Like Juan, I didn't need one long ago. As a veteran geezer, I do. The favorite is a cheap one that functions like the Patterson Mick linked to. An old Mitchell Unicolor that can be used anywhere in the image area is good for checking enlarger alignment. A 14 inch tall Patterson lets me reach the focusing knob when making big enlargements. The Pattersons let one focus on the grain instead of on the less magnified image like the Magna Sight or the similar ancient Baush and Lomb..