I'll try to answer, item by item. Hopefully I'll get the format right..
The idea of a "flat field" is a primary concern. Even though there are "primary design criteria", there are also "secondaries" ... and those characteristics that are inherently "there". Every "flat field" lens will have a finite depth of field or depth of focus. Example: Mount an enlarging lens onto the lensboard and use it as a taking lens. You will see that everything focuses in much the same way as a taking lens - there will be no razor thin "plane" where everything "snaps in" and then "snaps out".Originally Posted by Bob Carnie
??? Depends on WHY the edges are "unsharp." If sharpness isn't there in the negative, there is little hope for improvement. If there is an amount of misalignment - Stopping down most certainly will increase apparent sharpness. Try tilting the easel -- something I've done to correct perspective. One focuses near the center, and then stops down to gain a suitable amount of "sharp".I have never corrected un - sharp edges of a print by closing down, alignment of the lensboard negative stage and easal is the only way that I am aware of getting sharp edge to edge.
True ... there is an "optimum" aperture for all photographic lenses - part of the design criteria. Has little to do with alignment, though.As well try putting a lith negative of very small sharp type into your enlarger and project , and then start wide open , middle f stop, and very closed down.
adjust the density so that each print equal.
You * will * find that some of the prints are much sharper and ledgible to read.
I will, though, take exception to your use of the word "much". With my lenses, Rodenstock, Schneider, Omega, Elgeet(!) - what the heck is on that old Federal ? - It cannot be described as "much". At least not much-- and I have done a fair amount of lens testing (Optical Quality Assurance Specialist - in a previous life) in my opinion.
Come to think of the "Elgeet" lens -- that is on an old DeJur enlarger I got as a gift. It needs to be rewired ... the electric cord accompanying it disintegrated with time. It has a tilting negative carrier, used for perspective control, and a much neater way to do it than film cans, bricks, sundry other items under the easel to obtain the necessary amount of tilt.
I guess one could characterize the use of that carrier as, "Aesthetically Adjustable Misalignment".