I have a few lenses with with a rear filter holder so I would be curious to know how the rear "gel" filters compare with regular front-mounted filters in terms of optical performance.
Until recently, I had no idea about the variety of rear-filters available. I understand that it is not as easy to work with these filters and perhaps this is why they are a dying breed? (The Canon 17-40mm has a slot on the rear while the Canon 16-35mm II does not.) Not being able to easily mount filters on some super-wide and fish-eye lenses however (esp those with a large rounded front element) have prevented me from buying these lenses in the past. But perhaps I was wrong in assuming that a rear 10 stop ND filter was not a possibility without a $400 + $140 LEE solution.
I've read a few discussions in random forums (examples here and here) where some suggest that there are benefits (less vignetting) and others go as far as to say that rear mounted filters are superior to front mounted. Unfortunately none of these statements are supported by facts and hard evidence and there does not seem to be much information about rear filters in general.
It also appears to be difficult to find and buy these filters. B&H and Adorama offer mostly colour filters as well as some ND but I can't find a place to buy a 10-stop ND gel filter. I am most interested in the KODAK WRATTEN 2 Filters - filter number 3 and 4. (4 should be darker then the Lee big stopper) A link to a retailer (US, Canada) would be much appreciated.
I would also appreciate if someone could share how to handle, cut, and clean these filters as well as some other tips and tricks.