The text is about photography. In the beginning, cameras were very large, but then they became smaller. This meant that the camera user was less obvious and could move around while taking pictures. So, the missing words are: as they took pictures.

SCRIPTS

These days nearly everyone with a mobile phone is able to take photographs or even make a video, but originally cameras were so large and heavy that photography's appeal as a pastime was limited to a few enthusiasts. Also, the time needed for the exposure meant that your subjects, if you were photographing people, had to remain still for what must have seemed like a very long time. Movement would come out as a blur in the picture or, if someone walked across the view of the lens, would not register at all, which is why early photographs of city streets appear deserted. In other words, all pictures had to be posed. However, as early as the 1880s, manufacturers in both Europe and America began producing miniature models, some of them small enough to be hidden in people's clothing. Cameras came in all shapes and sizes. Handbags, walking s tick handles and tie-pins were among the oddest and were collectively known as "detective cameras". This was not because they were used in police work, but because the user could move about without attracting attention (BEEP).