What annoys me about MY enlarger (D2 and DII) is that I spend time aligning it just right, then move the caridge up and down, and measure again, it's OFF AGAIN! Extend the bellows and collapse, repeat, and measure, it's OFF AGAIN. Remove the neg carrier out and put it in again.... you guessed it, it's OFF AGAIN!

My prints are much better than before the alignment but it keeping PRECISE alignment, it's not possible.