Pancake Lenses

Pardon my ignorance, but I occasionally see for sale 40mm 'Pancke' lenses. Can any one tell me the difference between a pancake lens and any other 'normal' lens.

The SMC Pentax-M 40mm f2.8 lens was introduced with the MX and ME models. It is called the "pancake" lens because it is very thin and only adds the slimmest amount to the depth of the camera, making these two SLRs very "pocketable".

It consists of 5 elements in four groups, focuses I think to around 2 feet and produces stunningly sharp pictures. The only disadvantage is that the focusing ring is so thin (just a strip of rubberised grip) that with a lens hood in place it can be difficult to find room for the fingers to focus the lens!

Another option in pancake lenses is the Vivitar 40mm 2.5. After looking at the prices of the Pentax 40mm on eBay, I bought the Vivitar lens. I've been very happy using it on my MZ-5n. The focal length is just different enough to give a different perspective on scenes that I might shoot with a 28mm or 50mm. Plus, by leaving it focussed at the hyperfocal distance, it make a great point-and-shoot lens.