Ronald, sometimes it all feels like alchemy! Only the other day I did three identical prints and found perfect focus slighly fell away though the three. The enlarger was locked down perfectly so who knows why, but these odd things happen! why these longer lenses work so much better I do not know, but suspect it is something to do with printing short FLs on a 10x8 enlarger. even with a recessed panel, something perhaps did not jive properly.