Gel filters and the Sigma 15

I've had such great success recently with the D800 in natural light (eg hammerhead shots) I thought I'd fit a filter to the back of the Sigma15 and do more. Has anyone noticed any deterioration of sharpness doing this? I know a lot of you use Magic filters. I still have a large stock of Wratten filters from my professional photography days but of course I used them with big cameras not these little DSLRs.

Good question John, i was asking me the same question but never made some tests, mainly because of the lack of filters here where i live.
I would like to try shaping a glass red filter to the size and thickness to fit this tiny filter holder the Sigma 15mm has.

I believe that the Sigma with the right colored red filter on a sunny day and our excellent Sardinian viz will produce excellent pictures of our wreck here.
I may should give a try to Lee gels as i should be able to get a piece here.

Chris