Why portraits are the hardest tattoo to get right
Every other style has wiggle room. A rose that's slightly off-model is still a rose. A face has none: likeness lives in millimetres of proportion, and everyone who knew the person can see instantly when it's wrong. Add the smooth shading and contrast control that realism demands and you get the most technically unforgiving genre in tattooing, which is exactly why 'portrait tattoos gone wrong' is an entire category of internet content.
The uncomfortable truth behind those fails: most are competent all-rounders taking on a discipline they don't specialise in. Portraits are not a style you price-shop or book casually. They're the strongest case in all of tattooing for finding the specialist, paying their rate, and trusting them.
The reference photo decides half the outcome
A portrait can only be as good as its reference, and no artist can invent detail that isn't in the photo. What works: the highest-resolution original file you have, sent uncompressed rather than texted, with the face filling the frame, in soft natural light, straight-on or at a slight angle. What fails: filtered or beautified shots, which strip out the facial structure the artist needs; harsh flash that flattens features; and blurry crops from group photos.