Holga make various lenses for use on their cameras and one popular type is a pinhole lens. Pinhole photography is an art in itself and experimentation is needed to get it right.
First I should explain what a pinhole lens actually is. To describe it as a ‘lens’ isn’t strictly correct because it contains a pinhole aperture measuring 0.25mm (that’s a quarter of a millimetre) and no actual lens inside. Also, no focussing is necessary.
There’s an interesting description of a pinhole camera here: http://en.wikipedia.org/wiki/Pinhole_camera
There are no electronic contacts so when you mount it on your digital camera it will display an error message saying that the lens hasn’t been detected. It’s a fixed aperture, so all you need to consider is setting the shutter speed. I’d also set the ISO speed to 100.
To obtain the best results, mount the camera on a tripod and set the camera manually on a slow shutter speed. It will involve experimenting and taking several shots, but that’s part of the fun of using Holga lenses.
In bright conditions, start with setting the shutter speed to around 0.5 (½ a second) and gradually increase or decrease the speed to obtain your desired result. Remember that a tripod is essential for very slow shutter speeds.
I would also recommend that you set the self timer with a few seconds delay, because the camera is likely to move slightly when you press the shutter release button.
RCS sells the complete range of Holga lenses here http://www.rcssales.co.uk/index.php?main_page=index&cPath=47_7
I’m using a Canon EOS Digital SLR and here is the Holga Pinhole lens http://www.rcssales.co.uk/index.php?main_page=product_info&cPath=47_7&products_id=88