I place grads and polarizers up front. They will induce focus shift on the rear. Sure, you could put them on the rear and then focus, but I have found it difficult to focus through anything that has any measurable density. I used to put Wratten CC gels on the rear, but if using 2 filters, that was prone to a ghosting, or double images in highlight areas. After reading the photonet posts, I don't think I will ever put anything on the rear again. Here are some links from photonet that dealt with this:

http://www.photo.net/bboard/q-and-a-fetch-...g?msg_id=0033SN
http://www.photo.net/bboard/q-and-a-fetch-...g?msg_id=0039TF
http://www.photo.net/bboard/q-and-a-fetch-...g?msg_id=00392R
http://www.photo.net/bboard/q-and-a-fetch-...g?msg_id=003FaN

John Luke
APA/ASMP