Editing Images
Destructive vs non-destructive
Work on a copy and keep the original. Adjustment layers, masks and smart objects let you change your mind; direct pixel edits (erasing, painting, flattening) are destructive. History or undo steps are limited and usually lost when the file is closed.
Geometry: four different operations
- Crop removes pixels and changes aspect ratio and composition.
- Canvas size changes the document area, adding or trimming space without resampling pixels.
- Resize / image size changes the stated print dimensions and therefore ppi, keeping the pixel count.
- Resample changes the actual pixel count, up or down.
Confusing these is the most common exam trap. Straightening a crooked horizon usually implies a small crop.
Tone and colour
Brightness shifts all values; contrast stretches them apart; levels set explicit black and white points; curves give finer control. Hue/saturation changes colour and intensity; colour balance removes casts. Sharpening increases local edge contrast - apply it last, lightly, sized for the output. It cannot repair an out-of-focus shot. Blur reduces detail deliberately.
Retouching and selection
Selection tools isolate areas so edits stay local. Clone copies pixels from a sampled source point; heal blends surrounding texture and tone, better for skin and small blemishes. Red-eye from on-camera flash is fixed by darkening the pupil while keeping the catchlight.
Worked example: a 2400 × 3000 px file is 8 × 10 in at 300 ppi. To print 4 × 5 in at 300 ppi you need 1200 × 1500 px, so either resample down or crop - resampling to a bigger size would add no detail.