Here is my review of brand new pentax smc 40mm f/2.8 limited lens (usually called pancake lens)
Believe it or not it is simply outstanding lens. I just cannot figure out how pentax created such a thin and light lens which produces quite a good result.
The lens is very thin and very light so for the first time i put it on my pentax I did not notice much difference when lens is mounted and when not. lens cap is metal and very solid . Whole lens build quality is a top noch as there is no moving parts and everything goes smoothly.
For those who are advanced it may be a bit uncomfortable to hold a camera as there is no need to support lens with your left hand as you used to do with 'typical' size lens.
AF speed is OK however I did not notice speed improvement over kit lens. Simply it is fast enough but not speed daemon.
AF accuracy mostly was correct however there were some situations when the same picture was focused differently. there was no back, nor front focus though.
Picture quality is very good and the peak resolution is reached at f/5.6. At f/2.8 it is very good too. 'Bokeh' is ok though sometimes it may look a bit nervous or too little washed out. still looks good.
Generally I was quite happy to test this small thin lens which is really good and produces nice results. It's size may not be comfortable for your hands but it saves space in your bag. Definetely worth buying and I am thinking to buy one.