Quote Originally Posted by Bob Carnie
Hi Ed
sorry to open old discussions,
but on a previous thread, you suggested that if one focuses an enlarging lens wide open and notices the edges unsharp of the easel, all one would have to do is close down the apeture and the edges would *snap* into focus.
Since that discussion I have consumed quantities of beer to wrap my head around that statement .
Sounds to me to be a terrible oversimplification of what I must have written. That statement is not necessarily true at an *absolute* level.

In operation, we use the Grain Focuser with the Aperture wide open, to minimize the effect of "depth of focus" - in this case a we want everything to "snap" into focus - as much as possible. Closing down the aperture will increase the depth of focus ... making the "best" focus more difficult to determine.

If something on the borders of the field is not sharply "in focus" to start ... it will not NECESSARILY be corrected, invariably and always, by using a smaller aperture. There will be correction TO A DEGREE of "curvature of field" and "perpendicularity to optical axis" errors ... but not compete enough to say "everything will snap into focus" without reservation.

But ... I'm not averse to quaffing a pint or two at times.