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Posted: 7/2/2006 8:24:12 PM EDT
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The varaible brightness of the PVS 14 is unique to the PVS 14. No other Night Vision mono out there uses it. It is accomplished electronicly via a variable potnetiometer in the battery pack coupled to a special power supply only found in the MX 11769 tube. You turn the knob and the image gets brighter or dimmer. Simple right? Truth of the matter is ITT made it way more complicated than it had to be and they lost an impotant advantage by doing this via electronics. Now... it is possible to vary the brightness in a NVD in another manner. By placing an aperature of some sort in front of the lens and decreasing the size of the opening the amount of light entering the device gets restricted and the image gets dimmer. NOW something else happens... Depth of field increases. What you ask is Depth of field? Well simply (or not so simply) put, it is the area 1/3 in front and 2/3 in back of the plane of sharp focus for a given lens at a given focal length... With a decrease in the aperature this "depth of field" or area of sharp focus increases. How is this benifical in the short focal length lenses used in NVD? Think about this, If the lens is focused at infinity, then the 2/3rds of area that is in sharp focus behind the plane of sharp focus (imaginary line where a lens is sharpest) is wasted. So if you could place an aperature over your objective lens and then pull the focus back a little you would get your close fore-ground in focus. For instance with your pin hole day cover on your objective lens just about everything you see is in focus. Problem is, if you put your day cove on at night you almost loose the image. The hole is too small. How does all this translate into the real world? Well... walking about with your NVD head mounted, generally the ground at your feet is out of focus. If you adjust your objective lens to focus on the ground, your infinity focus goes to hell. So if you could add a small (but not too small) aperature things would darken up a bit but you would be able to see more. I am going to look into a way to do this with modern gen 3 objective lenses. IPSC_GUY SIERRA II ALPHA |
Yes the C-mount lens will work but they are not waterproof and I have only ever seen one custom made PVS 14 housing that adapted to c-mounts. Also C mounts when removed expose the front of the tube. I am thinking something that slips on and easily adjusts like the aperature ring of a camera lens. IPSC_GUY |
This is why I liked the NAIT NVPS-10 for so many years....It was the first NOD I ever bought in 1992 and their units are still C-Mountable...Their tube quality has gone down over the years, they just mainly deal with remanufactured ones. 
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| Frankly I never considered variable gain to be all that cool. But then again I don't use my nvgs in a brightly lit enviroment. The only really nice thing about the pvs14 is the ability to dim the image enough so it "blends" better with your unaided eye. Which I think was more of the original design purpose than what you are suggesting. Also in terms of my experinces with the apature type adjustment I didn't really notice much if any gain in depth of field nor did the image intensity change a whole lot compared to what you can do with the pvs-14. So I think it was mainly design purposes. |
With a small aperature, you wouldn't notice any real improvemnet unless you pulled the focus back. In essence you are facusing the lens closer and artifically extending the focus. My goal here is to not have to focus and refocus the objective lens for close work or walking. IPSC_GUY SIERRA II ALPHA |
I really look forward to your experimentation with this and to your results. It would be really nice to be able to increase the depth of field for moving through brush/trees, then flip it back to maximum brightness for observation. However, I wonder about the loss of performance with an aperture small enough to make a big difference in the DOF. Perhaps a lens aperture from an old 35mm lens affixed to a sacrificial window would be sufficient for testing. |
Well that sounds like an admirable goal. Let us know how it works out. The apatures I was using could be adjusted from full open to full closed, and even with them 1/4 closed I didn't really see any difference in depth of field. I think you would loose most the photographic type effects like depth of field when it all gets pumped through the tube, which I don't think can't be thought of as another type of lens. |
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