Using NVG’s for a multitude of Law Enforcement tasks presents some challenges from typical white light jobs. One of those challenges is that the NVG is focused at a given distance, typically at infinity. The problem with this is when you need to see or examine something at close range. This requires that the NVG be manipulated to change the focus point, which in turn creates a new issue being that you cannot discriminate something at distance because it is blurred.
There are a few remedies for this, one of those remedies is to make a tight “OK” hand signal and place that over the objective lens. This blocks the number of photons entering the tube and changes the focus to being much closer. You can adjust the size of the “aperture” by simply making a tighter or looser “OK”. Benefit is it’s free and you always have it, down side is trying to determine the right size and also manipulate other things with the hand that is occupied making the “OK”.
Several manufacturers make focus caps, they range in price from over $200 to $9. They all do the same thing, reduce the numbers of Photons entering the tube by way of a hole, or aperture, in the cap. The $9 Butler Creek “poor man’s” covers must be home made, typically the hole size will be 5.56 or 7.62 in diameter and these will absolutely do the job. Benefits are they are cheap so you don’t care if you lose or break them. Downside with them and others, is the diminished amount of photons that can enter the tubes. So the image is considerably darker when you flip the cap up for close range focusing.
Now the question becomes, “Why do I need to focus at close range anyway”? There are a myriad of tasks where this is beneficial; reading apartment/house numbers, removal of the power meter, confirmation of malfunction clears, ballistic breaching, placing of XB charges, handcuffing, prisoner searches and so forth are just a few examples. In short, I believe every cop using NVG should have them since they don’t take up additional space, they are always there if you need them and they offer additional protection of the objective lens when stored.
I was recently exposed to the new Steiner – Universal Refocus Lenses. Unlike previous offerings, Steiner approached this problem from a completely different perspective and I think they have hit it out of the park. Notice that the name is not Refocus Caps, it is refocus lenses.
The box contains one lens and two small Ranger bands. The Ranger bands are for additional friction on objective lenses that are abnormal in size. I was able to securely mount them to BNVD-G’s, ANVIS 9 and PVS-14 without using the bands and they stayed put. The biggest difference between these and caps is the lens. When you use them, the lens allows photons to pass through them and this in turn results in image staying as bright as it is without them in the down position. They are attached by squeezing the tension band and sliding them over the objective lens. When you release, they apply clamping pressure like a modern hose clamp. In addition, there are ridges inside that correspond with the ridges on the objective lens affectively locking it in place. This allows you to grab the entire cap and use it to adjust focus. They are very secure and it would be hard to knock them off.
The question then is whether or not they work. The following pictures show a view with caps up and down. The NVG is focused to infinity for both pictures and the change is clarity is simply from flipping them up. The caps have a detent system so they positively lock in the up or down position, but they move easy enough when you need them to.
These things work very well and I highly suggest you take a look at them. Everyone is going to ask about cost, suffice to say that nothing in the NVG world is cheap, and you get what you pay for. They are competitively priced with a different manufacturers cap, but that cap does not have look through capability like the Steiner Refocus Lens. Do you really need the ability to change focal planes quickly? In my opinion you do, for all the reasons mentioned above and for 101 more as well. Changing radio channels, reading info, looking at a booking photo, etc. I recommend some type of cap, these lenses from Steiner are the best version of this that I have messed with. Other types might work, but they are not “just as good as”.