29. August 2002
What resolution do I need from my (new) scanner?
Well, I admit, I can not really answer that question. What this article really does is, that it shows how film looks scanned at different resolutions.
The idea here is, that this should give you, the reader, an insight in how much information can be extracted from a piece of fine grain film.

A rule of thumb
A rule of thumb says that you should at maximum need an output of 360dpi on your printer to get photographic output quality. Less will also do, but if you can scan your film to match this output, then you should have no problems.
Now, what size of output do you plan on using?
An example:
I'm using a 13″ x 19″ printer and often prints 12.5″ x 12.5″ pictures from my square format 6x6 frames (remember that Rolleiflex TLR's are my favorite cameras).
The so-called 6x6 frames (in cm) are in reality 2.25″ x 2.25″. For a 360dpi print at 12.5″ x 12.5″ I need to scan at : (12.5″ x 360 dpi) / 2.25″ = 2000dpi

What if I want to print a 24x36 frame on the entire 13″ x 19″ (A3+) paper that my printer can handle?
(36mm = 1,42″)
(19″ x 360 dpi) / 1.42″ = 4800dpi

dpi or ppi?
I've used the term "dpi" above which means "Dots Per Inch". When talking of scanners it might be more appropriate to call it "ppi" - "Pixels Per Inch" since there aren't really any "dots" involved.
But don't get confused, it's all the same.

Scanning examples
Now to the main purpose of this article, namely showing you how a piece of film looks scanned at different resolutions.
Although this might not answer any questions about what scanning resolution you'll need for any special purposes, it still shows how much (or little) information and details can be extracted from film.
I've used a medium format frame, the film is a Fuji Provia 100F (RDPIII) and the scanner used is a Minolta Dimage can Multi Pro medium format film scanner.


You will see area inside the red-square (the top of the windscreen wiper) enlarged below.
There is no sharpening or other kind of software manipulation involved, except for the conversion to jpg. All resolutions was scanned directly on the scanner.


600 ppi


800 ppi


1200 ppi


1600 ppi


2000 ppi


2400 ppi


3200 ppi


4000 ppi


4800 ppi

What can be seen?
It's not until 2000ppi or perhaps even 2400 ppi that you actually begins to see any film grain. Keep in mind though that the Fuji RDPIII is a VERY fine grained film.
And then even at 2400 ppi you are not seeing grain in ALL parts of the picture. It's not until 3200 ppi that you actually see grains in all parts of the picture.
Although the amount of actual image details doesn't really increase at around 3200 ppi and up, you do actually still see a bit more film grain all the way up to 4800 ppi.

Here's another way at looking at it:


600 ppi


800 ppi


1200 ppi


1600 ppi


2000 ppi


2400 ppi


3200 ppi


4000 ppi


4800 ppi

What can be seen now?
Above I'm just letting the browser enlarge the images, there's no bilinear interpolation going on.
Now it becomes evident that the difference between 3200ppi and 4800ppi actually isn't that big. And maybe we got all the grains already in the 3200ppi scan?

Conclusion
There isn't no real conclusion to be drawn here. The idea was to show how much detail can be pulled out a scanned slide at different resolutions.
As many times before, it seems that 3200ppi will get almost everything out and in many case even 2000ppi or 2400ppi might do the job!

Happy scanning!

Links
I've written some related articles that can be found here:
• Minolta Dimage Scan Multi Pro - 4800 dpi on 120 film
• Medium format vs. 35mm vs. digital