Fixed Focus Lens have a fixed focus, so they have a constant distance setting. In Digital Photography, Fixed Focus Lens almost always found in Mobile Cameras. In older days, Fixed Focus Lens had a better position, because Fixed Focus Lens is now actually used to reduce the cost.
What is a Fixed Focus Lens ?
It is common mistake to get confused with the terms Fixed Focus Lens and Prime Lens – Prime Lens is a Fixed Focal Length Lens. Even the cheapest Prime Lens, Canon EF 50mm F/1.8 II costs nearly $100. But these Fixed Focus Lens are not for so artistic work – they are used on $100 to all camera mobiles.
Lenses form sharp objects in a plane, but you can accept a degree of uncertainty, so that in practice, in a focus area, the depth of field appears. With a film camera this blur circles up to 1/30 mm. The depth of field depends on the focal length and the aperture value, shorter the focal length, higher the aperture value and greater the depth of field. This can be exploited for so-called snapshot settings in photography. This allows fast moving objects photographed without being stopped by the Focus.
Image : Excellent demonstration of a Fixed Focus Lens by popsci.com
Now you have understood why those cheaper than a full frame DSLR with a prime Lens in a newbies hand, sometime creates great looking photograph. Definitely, there are noises, the resolution is less, but looks somewhat great. As a matter of fact, a donkey can not be compared with a mule.
Other Notes on Fixed Focus Lens
The idea of the snapshot setting can be utilized in simple cameras. This method was been utilized to the box cameras in old days. Ideally, if you want to power up the project with a moderate wide-angle lens, telephoto lenses etc. however,they will simply not work.
CCD sensors with Fixed Focus Lens are especially helpful because they can have an extremely small size. This results in very short focal lengths, can in turn deliver a great depth.