Sharpness and resolution: Drum scan compared to enlarger

[Bruce Watson]

So in a nutshell sample size is altered to suit ppi setting or at least there is a close link. So my next question is:

When doing a drum scan for say a 20x16 print, I assume you set scan ppi based on what print dpi you want to use and then you set aperture size to suit the scan ppi. Is that correct?

Pretty much. Almost all drum scanner software will automatically set the optimum aperture for you. There are some reasons to override the default to use bigger apertures on occasion but not really all that often.

[edit]
and just for example:
so if I want to print at 720dpi I would need 20x720 = 14400 so from a 4x5 neg I would need a scan at 2880ppi and an aperture approx 13microns ???
[/edit]

Pretty much. As long as you don't confuse the output file's resolution in ppi with the printer's use of actual print dots you'll be fine. That is an Epson printer printing at 2880 dpi will use eight ink drops on the paper for each pixel from the file regardless of the pixel density (ppi) of the file. PPI and DPI are not the same thing and are not really interchangeable although people do use them interchangeably. Sigh...

It's sort of a moot point anyway. All most drum scanner software wants to know is output resolution and the amount of enlargement. That is, it wants to know that you need 720ppi and 4x enlargement. The software does all the math for you. With the software I'm using with my scanner, you could alternately tell it the output resolution and the target print size (that is, 720ppi and 20x16 inches) and it will do the rest. And of course you'll also have to specify 8 bit or 16 bit file saves. I don't know of any drum scanner software that would let you set your scanner resolution directly (say, at your 2880 ppi). The software wants to do the math so you don't have to I guess.

[robc]

well I think thats one huge benefit of a drum scanner in that the ppi and aperture are truly variable whereas with a flat bed the hardware resolution and sample size is fixed so unless you want to print at the hardware resolution then you have to alter resolution with software which is a destructive process.

[robc]

Thanks again Bruce, I suspected that was how it worked but wasn't sure. Doesn't look too complicated at all...

Shame I can't justify the cost of one?

[robc]

Back on topic of original question.

One thing that is often forgetten when printing with an enlarger is that all enlarging lenses are optimised for a certain enlargement factor. My Rodenstock 150 APO Rodagon N is optimised for 6X enlargement. Print smaller than that and because of less enlargement it will still be very sharp but take it down to only 2X enlargement then you are pushing your luck. That means for optimum 10x8 prints from a 4x5 neg you should be using a lens optimised for 2X enlargement. My guess is that most 150mm lenses are optimised for 4X to 6X enalargement.
And then again if you push the enlargement way over the optimum of the lens, results will not be as good as using a lens designed for that enlargement factor.