Trying to use a b+w 87c IR filter on a Nikon 55 f2.8 macro on my D70. In particular, doing flash stuff after Bjorn made me happy by pointing out flashes emit lots of IR. And I am having the darndest problem. I carefully focus without the filter - even calibrated the mirror to make it spot on so what I see is what I get. Then I gingerly mount the filter and take the shot. But they invariably come out back focused. So this led me to try a very controlled situation where I took a non IR shot, IR shot, then non IR shot again and it turns out that when the IR filter is mounted, the DOF moves back. Significantly. Why is this? This is a b+w model 093 87c filter. Or is this something about IR that I just didn't know about until now? And if this the case, how the heck do I focus?