What size can the photos be printed up to after using a Plustek 35mm film scanner?

What size can the photos be printed up to for good quality after using a Plustek 35mm film scanner? asked Claire. In short you need a 3600dpi film scanner for A4 or A3 size prints and a 7200dpi film scanner for A3 size prints when the film is cropped. Here are my calculations:

• 35mm film is 36mm (1.41") horizontal x 24mm (0.94") vertical. 

• For high quality prints you need to print out typically using 300 pixels per inch.

An A4 print (11.7" x 8.3") needs:- 

• 11.7" x 300 pixels = 3510 pixels horizontal 
• 8.3" x 300 pixels =  1692 pixels vertical

An A3 print (16.5" x 11.7") needs:- 

• 16.5" x 300 pixels = 4950 pixels horizontal 
• 11.7" x 300 pixels vertical = 3510 pixels vertical.

The optical resolution dpi (dots per inch) of film scanners varies. Both horizontal and vertical pixels must be sufficient to give high quality prints. 

• 1800dpi film scanner
• 1800 x 1.41" = 2538 pixels horizontal (insufficient pixels to print to A3 or A4 size)
• 1800 x 0.94" = 1692 pixels vertical (sufficient pixels to print to A4 size, insufficient for A3 size)
• 3600dpi film scanner
• 3600 x 1.41" = 5076 pixels horizontal (sufficient pixels to print to A3 or A4 size)
• 3600 x 0.94" = 3384 pixels vertical (sufficient pixels to print to A3 or A4 size)

Rarely do you want to print to larger than A3 size so why are some film scanners 7200dpi optical resolution? The reason is that if you crop your film you will have less film to be scanned and therefore need the 7200dpi resolution. For example, if you crop your film by 50% then:-

• 7200dpi film scanner
• 7200 x 1.41"/2 = 5076 pixels horizontal (sufficient pixels to print to A3 or A4 size)
• 7200 x 0.94"/2 = 3384 pixels vertical (sufficient pixels to print to A3 or A4 size)
• 3600dpi film scanner
• 3600 x 1.41"/2 = 2538 pixels horizontal (insufficient pixels to print to A3 or A4 size)
• 3600 x 0.94"/2 = 1692 pixels vertical (sufficient pixels to print to A4 size, insufficient for A3 size)

Download the Excel spreadsheet Scanning Calculator by Dennis Curtin at shortcourses.com for:-

• pixels required per inch of print (as above)
• file sizes (depends on colour or bit depth of the scan eg 24 or 36 bit depth)