From the LensRentals Blog (article by Roger Cicala):
Well, I’ve written (with some misgivings because it has a tendency to create rioting in the streets) several articles about protective filters. Articles that say sometimes you shouldn’t use protective filters, and others that say sometimes you do need to use protective filters, and most recently, one showing how cheap filters can ruin your images.See how the filters performed on the LensRentals Blog's full post.
Because no good deed goes unpunished, the result of all this has been about 762 emails asking if this filter was better than this other filter. I answered most with I don’t know for sure because I don’t test filters and, of course, everyone asked me to test filters. To which I said no. Life is too short.
Even Drew, who I sort of work for, asked me to test filters and write up the results. I told him I’d need at least $1,500 worth of filters to make even a basic comparison, which I thought would end the conversation. But next thing I know Drew was ordering $1,500 worth of filters. I told him I’d get around to it some day.
Then Brandon, who sort of works for me, emailed and said he could build a gadget to measure transmission and polarization through filters if I wanted to start testing filters. I told him I’d get around to it some day. Then he said it would have lasers. "Someday" became "right now" because of lasers. We’ve got lots of cool toys at Olaf and Lensrentals, but no lasers.
So today I will show you the results of testing a couple of thousand dollars worth of clear and UV filters using a couple of thousand dollars worth of home-made laser light transmission bench and a lot of thousand dollars worth of Olaf Optical Testing bench. So that we get this out of the way now: please don’t email asking me to test your favorite $6 UV filter. I’ve opened up Pandora’s Filter Box with this, and it’s already going to lead to way more work than I wanted to do. I’ll maybe do some testing of circular polarizing filters later, and maybe some testing of variable neutral density filters after this. Maybe not. I’ve got ADD, and I get bored easily. Even with lasers.
I like to keep these articles, well, no geekier than they just have to be. But I also want our methods to be transparent. So I’m going to give an overview of methodology in the article and put the geekier stuff in a methodology addendum at the bottom.