tl;dr

The scientific answer is focal length. The right answer is "No, you don't."

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The Technical Truth

Wide-angle lenses let you fit more into the frame from up close. That's why phones use them for selfies. But the closer you are to any camera, the more exaggerated the nearest features become. Your nose, chin, or cheeks will look proportionally larger. It's not the lens distorting you. It's the distance.

Stand further away, or use your phone's 2x zoom which mimics a longer focal length, and the effect disappears.

Professional portrait photographers use longer focal lengths for exactly this reason. They compress perspective, flatten features, and create a more accurate representation of how we actually look.

So yes, the camera can add ten pounds. It's not in your head.

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The Real Answer

My aunt asked me why she looked fat in a photo I'd shared with her.

I told her all of this.

She replied: "All I wanted to hear was 'No, you don't.' You failed."

She was right. The correct answer isn't about focal lengths. It's: "You look great. The camera doesn't do you justice."