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Has anyone fitted filters to the rear of a Canon 17-40L?
This photo shows the rear filter slot (it's not mine - I'm not selling! - I borrowed it from here just to show the slot http://i247.photobucket.com/albums/gg149/bpnews88/Personal_Use/Shooting/IMG_3531.jpg ). It's intended for gel filters that you cut up and slide in, but I was thinking of something different.
I am thinking of unscrewing the little clip-mount thing and fitting a glass 10-stop ND filter there, cut to size by those nice people at Lee Filters. I'm thinking of using that sticky-backed magnetic sheet (fridge magnet style) cut into two rings, one stuck on the lens and the other to the filter. The magnetic sheet is very thin, about 0.5mm. That should hold it on okay, seal it from light leaks around the edge, and there is just room for everything to clear the mirror at the back of the lens on my 5D2 - there's about 3-4mm of space.
I can't be the first person to have thought of this, but google doesn't bring anything up. The advantage would be a) you can put a second filter on the front, polariser or grad maybe, without vignetting, and b) because of the way the image is projected from the back of the lens it would dramatically reduce the optical vignetting you inevitably get with a dark ND filter on the front (because a super-wide looks through the side of the filter at an angle towards the edges, making a 10-stop ND a lot darker than that at the periphery).
Apart from whether it's possible or not, I'm worried about reflections coming off the sensor and bouncing back off the rear surface of the filter, it being so close. I wondered if anybody has any experience?
This photo shows the rear filter slot (it's not mine - I'm not selling! - I borrowed it from here just to show the slot http://i247.photobucket.com/albums/gg149/bpnews88/Personal_Use/Shooting/IMG_3531.jpg ). It's intended for gel filters that you cut up and slide in, but I was thinking of something different.
I am thinking of unscrewing the little clip-mount thing and fitting a glass 10-stop ND filter there, cut to size by those nice people at Lee Filters. I'm thinking of using that sticky-backed magnetic sheet (fridge magnet style) cut into two rings, one stuck on the lens and the other to the filter. The magnetic sheet is very thin, about 0.5mm. That should hold it on okay, seal it from light leaks around the edge, and there is just room for everything to clear the mirror at the back of the lens on my 5D2 - there's about 3-4mm of space.
I can't be the first person to have thought of this, but google doesn't bring anything up. The advantage would be a) you can put a second filter on the front, polariser or grad maybe, without vignetting, and b) because of the way the image is projected from the back of the lens it would dramatically reduce the optical vignetting you inevitably get with a dark ND filter on the front (because a super-wide looks through the side of the filter at an angle towards the edges, making a 10-stop ND a lot darker than that at the periphery).
Apart from whether it's possible or not, I'm worried about reflections coming off the sensor and bouncing back off the rear surface of the filter, it being so close. I wondered if anybody has any experience?