Technical Comparison: RPO 3.0 vs Carson
It is necessary for us to apologize for the time since our last technical piece. Our business has been rapidly expanding and we have been working hard to keep up. Recently we have relocated and built a product assembly facility, complete with a positive pressure clean room. This is part of our continuing efforts to improve night vision build standards and ensure the highest level of qualit yand accountability for our clients.
RPO or Rochester Precision Optics is an American optical lens manufacturer that is known in the nightvision sphere for producing the high end lenses for the PVS-31A, GPNVG-18, and the next generation BNVD-Fused system for the US military. They are a well known and well established company.
The RPO 3.0 lens system was released roughly one year ago in 2022 and was met with lukewarm reception. This was not completely undeserved as it was not a clear upgrade to their 2.0 system. The previous RPO 2.0 system was known to offer higher light transmission along with lighter weight over standard milspec lenses such as Carson but the tradeoff was a higher degree of lens flare.
We understand that people are still having a hard time deciding which lenses to purchase and we receive regular inquiries regarding whether RPO 3.0 is a good choice over Carson.
Our intention with this post is to share pros and cons we have found using the best testing equipment and practices we have available to us, as well as documenting those findings to the best of our ability. Hopefully as you gather information in regards to what lens best suits your needs, this article stands as a valuable resource to allow you to make an informed decision.
We have always maintained that RPO 3.0 offers the following:
- Improved light transmission over Carson
- Improved lens flare over RPO 2.0, similar to Carson but with unique characteristics
- Significant weight savings over Carson
- Marginally higher zone 1 and 2 resolution at the cost of zone 3 resolution
- Slightly shorter eye relief with a slightly smaller eye box than Carson
Lens Test Parameters:
The US military specification for a PVS-14 eyepiece is that it must have an eye relief no shorter than 25mm when tested with an aperture of 6mm, same as the sought after PVS-31 system used by USSOCOM which is also produced by RPO. 25mm is long enough for use with most eye protection, 6mm is the size of the average human pupil. At night a human pupil can go up to 8mm in darkness, however when viewing an image as bright as the image from a nightvision device it will typically not exceed 6mm.
Why does this test parameter matter?
To make this test as relevant as possible, our goal should be to place the aperture of the camera lens no more than 25mm from the lens of the night vision device, to mimic the same distance your eye is likely to be when operating your night vision.
Test scenarios that deviate beyond the intended design parameters will not perform as intended and will compromise test results. While it is true that the RPO 3.0 eye box is slightly smaller than a standard Carson eyepiece, testing outside of milspec parameters will skew results.
Many higher end camera lens designs force the aperture blades much deeper into the lens assembly than we would like for proper testing as this would simulate having your eye further back from the device than you normally would. What is necessary for our application is a lens with a design such that the objective lens element is compact with the aperture blades placed close behind the glass of the lens element.
An example of a lens that mimics the relationship of the human eye to the night vision device, in this case a Sony/Zeiss Sonnar 35mm. Note the small objective lens design with aperture blades right up against the objective element. This setup was given approval by a RPO representative.
An example of a lens that is less effective at mimicking the relationship of the human eye to the night vision device, in this case our Sony G-Master II 24-70. Note the distance from the front of the lens to the apertures blades. This distance is further than your eye would be from your night vision device.
Taking photos through night vision using a full frame camera is tricky to do correctly because there are many factors that lead to errors when proper consideration is not taken. Many high end camera lenses have a very large objective lens element in order to collect more light and give more depth of field. Unfortunately using lenses like this for testing will capture distorted data such as off axis distortion, significantly exaggerated vignetting, and extreme edge distortion.
Our Testing Equipment and Procedure
For our testing we use a Sony A7R5 as it captures 8k resolution still photos. We will be using cropped sections to highlight points in this article, however we will provide a Google Drive link with all original 8k resolution photos available. For side by side comparison, we use two BCO LPMR 4Ks.
Our test devices are two Carson Industries PVS-14 monocular bodies with two matched Photonis 4G+ tubes. One set uses standard Carson lenses (Fujinon Objective with Qioptiq Eyepiece) while the other uses RPO 3.0. It is impossible to get tubes with identical specs, so it is important to note that we decided to use the slightly higher spec tube with the Carson lenses. We recognise that each tube will have slightly different brightness and tint characteristics which may affect perception of some results, however will not affect resolution. For the brightness test only one tube was used.
|Tube||Photonis 4G+||Photonis 4G+|
|Resolution||70 lp/mm||70 lp/mm|
Each photograph was taken on the same camera in a controlled setting. Light input is controlled by our Hoffman Engineering ANV-126A Nightvision Test Set inside our dark adapted clean room. Each PVS-14 was manually focused and verified by eye as well as the Hoffman Engineering HVS-126 Digital Test set to ensure proper focus. The camera was manually focused to the PVS-14 each time. Resolution tests were attempted multiple times each to ensure consistent results, only the best results for each system were used in order to avoid sampling bias.
Light Transmission Comparison
For a quick gain comparison, we used an autogated NNVT tube at around 1600 FoM. Specific specs don't matter as it was the same tube, the only part changed was the objective lens using the same tube and eyepiece. Only the objective lens was tested here as only input affects tube noise, clarity, etc.
Gain was measured at 0.100 mfL (milli foot Lamberts) using a Hoffman Engineering ANV-126A's Gain Probe. Only the objective lens was changed between tests, and was manually focused prior to each test as focus affects accuracy of the gain reading. As gain constantly fluctuates slightly due to randomization of scintillation, 5 consecutive readings were taken to find an average.
|Objective Lens||Carson (Fujinon)||RPO 3.0|
What this means is that at this light level (default gain test level on a Hoffman ANV-126A) the image from a RPO 3.0 objective lens is 3.9% brighter than the image from a Carson lens. This is due to the improved light transmission RPO lenses offer due to their advanced aspherical element designs that reduce the number of elements inside the lens assembly.
Improved Lens Flare Suppression
The RPO 3.0 system, being more comparable to Carson, has improved lens flare over 2.0. The cost is reduced light transmission, so while RPO 2.0 offered 9% improved light transmission over Carson on the objective lens alone, 3.0 is reduced to around a 4% improvement over Carson.
During our testing we found that there are instances where RPO 3.0 displayed more dramatic flaring than the Carson, and there were instances where Carson displayed more dramatic flaring than RPO 3.0. Lens flares occur when light is scattered by a lens system. In an effort to ensure both the Carson and RPO 3.0 setups experienced the same lighting conditions a pair of BCO LPMR 4K recorders on PVS-14s on a MOD Armory Bridge were used. This way a side by side real time comparison the difference between Carson and RPO 3.0 can be seen in real time.
It is easy to observe in the screen grabs below, there are moments where Carson sometimes had the advantage, and there were also moments where RPO 3.0 had the advantage.
The reality is they both have similar degrees of lens flare but have specific characteristics in specific situations at specific angles. If there has to be a winner, I would say Carson has slightly better lens flare characteristics overall.
Weight and Size
This part is quite simple
|Objective and Eyepiece (O-Rings, no Diopter)||96 g||60 g||36 g|
|Objective and Eyepiece (O-Rings, with Diopter)||116 g||68 g||48 g|
What this means is that in a monocular setup weight is reduced by 36 grams just by using RPO lenses. A further reduction of 48 grams can be had by also using our 3D Optimized Lightweight Diopter. On a binocular setup 72 grams can be saved on the lenses alone, and a total of 96 grams using diopters on a binocular setup. This is significant, as on a binocular setup you are reducing system weight by 15-20% depending on the setup. For example a BNVD-1431 Mk.II using NNVT tubes is reduced down to 482 grams.
RPO lenses are also significantly more compact than Carson. Left is a RPO PVS-14, right is a Carson PVS-14. Same tubes, both focused to infinity with diopters set to 0 using a Hoffman Engineering Diopter Scope.
Distortion and Vignetting
Distortion is a normal occurence in any lens design. All have it to some degree, we just get used to it. RPO has slightly more zone 3 distortion than Carson. Here are comparison images taken with a Sony A7R5, Sony/Zeiss 35mm showing distortion using the distortion reticle in our Hoffman Engineering ANV-126A.
Vignetting is also a normal characteristic of all lenses, some more than others. Note that the vignetting effect on Carson is more gradual than RPO. Carson darkens out more in zones 2 and 3, whereas RPO maintains even brightness for most of zones 1 and 2 and then falls off in the outer half of zone 3. For this reason, RPO vignetting is more apparent at first glance due to the fall off at zone 3, despite having less vignetting in zones 1 and 2.
Carson Zone Reticle:
RPO Zone Reticle:
Carson Blank Background:
RPO Blank Background:
Note that both tubes exhibit uneven screen brightnesses with irregularly shaped brighter regions in the center. This will in some cases cause some comparisons to appear that Carson is brighter or RPO is brighter depending on the region that is cropped, however overall RPO maintains a slightly brighter image. Do note that brightness is not being measured here as different tubes will have slightly different tints that cameras will pick up differently. That is why our brightness test was conducted using the same tube.
Our resolution test uses our Sony A7R5 which is capable of taking 8k resolution still photos. This greatly exceeds what any night vision device is capable of, including the human eye for that matter at this focal length. Cropped images will be used here, Google Drive link with all RAW files will be at the end of this article.
There is also some necessary clarification regarding chromatic aberration as there seems to be a gross misunderstanding of what this is. This phenomenon is colour separation when light travels through optical elements caused by different wavelengths refracting at slightly different angles through the element. Blues are known to be more difficult to focus uniformly. The bottom image shows a close up of chromatic aberration through a Carson eyepiece, the colour separation on the right and left sides of the box can be clearly seen.
Center aberration comparison:
Center Resolution Test
The number 5 designates the entire column of line pairs as Group 5. The Element number is the row. We can see that both lenses perform well and can read Group 5 Element 4. However with RPO you can somewhat make out Group 5 Element 5, whereas with Carson Group 5 Element 5 is not able to be deciphered.
Note that horizontal elements are clearer than vertical line pairs. The reason for this is Group 5 line pairs are slightly to the left and right of center. All lenses exhibit some degree of chromatic aberration. This can bee seen as the slight colour separation at the left and right of the reticle edges. Aberration and image separation occurs outward from center.
The Milspec (Carson) eyepiece design has remained relatively unchanged for 20 years, and was optimized for green phosphor. RPO was designed with white phosphor in mind, which is why there is less colour separation on the eyepiece side when viewing white phosphor images.
Edge Resolution Test
The same test was conducted but with the resolution reticle shifted to the edge of the screen. To landmark and make the test consistent, the collimation box was set to the edge of viewable area.
Both systems are excellent in that they maintain a good resolution throughout the image. Group 5 Element 2 is decipherable on both sides. However with the horizontal elements Carson is legible up to Element 3 whereas RPO is somewhat decipherable to Element 4. Though, it should be recognized that this part is slightly subjective.
At the edge, chromatic aberration in Carson is more apparent as well which contributes to resolution loss.
It is reasonable to conclude that RPO has marginally better edge resolution in this area, the border between zones 2 and 3.
While not part of this test, it should be noted that both systems lose a lot of resolution in the outer half of zone 3. It is perceived that RPO loses a bit more in the extreme edge. Note that in this area of zone 2 RPO displays less vignetting than Carson.
No lens system is perfect, each system has unique offerings. What lens you decide to choose comes down to a number of different factors and trade offs and hopefully this article helps you decide which best suits your needs from a technical perspective.
Anecdotally, I enjoy RPO lens systems enough that in my current personal collection of 10+ devices, none use standard Carson lenses. They are all either RPO or some sort of rare connoisseur/collector lenses.