I actually used the drop in circular polarizer on my 600mm f/4 for the first time in a couple of years last night while photographing some Beavers. The sun was setting behind me over my left shoulder and lighting the scene but also throwing a ton of glare on the water. I had to dial in a couple of stops of negative exposure compensation to keep the glare on the water from completely blowing out and that pushed the dark Beaver even darker. So I dropped in the polarizer, dialed it to cut as much of the glare as I could and took some more shots.
Here's shots with and without the polarizer
This was one of those rare times where the polarizer for the 600mm f/4 was handy but it met the conditions:
- Scene that would be helped by a polarizer due to the glare off the water
- Patient subject that stuck around while I messed around with the polarizer
- Enough light. - This one wasn't completely obvious as the polarizer and TC pushed my ISO sky high to 12,800 where without the polarizer the ISO was down at 2800 (both images shot at f/5.6, 1/640") but without the polarizer I had to do a hard pull on the shadows to bring out any detail so they both ended up with similar noise and I ran both through Topaz DeNoise to clean them up.
It might be another couple of years before I find myself using the drop in polarizer again but in some situations it can make a difference.