On the back of your Holga 120 Film Camera there’s a black rectangle with a sliding arrow. If you slide it to the top, it points at the number 16. If you slide it to the bottom, it points at the number 12. What does this do or mean?
First off, it’s important to remember that the Holga 120 takes 120 medium-format film. That’s what the 120 in the name means. It’s not the 120th generation of Holga :). 120 medium-format film is larger than 35mm film, as seen here.
Because the 120 film is nice and large, it works very well with big square images. That’s how most Holga photos are taken, in square composition. Because each image is so large in terms of the film space it takes up, you get a mere 12 images when shooting in this mode. That is what sliding the arrow to point at the 12 means. It means you are going to see the number counters that coincide with the 12 locations on the roll to fit 12 squares per roll.
If you instead slid the arrow to point at 16, you’d see an entirely different set of numbers. Now the numbers would come more quickly and they would count up to 16.
Note that sliding this arrow does NOT change at all how the Holga takes the photo. It only changes how the film counter is displayed. In order to actually get 16 images per film roll, you also need to put an insert into the camera itself. The insert narrows down the amount of film the Holga uses per shot. It in essence trims off the left and right side of the image. So you are now shooting vertical rectangles.
It’s important to realize that the viewfinder doesn’t change at all. It still shows you a square. You have to manually figure out how much of the image you’re going to lose on the left and right when you compose your shot.
So, to summarize, the arrow on the back should point at the 12 when shooting normal square images. You should slide it to the 16 when planning to shoot rectangular images with the rectangular insert. But sliding this arrow does NOT change how the camera takes the photos. It only changes the counter you see as you manually increment the film. You must also add in or remove the rectangular insert so the way you are using the film matches how you are advancing it.
What happens if you mis-match the insert size with the counter pointer? Read here!