GoPro Hero 4 Silver
and Backbone Rib-Cage modification kit  

The several models of GoPro Hero are probably the most popular and well-known action cameras. Make a YouTube search, and you will find thousands of movies, including many of the craziest extreme-sports movies, shot with these cameras. The Hero cameras are small and lightweight, and are extremely tolerant of vibration and acceleration (read: dropping and bumping) because they have no moving parts except for the three switches (which don't really count as mechanical moving parts).

Gopro introduces new Hero models once a year. This allows this camera to retain a slight but significant edge on the competition. Hero cameras are more expensive than their direct competitors, but they are technically more advanced and more capable, and on average also smaller and lighter, than the competition. Hero competitors seem to concentrate on two types of action cameras: dirt-cheap lookalikes of much lesser capabilities, and advanced models that are almost invariably bulkier and heavier than the Hero, and sometimes even more expensive.

Hero 4 Silver in waterproof case, front.
Hero 4 Silver in waterproof case, rear.
Hero 4 Silver, front.
Hero 4 Silver, rear.

Currently, the Hero 4 is available in a Black and a Silver model. The Hero 4 Black is top-of-the-line and expensive, and offers the largest number of video modes and the fastest frame rates. The Hero 4 Silver is cheaper and provides roughly the same capabilities of the Hero 3+ Black, last year's top-of-the-line model. In addition, the Hero 4 Silver is the first model to provide a built-in, touch-sensitive LCD screen, and this without increasing the camera size. Previous models, and the Hero 4 Black, do accept a piggyback LCD screen, but at extra cost and size.

Several small accessories are included. They are mostly simple attachments for a variety of substrates, including flat and convex (read: helmet) surfaces.

For cost-conscious buyers and "suicide" missions where the risk of destroying the camera is very high, there is the Gopro Hero (it has no 4 in its name), the first lower-cost model with integral, non-removable waterproof case and more limited capabilities. In spite of being Gopro's entry model, it is more expensive and more capable than many of the lookalikes currently on the market.

Resolution and image quality

The camera records still images and video in wideangle mode by using the whole sensor surface. A variety of different resolutions is available in wideangle mode, but I don't know exactly how the lower resolutions are achieved. Most likely, in order to limit the amount of in-camera processing, lower-resolution images are undersampled, i.e., only one every n lines and columns of pixels is read from the sensor and becomes part of the recorded image. There are several ways of undersampling an image (e.g., with respect to how many adjacent sensels are sampled and made available to the demosaicking algorithm), and I have no detail on how the Hero 3 works in this respect. The alternative method that provides the best image quality is not undersampling, but capturing and demosaicking a full-resolution image, and then reduce its resolution in-camera with one of a variety of algorithms (bicubic interpolation is one of the most effective simpler algorithms). However, this requires a substantial amount of in-camera processing, and I don't think that this power is available to process video in the Hero 4.

Medium- and narrow-angle images are generated and recorded by the Hero 4 by reading data only from the central portion of the sensor (depending on image resolution and angle of view, sometimes in combination with undersampling). For this reason, there is an upper limit to the resolution of images in these two modes, i.e., the recorded image cannot have a resolution higher than the native resolution of the utilized portion of sensor. Thus, the full 12 Mp resolution is available only in wideangle mode.

In video mode, the tradeoff of reading data from just part of the sensor (the central portion and/or an undersampled set of sensels) is that a higher frame rate is possible, at least if there is enough light. However, if a fast frame rate is not necessary, recording at higher resolution and then reducing the resolution of video in post-processing with an algorithms that averages the values of adjacent pixels (as opposed to simple undersampling, which discards part of the pixels) produces a significantly lower image noise than by directly recording at lower resolution on the Hero 4. Another type of noise reduction averages two or more successive frames. At least in principle, this is equivalent to recording with a lower frame rate, but doing the frame-speed reduction in post-processing may be more versatile. At least in some applications, it may be worth testing which of these methods gives the best final image quality.

In still images, there is no particular advantage in recording pictures at less than the full resolution, aside from faster image sequences and savings in storage memory. In fact, a significantly better image quality and lower image noise can be obtained by recording still images at full resolution, and then reducing the image resolution in post-processing with a more powerful algorithm than in-camera undersampling. Cropping to reduce the angle of view can also be done more intelligently in post-processing.

Hero 4 limitations

The Hero 4 is not meant to be a professional, or even semi-professional, movie or still-image camera. It is not meant as a system camera with replaceable lenses. It is designed to be easy to carry, attach to the body or sport equipment, and use with minimal configuration complexities. It can be configured to start recording video or still sequences as soon as it is turned on by pressing the front button. In fact, the configuration of the Hero 4 is so streamlined that it can almost be regarded as a Zen-inspired device.

The standard lens is a fisheye design and does not require refocusing, except for very close subjects. At the widest angle setting, movies and pictures may need to be de-fished in post-production.

The main limitations to a professional use are:

  • No autofocus.
  • No manual focus.
  • No manual exposure.
  • No configurable spot/integrated metering except a simple, two-choices setting (spot meter on, and spot meter off, which roughly means integrated).
  • No manual ISO setting except lowlight off (= the camera massively underexposes the whole scene in low light to avoid turning everything into a noise soup) and lowlight on (= the camera tries not to underexpose too much, which may correctly expose the brightest subject areas).
  • Only primitive white-balance (WB) settings. A few WB presets are available, including "flat" which supposedly record with the sesnor'd bult-in preset.
  • Primitive sharpness and color saturation settings, but there is a ProTune mode that you should always leave on, together with low sharpening. Video uses more card memory, looks slightly less sharp (but only because artificial sharpening is not added), color looks less saturated (for a similar reason), but dynamic range is higher and leaves more room for changes in post-processing. ProTune cannot improve low-light performance, through. You must turn on ProTune twice: first in the Movie menu, and then in the Picture menu. See abekislevitz. for ProTune in picture mode on the Hero 4.
  • Movies and still pictures are stored on the memory card in a proprietary format, and you need to convert them in post-processing with the GoPro software. Still pictures are stored as individual frames in a movie sequence, for instance.
  • No JPG configurable quality (= compression) and no raw shooting (the raw white balance setting is something else entirely). Use ProTune for the best quality on the Hero 4.
  • No wired remote shutter trigger. However, an optional wireless remote control is available. A free smartphone app allows in addition the complete configuration of the camera and the inspection of live view and playback.
  • No flash synchronization.
  • No image stabilization.
  • Exposure compensation is available, but is not indicated in the front/rear display. You may forget that you set it and overexpose or underexpose everything until you discover your mistake, usually when you are already back home and are checking the day's shooting on a computer.

The Hero 4 is not completely hopeless for professional use, because it provides good video quality and reasonably good still image quality (at any rate, not worse than many compact digital cameras). Live view on an external monitor connected via cable is possible at full HD resolution, which is almost good enough for precise still-picture focusing, and good enough for manually focusing HD video. It pays off to experiment with all available video modes. Some give better image quality than others, often in counterintuitive ways. See for example abekislevitz. It discusses the Hero 3, but similar conclusions apply to the Hero 4.

The lack of a mechanical shutter means that the Hero 4 is suitable for still-picture applications where shutter vibration would be a problem, like when the camera is mounted on a microscope or telescope. However, these applications are limited by an intrinsically modest low-light performance.

No mechanical shutter means also that the camera is completely silent, and therefore can be used for surveillance, hidden-camera and wildlife still-image photography. A negative consequence of electronic-only shutter is that there is a significant time lag between pressing the shutter release button and the actual exposure. The time lag is caused by the fact that the sensor must be cleared immediately before the exposure begins. This problem may be partly compensated by shooting a rapid sequence of still images, rather than a single image. In this mode, instead of triggering the camera at exactly the right moment, you must try to anticipate the action and press the shutter release between one and two seconds before the moment you intend to record. You will then be able to select the best image(s) in post-processing.

The USB port used for charging the internal battery allows use of the camera for long intervals with an external USB AC adapter or a battery-powered USB power bank. My favorite power bank used two 168?? batteries and is enough to power the camera for several hours. At the same time, the internal Hero 4 battery is also recharged, so it becomes possible to disconnect the power bank after a while and continue to shoot on internal power.

A proprietary piggyback extra battery module (which unfortunately covers the LCD monitor when mounted on the Hero 4 Silver) can be mounted at the rear of a Hero 4 Silver or Hero 4 Black to extend the shortish internal battery life. The camera with piggyback battery can be enclosed in the waterproof case by using a special, deeper rear case cover. A similarly sized piggyback LCD monitor module is available for the Hero 4 Black, which has no built-in LCD monitor. It is not possible to stack a piggyback LCD screen onto a piggyback battery in order to simultaneously use both.

Internal components can get warm during extended use. There are reports of heat accumulating inside the Hero 4 Black and compromising its functions. The sensor is soldered onto a rather small PC board that functions also as heat sink, but this PC board is not attached to any heat-conducting structures. Three IC packages on the main PC board are covered with heat-conducting pads and pressed against the innermost of the two layers of the front bezel, which is made of aluminium alloy but quite thin. The outermost layer is plastic and covers most of the inner bezel. The rest of the case is made of plastic with a rubbery feel.

Hero 4 for multispectral photography

The sensor is the same in the Hero 4 Black and Silver models, and most likely the Hero 3+ also uses the same sensor. This discussion is based only on my specimen of the Hero 4 Silver.

The sensor is equipped with on-chip Bayer filters. This sensor does not seem to have a hardware anti-aliasing filter, and most definitely it is not covered with a NUV- and NIR-blocking filter. In theory, this means that the camera is ready for multispectral imaging out of the box. In practice, however, a NUV- and NIR-blocking filter, probably of the interference type, is built into its standard lens. The rear lens element is visibly convex. Therefore, the built-in NUV- and NIR-cut filter is not a flat glass window simply added at the rear of the lens. It might be a flat window placed somewhere between lens elements, or it might be a multi-layer interference filter deposited on one of the lens elements. It is therefore indispensable to replace the Hero 4 lens for multispectral, NIR and NUV photography.

The Hero 4 comes with a lens in S (12 MA thread) mount. The lens is focused at the factory (at the parfocal distance) and fixed in place with a strong thread sealant. In some cases, after removing the collar that surrounds the lens, the latter can be unscrewed by hand. In other cases, a large amount of thread sealant or glue has been used, and forcing it requires the use of pliers and is likely to mar the lens barrel, or even to break the plastic ring that holds the front lens element in place. The method recommended by most third-party companies involves removing first the internal lens holder with the lens still in place in this holder(which requires the complete disassembly of the camera down to disconnecting every cable from the main board, and the removal of the latter) and the use of a hot air gun on the lens and its holder to loosen the glue. I opted for mechanical twisting of the lens with pliers, without removing the lens holder from the camera. This succeeded but slightly deformed the front plastic ring of the lens.

At this point, the simplest way to use this camera in multispectral photography is to replace the lens with another S-mount lens suitable for this application. Surveillance S-mount lenses for VIS and NIR imaging are indeed available, and many of them are not physically nonger than the standard Hero 4 lens, and consequently can be mounted in the waterproof Hero 4 case.

For imaging exclusively in the NIR, a NIR-pass, VIS-cut filter must be mounted at the front or rear of the lens. Mounting such a filter at the rear of the lens can be problematic, given the short distance between rear lens element and sensor.

[figure]

S-mount lenses do not normally have a front filter mount. However, accessories are available for friction-mounting (somewhat loosely and insecurely) a 37 mm or larger filter around the lens collar of the Hero 4. This is possible only if the lens barrel is not physically too long, and prevents the use of the waterproof case.

[figure]

Alternatively, aluminium cases (not waterproof) with a front filter mount are available from third-party makers. Also in this case, the smallest feasible filter size seems to be 37 mm. In principle, with a lens of sufficiently long focal length to avoid vignetting, a smaller filter could be used via a step-down adapter.

[figure]

Filter adapters are also available to mount a filter at the front of the waterproof case. In this case, the smallest practically usable filter size seems to be 52 mm. The filter adapter is securely attached via one or two locking screws.

UV imaging with any of the solutions discussed so far must presuppose that a suitable S-mount lens for UV imaging exists. Universe Kogaku might have such a lens, or might have had one, or might plan to make one available. The only thing I know is that a small S-mount lens is pictured on their web site together with their UV lenses in C mount, albeit is not described in the text.

In conclusion, there are no simple solutions for UV imaging with a Hero4, short of converting it to accept lenses in CS, C or larger mounts.

Rib-Cage by Backbone

Backbone markets a well-made and overpriced conversion kit for the Hero 4 Silver and Black. This kit currently costs roughly as much as a Hero 4 Silver.

The purpose of this kit is to allow the use of S-mount, CS-mount and C-mount lenses on the Hero 4. The replacement front bezel of the kit is thicker than the original one, and the modification involves moving the camera sensor from its original position, deep within the camera, to flush with the replacement front bezel. A custom S mount is then attached with four screws at the front of the bezel. A CS adapter screws around the custom S mount, and a C adapter into the CS mount. Once converted, the camera no longer fits within its waterproof case. You should regard the conversion as permanent. If you need to put your Hero 4 in the waterproof case, you will need a second, unmodified one.

This kit is a CNC-machined modification for the Hero 4 Silver and Hero 4 Black. A similar but not identical kit is available for the Hero 3+, and should not be used on the Hero 4. The name of this kit is Rib-Cage. In keeping with the anatomy-based nomenclature chosen by this company, the kit carries the stylized image of a human backbone laser-engraved on the right side of the front camera bezel (where it is partly hidden by lens mount and tripod bracket of the kit), and includes a replacement front button for the Hero 4 that carries the relief image of a human-like skull. A few plastic pieces to modify and re-use the original Hero front button as an alternative are also included.

The kit consists of three main parts (a two-piece front bezel and a bracket with tripod sockets) and several screws, plus a flexible PC board to insert as an extension between sensor board and main board.

The kit contains also four small hex keys to tighten the included screws. These keys are of poor quality, and one of their ends does not fit properly in the screw head sockets. It is a good idea to use keys of better quality rather than risk damaging the screw heads.

An ordinary steel paper clamp and a couple of custom-shaped plastic pieces are included to help in separating the front LCD display from the original case. A piece of double tape for reattaching the front LCD display within the new bezel is included as well.

The kit includes also a few black round stickers of two different sizes, the purpose of which is unexplained in the instruction video (perhaps meant to be used as electrical tape?), and a couple of Rib-Cage-themed stickers for boasting purposes, but not meant to be attached to the camera itself.

Finally, at the bottom of the kit case, there is an assistance code that you can probably use to contact Backbone if anything goes wrong during the conversion and you need instructions not found in the video tutorial.

Necessary tools and materials not included in the kit are a small cross screwdriver (size 0), heatsink paste (not heatsink cement!), a few bits of electrical tape, and propyl alcohol. Choose a type of alcohol that contains no oil and avoid anything labelled "for massaging" and the like. I used instead denaturated ethyl alcohol, which also works. Do not use ether or acetone. A few other small odds and ends may be needed. It is a good idea to watch the whole tutorial before actually starting the conversion. Then watch the tutorial one small step at the time, and pause the video while executing each step.

For picking up and positioning screws and other small parts in tight quarters, I found it useful to have at hand straight needle-nosed tweezers, a curved steel dental pick, a wood toothpick, and the thin blade of a pocket knife to insert between case and connectors in order to take the main PC board out of the case - use the blade carefully, keep it flat and do not twist, or you will cut something. Work on top of a shallow tray so that any small loose parts will not roll off the table, never to be seen again. I like to place a few small Petri dishes on the tray to keep parts separated while I am working.

Two NUV- and NIR-cut filters are part of the kit. They (or just one of the filters, plus an O-ring) can be placed semi-permanently between the S mount and the front plate of the kit. In this position, the filter(s) are a very short distance from the sensor. These filters obviously must not be used in a multispectral conversion. However, simply omitting the filters leaves most CS and C lenses incapable of focusing to infinity, a problem common in this type of conversion. It is necessary to file away about half a mm from the rear (not front) of the CS adapter to restore infinity focus. The thread of the adapter is several turns, and can be used to finely adjust the infinity focus. The thread is long enough to restore the normal infinity focus if you should decide later to use the internal filter(s).

A CS mount screws around the perimeter of the S adapter and is fixed in position by a grub screw. This thread is meant as a focus calibration for C and CS lenses. A CS to C adapter can then be screwed onto the CS mount. The C mount allows the use of one the many adapters designed to connect still camera or movie camera lenses to C videocameras.

The instruction video for the conversion of the Hero 4 Silver is good but not faultless (parts of it show the innards of the Hero 4 Black, which differs internally from the Silver and has fewer connectors). The conversion took me over one hour, and I had to partly disassemble the converted camera twice to reseat the internal power and rear-LCD connectors. It is a good idea to test the camera a few times during the final reassembly, starting from when all connectors have been reattached (including the sensor, which comes last). At this point, if the camera does not turn on, the internal power cable is probably not correctly seated (but other connectors may be involved). If the camera turns on and recording modes can be seen on the front LCD display and changed with the buttons, but there is no live view and no configuration menu on the rear LCD, the rear LCD connector is probably not well seated. If the menu can be seen on the rear LCD display but not the live view, at least one of the two sensor connections (counting also the Rib-Cage extension cable) may need reseating.

Above is the Hero 4 Silver after conversion, together with its (unmodified) CS and C adapters. Some problems still need to be solved at this stage, especially the plastic case not fitting properly around the replacement bezel (in the above figure, this is visible along the top of the bezel. In my case, removing both layers of the replacement bezel and checking for stuck cables and improperly seated boards, and additionally reseating the extension cable between main board and sensor board, resulted in a much better fit, with which I am now satisfied.

Mir 11 in C mount.
Vega 7 in Kiev 16 mount.
Soligor 35 mm f/3.5 in M42 mount.

Above is the modified Hero 4 with three third-party lenses of variable suitability for UV photography, in different mounts and with suitable C-mount adapters. These are the lens sizes I feel comfortable mounting directly on the modified Hero 4, without an additional lens support.

Anything heavier, like the Coastalopt 60 mm Apo in Nikon F mount, needs to be supported. The above figure shows the use for this purpose of a 15 mm rail system of the type commonly used for filming with digital cameras.

Warning

This conversion involves detaching six and reattaching seven extremely small connectors and delicate flaxible PC boards. The risk of irreparably damaging one of these is quite high if you have not done this type of work before. If you want to avoid the risk, you may purchase an already converted Hero 4 Black or Silver from Rib-Cage. This, however, will cost you roughly twice as much as purchasing a Hero 4 and Rib-Cage conversion kit.

UV performance

The lack of custom WB forces the use of one of the built-in color profiles. The "raw color" is a reasonable choice. Auto WB should not be used, because it is extremely unstable. In manual sequences shot with the same subject and illumination in this mode, subsequent shots displayed extreme and unexplained swings in WB.

The test images shoul that the modified Hero 4, together with lenses known to perform reasonably well in UV photography and UVA-pass filters with relatively broad transmission bands, produces results comparable with multispectral-enabled system cameras equipped with CMOS and CCD Bayer sensors.



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