The millimetre thresholds that decide whether a fine line holds or blurs, and how to check yours before you tattoo.
On screen a line looks the same at any zoom, but skin only sees the real size. A stroke that reads as crisp at 2000 px can land as a 0.15 mm line on the arm, and 0.15 mm is below what most needles and most skin can hold cleanly. Measuring in millimetres at the true tattoo size is the only way to know what will actually happen.
Fine-line work usually needs roughly 0.3 mm or more for a single healed line to stay separate. Lettering and traditional linework are more forgiving. These are heuristics, not laws - skin type, placement and healing all move the number - but a design sitting well under the threshold is a warning worth heeding.
Set your design to the real width it will be tattooed at, then measure the thinnest stroke in millimetres. inkPreflight does this automatically and flags every line below the threshold for your chosen style, so you can thicken, resize or warn the client with a number instead of a hunch.