+1 on Drew's comment.
Another caveat: Even the best grain focuser (i.e., the ones that focus on an aerial image) will give you problems if you don't focus the reticle using the eyepiece adjustment first and then make sure that the reticle is in sharp focus when viewing the print. It's possible to "look past" the focus point, where the reticle is, and get inaccurate focus. Ensuring that the reticle and the print grain are both in focus together is the key.
For magnifiers that project an image on a small ground glass (e.g., Magna Sight), you just need to make sure you have your eyesight correction on if needed
Best,
Doremus
Another caveat: Even the best grain focuser (i.e., the ones that focus on an aerial image) will give you problems if you don't focus the reticle using the eyepiece adjustment first and then make sure that the reticle is in sharp focus when viewing the print. It's possible to "look past" the focus point, where the reticle is, and get inaccurate focus. Ensuring that the reticle and the print grain are both in focus together is the key.
For magnifiers that project an image on a small ground glass (e.g., Magna Sight), you just need to make sure you have your eyesight correction on if needed
Best,
Doremus