Originally posted by JayR will a step up ring affect the optics noticeably?
Noticeably: No.
Very slightly: Yes.
I often use the +8 dioptre DCR-250 with a 49-52 step ring on my 49mm primes; the ring thickness doesn't seem to matter. But those primes have shallow insets. I'm not sure if your DFA100 has a shallow or deep inset. Well, I can't find my Raynox kit right now (I'm still unpacking gear from a long journey) but I put a +10 dioptre closeup adapter onto my MacTak 50/4, which *does* have a deep inset, about 50mm. I checked the focus on some text. Then I handheld the closeup lens a bit further from the MacTak -- actually, I held the closeup lens constant re: the text, and pulled the camera back. I could move it about another 50mm before I started getting vignetting. Magnification did not appear to change.
I substituted a ruler for the text. With the MacTak fully extended to 1:1, it sees ~23mm, as expected. With the +10 dioptre lens attached, it sees ~16mm, so magnification increased to ~1.34:1. The +8 dioptre Raynox DCR-250 will give a bit less magnification. (The 2x TC would take you into 2:1 territory, but with a severe loss of light.) I then tried pulling the camera away from the closeup adapter, and magnification seemed to decrease very slightly -- hard to tell for sure, what with shadows, my shaky hands, and the perversity of inanimate objects.
Short answer: Don't sweat it!