There seemed to be some interest in the use of apertures to increase DOF (depth of field) so I thought it might be a good idea to start a thread where we could discuss augmentation of I² images using commonly available tools.

I'll start with the Aperture:

I first became aware of the use of apertures used in conjunction with NVD's when I read a study on the feasibility of using the AN/PVS-14 for pilotage  that was conducted by the US Army. IIRC their main concern was limiting the amount of photons entering the system so they could make use of the MNVD during twilight hours when the pilots were generally able to fly unaided but wanted the advantage of augmenting their vision with the MNVD's. They achieved this by drilling a 4mm aperture hole in a COTS flip-open scope lens cover. When the twilight waned, the pilots simply flipped the lens cover open - fully exposing the objective lens. A byproduct of the experiment was increased DOF due to forcing the light entering the lens through the aperture. This is a commonly observed phenomenon and general information can be found on this topic all over the web.

No information was presented in the evaluation as to why the 4mm aperture size was chosen so I chose to experiment. I began by having discs cut with varying apertures, but of a standard outside diameter so they could be mounted in a filter lens frame. I chose standard photography F-Stop sizes to keep the experiment anchored a little. I swapped the apertures until I found the largest opening that increased the DOF enough that I could get marginal focus on Tritium pistol sights at arms-length while simultaneously having a practical long distance focus (7mm). This limits the amount of photons available to the PC so gain has to be increased considerably (increasing noise) to achieve a usable image. There is no free lunch, but this aperture can be used in a high ambient light night environment. Out in the deep dark, this aperture would be completely unusable. Some people are using Butler Creek flip covers (PN 30070) so they can have ready access to using the aperture for increased DOF while being immediately able to flip the cover open for wide open use. An LIF, UV haze filter or IR filter can be used in conjunction with the flip covers.

Remember the smaller the aperture, the better the improvement in DOF will be - with a commensurate apparent loss of gain. Additionally, although DOF is increasing, allowing for better overall focus, some maximum resolution will be lost due to increased noise relative to increased gain selection.

Examples of commercially available apertures:

Matbock Tarsier Eclipse: approximate retail price $215

Phokus Hoplite: approximate retail price $100

Modified Butler Creek Item # 30070 / 07 OBJ (Size 7): approximate retail price $10

That's my opening shot. I would appreciate if we could open a discussion and explore this and other optical effects.

Interesting...

I bit the bullet and picked up one of these http://soldiersystems.net/2014/04/02/tarsier-eclipse-9-month-review/

This gives you a fully adjustable aperture. It works great indoors where you might need to manipulate something small up close. I've actually opened my safe with no lights on for the hell of it. 

The only downsides i have have found are price and it takes a fraction of a second more to adjust vs a flip cap cover.

The Tarsier Eclipse ($200) and the Phokus Hoplite ($100) are both good options available to the mass market. $10 (Butler Creek) is a small price to pay to see if an aperture will provide any benefit to the end user. Personally, the 7mm aperture is where I usually want to be anyways, so I don't see an Eclipse in my near future. If the end user likes the aperture but thinks he needs more versatility, then he can always upgrade and pass his homemade aperture to someone else to try.

30mm UV Haze Filter

The next solution I would like to explore is the 30mm UV Haze filters from the photography discipline. They are economical at $5-$10 each, protect the lens while allowing a clear view with little flaring and are readily available. Further, they do not interfere with focus of the objective lens when used in conjunction with a LaRue PVS-14 mount like the OEM sacrificial window does, and they fit well behind a Butler Creek lens cap (that also works in conjunction with the LaRue PVS-14 mount). I prefer the type that are assembled using a threaded lock-ring because you can disassemble them and use them as a tool for experiments such as making your own apertures. The low cost allows me to not care about them - If they get dirty or fogged I just rub them with a grimy finger and keep moving.  Make sure to get a brand name filter. All of the brand name filters I have used have fit properly. The generic no-name filters that I have used have had erratic threads.

Yes, the Wilcox sacrificial window is a good option but its cost is much higher and the standard photography filters allow attaching another accessory lens in-series without removing your sacrificial window.

The LIF makes a good sacrificial window and fits nearly flush to the front of the objective when installed. It does, however, eat some of your gain - so it is not good for use in the deep dark - and needs to be removed for attachment of accessory lenses unless they use the external ring-mount.

My current preference for economical UV Haze filters for the PVS-14 is the Tiffen UVP-30mm. In addition to being a good lens it includes an excellent case that will store multiple lenses.

The offerings from Bower and Kenko provide suitable results in use but do not allow for disassembly/experimentation as readily.

I just use the butler creek flip up lense cap with a small hole drilled in the center.  It allows me to see anything within arms reach into perfect focus.  Great for door locks, looking through packs, etc...  Also, with the PVS 14's illuminator on, the overal visability with the cap down is still pretty good.

I haven't used any of the pricier options, but don't really see the need to.

When i purchased my ab nightvision housing set, it was less than half the cost of a pvs14 housing kit.
The objective is a c-mount security camera lens. In addition to the standard focus mechanism, it also has an adjustable aperture built in. The feature has proved to be beneficial in suburban light settings, especially when used with the mx10160 tube.

Regarding the use of filters:

In ordinary photography, color filters are used to enhance or subdue detail of various features in the scene (say, dyes and fabrics and particular kinds of foliage and soils and paints) , or of the film or sensor in the camera, from certain wavelengths of light to achieve a desired "look" in the viewed image. Sort of like Photoshopping the picture as it is taken.
Filters that limit the passband also sharpen the focus because the wavelengths focus at differing distances from the back of the objective lens. This is the reason there is a white tick and a red tick on the focus ring of a normal camera lens. Set distance to the res tock on the distance scale for red/IR focus, and to the white tick for "visible" light.

Somewhere there is a pile of scientific analyses describing how to enhance contrast or subdue features of particular man-made and natural objects of interest to us, by the use of filters and polarizers  in front of standard night vision sensors and light sources and by controlling the strike angle and polarity of incident light. Many fields of science, including defense and agriculture and forensics and environmental must be cranking out such documents continuously, because every one of the participants strives to be published to share the work and his own name.

I have never found one such document worth linking on this site except for some dealing with thermography . Do it, and you will save us all a lot of fooling around.

What size flip cap?

Augee recommended one but he said it was loose. I ordered a different size last night and I will update with a part number after I verify fit.

So far, for a fixed all-purpose aperture, the hive seems to think 5mm to 7mm is the right size. The US Army used 4mm but they were trying to bridge the gap at twilight and I don't think many of us have that concern.

Serious question.  I do some flying at night.  It can be interesting to thing about what happens when and if the motor quits.  Some people say head for the lighted areas, some say head for the dark areas. John King said wait till you get low, and turn on the landing light. If you don't like what you see, just go ahead and turn it off...

If there an NVG rig that could be used for night flying?  I know the Army uses NVG for helicopter pilots, but what about to assess possible landing sites?  Some need IR illuminators, and of course that wouldn't work.  But I would think there's a fair amount of ambient light around developed areas that amplified light could provide help.

Thoughts?

Thank you. I've been using the issue rubber cap for now. Works fine with more illumination. 

Tagged for outcome of whatever cap size you'll be trying. I already use the 09A (eye), but it is a loose fit. I did the electrical tape trick, but if I remove the cap frequently the tape eventually starts to come off.

I've bought several filters too. Kinda starting to wish I would have bit the bullet and bought the Hoplite to start off with. 

So I have about 1300 hours in an AH-64 with probably 500 under FLIR using my one eye ball with the monacle and one unaided, maybe 350 under just ANVIS goggles, and a bunch flying both with the tubes far out in front of my face and using one eye with the NVG tube and the other with FLIR, sort of weird but not bad if you want a bunch of situational awareness over an objective.

So in regard to flying with NODs, what is it in particular you want to know? Flying in Afghanistan with shit illum and the goggles just aren't good. Takes
About 30-40 percent illumination to get a good picture. In the states, with so much light pollution, you can get a pretty good picture no matter what. Landing at a lit runway with NODs can suck sometimes, as they will bloom and wash out from the runway lights when you get low.

As far as emergencies go, you might be able to scope out a spot if your engine quits, but honestly, once you get low in the trees with NODs, the shadows can be rough and disorineting, and once you get low enough, it's hard to make out much of anything in bad illum and shadows even with .mil grade tubes

I do like flying with night vision, but for most applications, even for an emergency, unless you are doing spook stuff, I can't see deploying a set of NVGs under stress and continuing to fly the aircraft. That is, unless you have them on your head and can flip them down immediately.

Feel free to ask any questions AV related or PM me

I also spend a lot of time on goggles while flying civilian HEMS. The only FAA approved goggle is the ANVIS systems, even then they are required to go in to an FAA authorized service center every 180 days for rectification. While focusing systems could be nice they are not allowed for civilian use by the FAA.

Our AO includes Yellowstone and the surrounding areas, some places that we fly are devoid any light at night, mainly off the SE side of the Park where the nearest road is 25-35 miles away. Maybe there is a camp fire or two in the area. On a rainy night or just night with heavy cloud cover there is not really any natural illum.

The thought of having to do real EP's in this remote of an area is scary under good conditions, on a dark night even more so. But I would not trade the advantage that goggles give us when we are working in area's like this. But then again we also have two who can get their eyes outside to help find a less hazardous place to place the aircraft, but even then the clock is still ticking fast.

We also have the fun of landing on road ways and near homes, this always presents the problems of wires and fences. Both of which are very, very difficult to spot from the air, it is much easier to spot the power poles or fence posts, T posts are a real pain to spot without any illum in the area. Head lights on vehicles are a mega pain, please if a helo is coming into an accident, or other emergency, that you are stopped at turn off your headlights! Even running lights are dam bright and will cause a momentary resolution change in the goggles. Sadly to stay safe we have had to go around and make a second attempt at landing more than a few times.

Butler Creek flip cover (PN 30070) is a perfect fit for the PVS-14 objective lens.

Telephoto Lenses - Selection

The military 3X afocal magnifier is a good option for adding magnification to the PVS-14. Its primary shortcomings are price and length/weight. When added to the end of the PVS-14 for helmet-mounted use, a significant amount of leverage is applied to the operator's neck, resulting in strain over the long term. One way to overcome both of the shortcomings of the 3X afocal lens is to substitute a telephoto lens from the photography market. The following masses all include adapter rings, if necessary. The length is the approximate length added to the PVS-14. Keep in mind that the further the lens stands from the axis of your neck, the more leverage will be applied to your neck.

Telephoto lenses inherently reduce DOF (depth of field) so more manual focusing may be required depending on your application versus an afocal lens.

Infrared light focuses differently from other light frequencies so many photographic lenses that provide suitable results in their intended application may not provide suitable results for us. Oftentimes, camera manufacturers add an IR block filter to prevent having separate focus points. Additionally, the focus-point of these lenses is typically unknown and I may have gotten better results if I had done more experimentation with spacers. An IR pass filter can be used to block non-IR light, sometimes resulting in better performance with some lenses in some circumstances. This results in a loss of light gathering, however, so I do not recommend them for general use in conjunction with a magnifier because you should not be dependent on IR filter usage. Here are some I have tested:

Lens: Military Afocal 3X MX-11683/U
Native Thread: 30mm
Length: 2-7/8"
Mass: 152g without friction adapter.
General Observations on Resolution through PVS-14: Excellent.

Lens: Tokina VTC-20S
Native Thread: 37mm
Length: 1-7/8"
Mass: 132g
General Observations on Resolution through PVS-14: Excellent.

Lens: Sony VCL-HG2037
Native Thread: 37mm
Length: 2-7/8"
Mass: 357g
General Observations on Resolution through PVS-14: Excellent but length/mass are above acceptable levels for extended headborne use.

Lens: Sony VCL-HG2030
Native Thread: 30mm - Direct PVS-14 mating
Length: 1-7/8"
Mass: 167g
General Observations on Resolution through PVS-14: Very good.

Lens: Sony VCL-DH1730
Native Thread: 30mm
Length: 1-5/8"
Mass: 83g
General Observations on Resolution through PVS-14: Poor resolution.

Lens: Sony VCL-R2037
Native Thread: 37mm
Length: 1-7/8"
Mass: 93g
General Observations on Resolution through PVS-14: Good.

Lens: Raynox AF-2000
Native Thread: 37mm
Length: 1-5/8"
Mass: 65g
General Observations on Resolution through PVS-14: Fair to good. Low mass allows for extended use.

Lens: Neewer (part number to follow)
Native Thread: 37mm
Length: (pending)
Mass: (pending- approx. 100g)
General Observations on Resolution through PVS-14: Very good. This is a current production model that is widely available at an extremely attractive price point.

I tried a few Sakar lenses. Though they are of high quality, none of them provided satisfactory results in conjunction with the PVS-14.

Telephoto Lenses - Use

Once the telephoto lens is installed you will probably soon notice a lack of orthogonality between the I² image and the unaided image. You're running magnification, making the DOF shallower and shifting the input of the MNVD even further forward - there's no free lunch. Extended use of a telephoto lens on the MNVD can cause eye strain, headaches and even nausea. Some people are naturals and suffer very little discomfort but most of us have problems. This is one of the rare instances that I recommend the use of the "pirate patch". Eliminating the competition from the unaided/unmagnified image can greatly help reduce the negative effects of running a telephoto lens. Since you are probably running an enlarged bell lens, I have a fun trick that I find better than the patch.

1) Install the appropriate Butler Creek flip-cap on your telephoto lens.
2) Rotate the cap on the bell so when you have the lens focused for the distance that you need to view that the cap cover falls in front of your unaided eye when the cap is opened.

Even in fairly dark conditions you will see a significantly contrasting circle of the lens cap against the night environment. With a 55mm bell I get a pretty good effect. I try to center my subject in the contrasting circle in front of the unaided eye - letting it fall where it will in the I² image. I find this setup to be very relaxing and I can use the MNVD for observation like this for a few hours at a time without too much discomfort. Once you become fully comfortable with this technique you may actually get the sensation that you are seeing the image with the unaided eye - this is my target condition. I want to feel like I am looking THROUGH the system - not like I am looking AT the system. It takes a little practice to get to this relaxed state but the payoff is enormous. Also, the better I have my focus set for the required distance and the better I have my diopter correction adjusted the more I am able to relax.

Many of the 2X telephoto lenses are significantly lighter and shorter (less leverage) than the military afocal lens but extended use may still cause some neck strain so a counterweight may still be in order.

Weapons Mounting

2X magnification seems to be the sweet spot for magnification wit a telephoto/MNVD combo. 3X would be nicer but then the length of the system matches a military afocal lens. With a 2X telephoto I find it much easier to get the MNVD to fit the rail space behind an Aimpoint/EOTech than I do with the afocal lens. 2X magnification is barely enough to get the rollbar of the EOTech to extend itself away from the center of the field of view and be reduced to blur, making its presence less obtrusive. The extended bell of the telephoto lens makes the necessary manual focusing that you will have to do a little easier. (The forward mounting and partial obstruction by mounts/accessories may make the native MNVD objective lens a little difficult to get ahold of.) You can use the same trick with the Butler Creek cap to help avoid eye strain in the weapon mounted configuration as I described for head mounted use.

I forget what butler creek cap I used for my 14. I listed it in here somewhere. It's not the push button one, and it is friction fit.

On the advice from a friend, I heated a 5.56 brass and used that to create the hole. I just looked up the conversion of 7mm to decimal and was pleasantly surprised to see it was fairly close to the mouth of that brass.

This used to be called the DARC trick, or something similar.

I used to know quite a bit about filtering with reference to film and electro-optical systems, but I don't remember anything really now.

I do know people have discussed using filters for hunting and surveillance at dusk but I can't remember if it was in conjunction with gain control or not.

Neutral Density Filters

ND Filters are typically available in values that halve the available light from the last previous filter. Examples:

No Filter: Full Value
ND2: 1/2
ND4: 1/4
ND8: 1/8
etc., etc. into values that are so dimming in nature that they are worthless to us. Multiple filters can be combined to provide fractional values to fill the gaps.

ND Filters are designed to diminish the amount of light that passes through them equally across the spectrum, as opposed to band-pass or band-block filters that select certain frequencies to modify. ND Filters can be used to limit the amount of photons entering a fixed gain tube to prevent oversaturation in a high light environment. They can also be used for experimentation purposes such as starving a tube of photons to see image quality degradation at higher gain without otherwise affecting the image as an aperture would (changing the depth of field). In my opinion, they do not provide much utility for the average end-user but are an inexpensive tool for those that like to experiment. Most end-users would probably be better served with an IR pass filter that blocks most visible light but still allows use of lasers and illumination tools without seriously degrading their performance.

Bump for moarr!

Coming Soon:
Polarizing Filters

Combining 2 polarizing filters and rotating 1 out of phase will act like variable neutral density filter.

FLD Filters
These are inexpensive and commonly available so people ask about them from time to time. In my limited testing of them I have found that they serve no usable purpose for our applications. They slightly degrade gain with no perceived benefit.