Placing filters at the back of a lens is a technique that was used “back in the day” of film photography (although it is still done nowadays).
It has drawbacks and advantages, one of the latter being there is almost zero risk of vignetting.
As far as I know, OWL is the only system that lets you do just that for modern photography on mirrorless systems. It sure is convenient, but not ideal because it does not have electronic control of the aperture nor AF.
I came across this article and video demonstrating a similar line of thoughts. The company is developing a new ND filter system that is placed inside the lens adapter, and electronically controlled.
It seems like a good idea on the paper, because it will be much more convenient than having to transport and put on / take off delicate filters. (trust me, it can be a real pain in the… neck). Also, they claim it will be possible to remotely control it, which is really cool.
A few questions arise from this tech though (I will skip everything that has to do with aperture and AF control here and focus on the new feature):
1/ It currently does not have aperture nor EF control, with may or may not be a problem depending on the lens or subject you shoot. But they mention in the interview electronic control will be available in the final product, so check.
2/ what will be the maximum density the filter reaches? For 3-5 minutes long exposure in day time, you need about 16 stops. They mention in the interview it will go up to 12 stops, which is a bit on the low side… But let’s see if they can push it darker.
3/ what about optical quality? and especially chromatic aberrations, color cast/shift?
4/ what about light leaks? Pretty obvious point, but when you have a 16 (or even 12) stops filter, you want your adapter to be really light tight. I guess it’s the case, but who knows?
Bottom line, I think it’s a cool product and every long exposure photographer out there should keep an eye out for it.
Lots of potential, some real questions and technical challenges.
Let’s hope we get to see a finalized product that will be high quality and we’ll all die to use.