I was asked by William if I suffered any loss of FOV when using my 2.6mm f1.0 lens in a cctv housing. Well the answer was yes, until I modified the housing. The presence of a very tall and bright streetlight at the end of my garden made it impossible to use a dome on the front of the housing, so I made some modifications to the housing so I could use the original flat glass window.
Most cctv housings have a small hood situated above the window to help keep rain off the glass, this was duly sawn off to open up the fov. Next, I removed the glass window from inside the housing and used silicone sealant to stick it back onto the outside front of the housing. I painted the inside of the housing matt black to increase contrast and also painted the glass edge black to prevent light artifacts through the edge of the glass window.
These modifications allowed me to position the lens as far forward as I could get it and still be able to open the housing. The distance is probably only about 3mm but this and the absence of the hood gives an almost unrestricted FOV with a 2.6mm lens. I only lose a tiny little bit on each top corner of the frame. As an extra bonus, water no longer pools at the bottom edge of the window recess but just runs off..
Hi Allan - many thanks for this explanation - very interesting (and creative)!
One slight note of concern relates to the proximity of the lens to the front glass and the fact the that glass is now positioned slightly further away from the demist heater. I had an issue last winter whereby I had the lens too close the front glass and as a consequence had insufficient flow of warm air from the heater passing directly between the lens and the glass. As a consequence the front glass was wonderfully clear of condensation except for a perfect circle directly in front of the lens.
Not saying it will be an issue for you ... mine's been OK since I increased the gap to around 5mm ... but you may wish to keep an eye on it.
That's a valid point William. To date I have had no issues. It may be that with the glass moved to the front of the housing the gap between lens and glass is about 6mm, and thus allowing the air to move freely between them. The main reason for relocating the glass window was because the lens was almost touching the glass and I was suffering vignetting on all corners.
Of course this modification to the housing is only really needed for the 2.6mm lens. I have no problems with the 6mm lens due to it being further back from the glass window. I suspect there will be no issue with a 3.8mm lens either.
I will be keeping an eye on this as the weather gets colder and frostier. Thanks for the heads up.
Post by michaelmorris on Nov 27, 2014 13:24:27 GMT
If there is an issue with fogging in front of the lens, you could try fitting a small 12v fan to distribute the warm air across the lens. The Museum of the History of Computing in Cambridge had some tiny 12v maglev fans goinf for silly money.
I like your idea od painting the front of the inside of the housing black. I've already painted the inside of the hood on my housing matt black, I'll give your idea a go.