Gail S. answered 04/29/19
Photoshop Teacher and Technician with Ten+ Year's Experience
In addition to Alexander's answer, you might be wondering about such touch up tasks as removing spots, cracks, and blemishes. Before digital photography, skilled 'retouchers' used very fine brushes and colored or black and white dyes to fill in areas on negatives and/or prints. Dust resting on the negative during the exposure of the photo paper would prevent exposure and create troublesome white spots. These were filled in with darker colored dyes. The tools and dyes are still available in well stocked photo supply stores. Troublesome spots may still be fixed this way on fine art digital prints. They occur due to dust on the paper when printed, or spots where ink has flaked off after printing, sometimes due to mishandling.