often to you take the lid off to "look"? I think that is playing a factor here. Just my 2 1/2 cents.
Take the lid off as few times as possible. Unless you see the temp go down drastically leave it on. Have your coals ready to go on, if you are using a chimney, have someone remove the grill, you add the coals, get them back on and close that lid ASAP. I find by using a combination of lump hardwood and briquettes I can maintain hotter and longer for quite awhile. I wish I took notes too - but it seems like a good 2 hours or more before I noticed a temperature change. Everything cooked MUCH faster because I was leaving the door closed and not messing with the coals as much.