Because Holga's two approximate aperture settings are identified as "sunny" and "not so sunny", much of Holga's flexibility is in the film.
© Dave Handler
You can shoot under almost any lighting condition by choosing an average speed film and pushing or pulling it as necessary. (Pushing film is a term for rating the ISO film speed higher than the film is designed for. The film is then developed for a longer time, increasing the effective sensitivity of the film, compensating for the under exposure in the camera. Therefore, you can shoot a roll of 100 ISO with the intention of having it processed at 400 ISO. Simply instruct your lab to, in this case, push two stops. Pulling is the opposite of pushing and is much less commonly used.)
TIP: On sunny days, use a 100, 160, 200 or 400 ISO B&W or color negative film. On gloomy days or late in the afternoon, an 800 ISO film might be necessary. For near sunset or indoor situations, an ISO 1600 or 3200 is recommended.
For those just starting out with the Holga, the easiest film to use is C-41 color or black and white film since it is more forgiving and can be developed at most 1-hour photo labs.
Slide films are recommended primarily for the Holga 3D Stereo. Slides are film positives that are viewed with slide viewers. Slide film can also be cross-processed to get printed photographs. (Cross processing is a development process where slide film is processed in chemicals intended for negative print film. Cross processed photographs are characterized by being contrasty and having unnatural or color shifted colors. Results may vary greatly from roll to roll and print to print.)
EXPOSURE TIPS: Holga's lens is fairly low contrast (it is plastic, after all) and tends to lose detail in the shadow areas. For black and white negative film, the rule of thumb is to expose for the shadows and develop for the highlights. Even color negative film can benefit from the habit of over exposing slightly.