A photo is 3000 pixels wide. You change its resolution field from 150 to 300 PPI and expect more detail. If the pixel count stays the same, the calculation instead gives a smaller print: you are packing the same pixels into fewer inches.
Calculate before resizing
PPI means pixels per inch. Divide the pixel width by the chosen PPI to get the width in inches; multiply by 2.54 for centimetres.
- At 300 PPI: 3000 ÷ 300 = 10 inches, or 25.4 cm wide.
- At 150 PPI: 3000 ÷ 150 = 20 inches, or 50.8 cm wide.
- After a crop leaves 1800 pixels across, 300 PPI gives 6 inches, or 15.24 cm.
These are size calculations, not promises of sharpness. Focus, compression, viewing distance and the print process still matter. Calculate from the final cropped image, not the original camera dimensions.
Check whether pixels are changing
In Photoshop’s Image Size dialog, inspect Resample. With resampling off, changing resolution redistributes the existing pixels over a different physical size. With it on, resizing can add or remove pixels. More calculated pixels do not prove that missing photographic detail has been recovered.
Work on a copy. Compare the pixel dimensions before and after, then inspect the actual print preview: a fit-to-page setting can change the output size again. Ask the print service for its requirements and try a representative print before ordering a large batch. A single PPI value is not a universal quality guarantee.