hi all, i have a question, any help is really apprecited...... i take underwater photos in various underwater housings, for those of you that do not know when you use a dome port in front of a wide angle lense this effects the close up capabilities of a lense and pushes the all important close up further from the film plane. to supplement for this you can correct this by using a close up filter on the front of the lense, the strength of the close up is determined by distance of the dome port. my problem....... i want to use a fisheye lense that does not accept front filters. does anybody know if using a +4 lense on the rear filter holder would work the same as a screw in type filter on the front optic. i will have to get this made up so i can't trial and error this one, i don't want to go through the expence if it would not work....... my thoughts is it will not work but can anyone shed any light?????? my lense is a nikon 16mm f2.8 afd fisheye but i would consider a sigma 15mm f2.8 ex fisheye if easier to get a filter made. i use nikon f100's and fuji s2 pro. thanks for your help.