Pentax Q 40mm homebrew telephoto
I chose this lens from my parts bin because it was a complete package including a helical focus system, unfortunately I don’t know what camera it came off but it appears to be a 40mm f2.8. That 40mm when converted to a full frame equivelent is 220mm. Once I figured out the distance it needed to be placed from the sensor I mounted it on the end of a piece of ABS tubing which also attached to a modified Q to C-mount adapter I had purchased on the internet. Initial testing was done with no aperture, it was wide open. The resulting images were soft and lacked contrast as you can see with these following images.
My next step was to line the inside of the lens with black velvet paper to keep down the internal reflections and create a fixed aperture to try to improve the lens performance. These changes had a dramatic impact on image quality.
But I wasn’t quite done. I made a new aperture that was a bit smaller and did some more testing. This is the state that it is at right now.
I was likely 100 feet away from the rebar in the one picture and you can see that the bridge, hundreds more feet away, was also in focus. That isn’t always a desirable quality especially if you want to use selective focus to separate different planes in an image but on the other hand this is a feat you can’t easily accomplish with a large sensor camera if it is what you want. And the lens is very small I might add.
A final note here is a comparison 100% crop from the Nikon P7000 at full zoom (200mm equivalent) vs the Q homebrew (220mm equivelent). The Q is on the left.
