Make & keep
Register two scans before comparing a photo strip over time
Align working copies so wear, cropping or print changes can be compared accurately.
A side-by-side comparison can reveal a change in condition or an edit, but different scan angles create false differences. Keep both originals and align only working copies for the comparison.
Match scale and orientation
Use a visible edge or frame border to rotate and resize the working copies. Avoid stretching one dimension independently. Record which files were used and the crop applied so another person can repeat the comparison.
Compare one feature
Inspect the same edge, handwritten mark or surface area at equal zoom. Note genuine differences separately from scan shadows, dust and colour shifts caused by different devices. Do not call a difference damage until you have examined the physical strip.
Save the comparison
Export a labelled side-by-side reference image with the two source filenames and capture dates. Keep it separate from the master scans. If the comparison suggests new damage, stop handling the object and consult a conservator before attempting cleaning or flattening. Use a colour-managed display if colour is part of the question.