“The camera adds ten pounds” is an old film and television adage that is still held by many. Camera angles and colors can make one to appear fatter or thinner.
 
“The motion-picture camera adds from 5 to 20 pounds to the appearance of a star’s figure” has been cited in print since at least 1929. “Ten pounds” has been cited in print since at least 1930, “fifteen pounds” since 1935 and “five pounds” since 1969, with “ten pounds” being the most quoted measure.
 
A joke from at least 1957 is, “Remember, the camera adds 10 pounds, so don’t eat cameras.”
   
   
25 April 1929, Lowell (MA) Sun, pg. 14, col. 1 ad:
Diet the menace of Hollywood
Stars wrecked seeking slimness
PHOTOPLAY MAGAZINE recently revealed that many motion-picture stars ruin their health seeking slimness. The motion-picture camera adds from 5 to 20 pounds to the appearance of a star’s figure, so that many of these celebrities become victims of dieting.
(The Sugar Institute—ed.)
 
30 August 1930, Hamilton (OH) Evening Journal, “Screen Test” by Jack Lait, Feature Section, pg. 1, col. 6:
The moving picture camera adds ten pounds to a girl’s appearance.
 
3 November 1935, The Plain Dealer (Cleveland, OH), “Urges Girls Not to Try to Look Like Movie Favorites” by Lois Leeds, Women’s Magazine and Amusement Section, pg. 14, col. 5:
The slenderness that is the rule at Hollywood is necessitated by cinema peculiarities. The movie camera adds fifteen pounds to one’s apparent weight.
 
5 March 1944, Seattle (WA) Sunday Times, “They Bet on Themselves” by Dee Lowrance, Magazine Section, pg. 8, col. 7:
The tyranny of the scales is just as prevalent in Hollywood as it is elsewhere. For it is even more damaging to have excess weight on your carcass in the film village as the camera adds 10 pounds to anyone it photographs.
 
25 November 1949, Dallas (TX) Morning News, “Judy Shaves Off Pounds But Intends to Keep Her Health” by Aline Mosby, sec. 3, pg. 6, col. 4:
“They’re right about losing it. The camera adds ten pounds. You don’t look nice on the screen if you’re fat.”
(Judy Garland—ed.)
   
Google Books
The Courts of Memory
By Frank Rooney
London: Bodley Head
1955
Pg. ?:
I remembered her telling me once that the camera adds ten pounds to an actor. Perhaps it does.
 
17 March 1957, Chicago (IL) Daily Tribune, “Radio-TV Gag Bag” by Larry Wolters, Magazine, pg. C33:
To TV actresses: “Remember, the camera adds 10 pounds, so don’t eat cameras.”
 
19 October 1969, Springfield (MA) Sunday Republican, “‘Model’ Way To Trim Inches” by Reba & Bonnie Churchill, Magazine, pg. 14, col. 3:
From Hollywood comes a tip to trim hips. Actresses, as well as mannequins, know the camera adds five pounds to the figure, so full hips appear fuller before a photographer’s telling lens.
   
Google Books
The Box:
An oral history of television, 1920-1961
By Jeff Kisseloff
New York, NY: Viking
1995
Pg. ?:
“Don’t be a dirty bird,” soon became part of the national lexicon, and his closing words of wisdom were quoted by columnists around the country. “This is Lonesome George reminding you that the camera adds ten pounds, so don’t eat cameras.”
 
Google Books
Taken to the Stage:
The education of an actress
By Mimi Kennedy
Lyme, NH: Smith and Kraus
1996
Pg. 68:
“You can’t just do stage work any more,” he warned me. “Today’s actresses must work in television and film. The camera adds ten pounds, you know.”
 
This adage is only slightly less ancient than “Don’t believe everything you read,” and only because the camera postdates the printing press. It was, and is, widely believed. I arrived in Hollywood a decade later to find it largely obsolete; where there’s a will, there’s a way to make those ten pounds disappear.
   
Slate
Smile and Say “Fat!”
Why does the camera add 10 pounds?
By Michelle Tsai|Posted Thursday, Feb. 22, 2007, at 6:49 PM ET
(...)
So why do cameras add 10 pounds?
 
Bad lighting, mostly. The flat, even illumination on the red carpet makes it hard for the camera to capture dimension, unlike in a photo shoot with flattering soft lights. Cast from an angle, light creates shadows that sculpt the face and body by hiding unwanted flesh. Softer lights can hide wrinkles and smooth out the skin for women, while harsher lights on male faces exaggerate lines for a chiseled look. Without the aid of shadows, however, light exposes the imperfections of the face and body and makes the resulting image bigger and flatter. That’s why everyone avoids white dresses—which cast fewer shadows under even lighting—except the thinnest actresses, like Nicole Kidman.