You can calculate the Pixel Resolution of your scanned analogue air photos using a simple calculation.

pixel resolution = photo scale / scan resolution
 
Note: only the denominator is needed for photo scale

Required information:

• Resolution used when scanning the photos (in dpi – dots per inch)
• Photo Scale

Example:

Photo scale = 1:4000
Scan resolution = 200 dpi

pixel resolution = 4000 / 200
pixel resolution = 20 inches
pixel resolution = 0.508 m

Note:
1 inch = 2.54 cm
20 inches = 50.8 cm
50.8 cm = 0.508 m

Therefore, for a strip of air photos flown at a scale of 1:4000 and scanned at a resolution of 200 dpi, in digital form, each photo pixel has a resolution of 0.508 m.