Godox Witstro AD200 electronic flashGodox is a China-based brand specializing in equipment for photographic illumination. Among other things, it produces a range of mains- and battery-operated electronic flash units. The feature that perhaps mostly distinguishes the Godox Witstro line of flash units is that they all have built-in receivers that accept radio (2.4 GHz) TTL commands from compatible masters (i.e., camera-mounted transmitters). Dedicated models of masters provide TTL operation with Nikon, Canon, Sony and, most recently, Fuji and Olympus/Panasonic cameras. Micro 4/3 cameras from Olympus and Panasonic share the TTL flash control protocols and physical flash shoe, but these protocols are not a part of the Micro 4/3 specifications, and therefore other Micro 4/3 cameras may not provide the same TTL flash functionality. I believe that each of the compatible Godox Witstro flash units understands all the different camera protocols. When Godox recently introduced the Olympus/Panasonic master, it also issued firmware updates to make the AD200, AD600B and AD600BM flash units compatible with these camera brands. As a result, you can use one of these units with any of the supported cameras, as long as the camera is wearing the right dedicated master on its flash shoe. Gone are the days when you needed to buy a dedicated flash unit for each camera brand. I do not see Olympus/Panasonic firmware updates for the Godox Witstro AD360 models. This might mean that the days of these models are counted. Godox might decide to replace them with an all-in-one, modular AD400 similar to the AD200 but larger and more powerful. An alternative is that the AD360 range may be retired soon (see below for a discussion of its clumsy design), leaving only the AD200 and AD600 as successors. The Godox Witstro AD600 models are studio-style units rated at 600 Ws, but contain a rechargeable battery that can be replaced with a mains-powered supply. Profoto makes a similar battery-operated, but much more expensive B1 model (a test on Petapixel compared the two units). For most of my studio photography, I can do with non-TTL, mains-powered Bowens strobes, and only occasionally need to use a separate flash meter. Carrying something of this size to the field is out of the question for me. Therefore, I am not tempted by the AD600 or B1. For field photography, over the years I used several brands of battery-operated strobes, most recently a couple of dedicated Metz models for Olympus cameras. There is a large gap in emitted power between studio strobes (which start at 200 Ws, although typical models are rated 400 Ws and higher) and battery-operated, portable strobes that mount onto the hot shoe of the camera (which generally top out at 50-80 Ws). Until recently, Godox and a few other brands (like Quantum) provided just a handful of battery-operated models of intermediate power between the two ranges (e.g. the Godox AD360 models).They are exceedingly clumsy to use in the field, because:
Bowens and other brands provide separate battery-driven packs for their studio units, but the resulting system cannot be called portable, and is euphemistically classified as "transportable" (even less so than the first "transportable" personal computers of the 1980s, which were the size and weight of a large sewing machine - I know because I carried around a TRS-80 4P and an Osborne 1). Working in the field with Bowens strobes means arriving on location with one or two large suitcases, setting up camp, and begin connecting cables between the separate pieces of equipment. 
Things changed early in 2017 with the introduction of the Godox Witstro AD200, an all-in-one unit rated at 200 Ws, not much bigger than a battery operated strobe and similar in size to an AD360 without battery pack. 
The above figure shows the AD200 side-by-side with a Metz 52 AF-1, a medium-large speedlight weighing 447 g with batteries .The AD200 with speedlight head and battery weights 899 g, i.e., twice the Metz speedlight. The AD200 is modular: you can use a speedlight-style rectangular head (without zoom function, with built-in LED modeling lights sufficient for illuminating a subject in macrophotography but not farther away), a bare-bulb head of similar power, with a proprietary reflector mount, or a LED head for continuous illumination (not so powerful, but more than the modeling light in the speedlight head). The speedlight and bare-bulb heads are included with the base unit, while the LED head is a separate purchase. Using the LED head requires the AD200 to be updated to v2.0 firmware, which is done through the micro USB port. My AD200 originally came with firmware v1.7. The update was fast and uneventful, once I understood that the AD200 must be connected to a PC first and then turned on (which leaves the LCD screen completely dark, but the unit is nonetheless on and capable of communicating with the Godox G2 update software). The AD200 is available as a rebranded product with multiple brands. It is even advertised under other brands, but still sold with the Godox brand. There are no internally customized versions that I am aware of. All rebranded items are identical to the Godox AD200, so choose on the basis of price and support/warranty policies alone. AD200 general specificationsThe AD 200 at full power with the speedlight head is rated at a GN (guide number) of 52 with coverage for a 35 mm lens on full-frame at ISO 100. The bare-bulb head with the standard 12 cm reflector is rated at GN 60 at 100 ISO and covers a 28 mm lens on full-frame. When comparing these specifications with those of speedlights, note that the GN of speedlights is often rated at ISO 200 and sometimes even with the zoom set to cover a 100 mm lens, i.e., a quite narrow emission angle. The AD200 speedlight head has no zoom. The GN measuring environment varies broadly among companies, and most companies provide measurements taken in optimal conditions (e.g. highly reflective walls and ceiling that help to give a high GN reading). This makes the GN ratings provided by different makers, at best, difficult to compare among different brands and not directly applicable to actual performance in real-life conditions. I took the following measurements with a Sekonic Flashmate L-308 S flash meter in short succession and in exactly the same setup. Therefore, the results should be directly comparable.
In practice, the AD200 speedlight head gives 1 more stop of power (i.e. twice the power) than the Metz 52 AF-1 with a similar FL coverage, the bare-bulb head with reflector two stops (i.e. four times the power). Zooming the Metz 52 AF-1 can make this speedlight more powerful than the AD200 when using telephoto lenses, since the AD200 has no built-in zoom, and no dedicated external zoom accessories are available. For this specific application, it may be better to choose another flash unit over the AD200. Recycle time of the AD200 is up to 2.1 s and the fully charged battery provides up to 500 flashes at full power. The speedlight head can be flashed in quick succession at full power for up to 40 times before requiring a pause for cooling down (Godox recommends 15 minutes after an overheat alarm), the bare bulb up to 50 times. The user manual provides more detailed data. The AD200 also has a stroboscopic mode (in manual mode), at a settable power level and a settable frequency of 1 to 99 flashes per second. The flash sequence continues until the user releases the shutter button on the camera, or the unit runs out of capacitor charge and is unable to keep up. This mode displays as Multi on the LCD screen. The AD200 can also be set as no slave (i.e. no radio control) and triggered in manual mode through a 3.5 mm input jack. As an additional alternative to the built-in 2.4 GHz radio control, the AD200 can be triggered through a radio receiver connected to a USB-A connector (in manual mode). The AD200 is not weather-proof. The speedlight head and main unit have ventilation slots, and connectors have no rubber seals. There is no temperature sensor inside the bare-bulb head (I have not opened the speedlight head yet). The AD200 does have an overheating sensor, but apparently only inside the main module. Updating the firmware requires the installation of the Godox G2 software (not the G1 software used to update other models). The right firmware image must be downloaded from the Godox web site and extracted from a RAR archive. 
The battery is rated at 2,900 mAh and 14.4v. It appears to be large enough to contain four 18650 cells, which in series would yield a nominal 14.8 V, close enough to the rated voltage. The charger displays the charging status in four stages. All four green LEDs are blinking when the battery is discharged. They light up steady one by one while charging, and when they all are lighted steady, the battery is fully charged. The battery has only two contacts, so the battery and charger are not swapping data. Hopefully, circuits in the battery casing or charger prevent overcharging. A rubber door near the power switch protects the Micro USB port used to upgrade the firmware. A second, USB-A socket on the opposite side is used to connect an optional wireless receiver (not compatible with the Godox 2.4 GHz system and only triggering in manual mode). A 3.5 mm socket nearby can be used for wired triggering or a generic wireless trigger.Configuring the AD200Manual configuration on the AD200 is required for matching the group settings of the master. Once the master can talk to the flash, settings related to exposure can be chosen on the master. Some operation settings can be set only on the AD200, for example to activate the built-in optical master to fire external flash units (only in manual mode). 
The AD200 has 5 buttons and a rotating dial for setting the configuration on the flash. The top right button is white and lights up red once the flash is ready to fire. Click this button to test-fire the flash (in TTL mode, it will fire at reduced power, while in M mode it will fire at the current power setting). Note that the flash fires when you release the test button, not when you press it. This still gets me now and then - I press the button, nothing happens, and I keep it pressed while wondering why. The LCD screen (not touch-sensitive) displays 4 lines of data. The screen is back-illuminated and displays white text on a black background. Above the rear panel is a red transparent window that contains an optical master. Power output in manual mode can be adjusted in 1/3-stop steps (full power to 1/128 power, or a range of 8 stops). There is also a setting to delay the flash firing for up to 30 s for rear-curtain operation. The display can be configured to show the flash duration (computed from triggering to 50% decay). Uniformity of outputUniformity of output in manual mode is a particularly important feature expected from a studio strobe, while speedlights may get away with slightly higher fluctuations. In any case, fluctuations higher than about 1/4 of a stop are detectable in images shot in a sequence (by measurement more easily than by eye), and fluctuations of 1/2 stop are at the limit of what I regard as acceptable. Even a 1/2 stop difference can easily be corrected in post-processing with practically undetectable losses in highlight and dark areas, but it takes time to do so. I tested the uniformity of output of the AD200 in manual mode, starting with a fully charged battery. For this test I used the bare-bulb head with 12 cm reflector and diffuser, and placed the flash meter 0.5 m in front of the flash head. Starting at 1/1 (i.e. full power), I manually triggered the flash with the test button (at the rear of the flash) ten times at each full-stop power setting (waiting for the flash to recycle at the higher power settings, while recycle was practically instantaneous at lower power), reading the output with a flash meter placed at 50 cm in front of the flash head. I then returned the power to 1/1 and repeated the test. The Sekonic Flashmate L-308 S flash meter I used for this test displays readings with a rated precision of 0.1 stops and three digits of displayed precision. I initially planned to generate graphs of the results, but after recording the actual measurements, I found that there is nothing to plot. At each power setting, the readings were extremely constant and the meter gave exactly the same three-digit reading in over 90% of the measurements. The remaining measurements display a maximum deviation of 1 unit of the last digit (e.g., 19 of the measurements at full power were f/64.3, and one measurement f/64.4). This speaks very well for the short-term consistency of both AD200 and flash meter. A quick testing with the speedlight head indicates a comparable performance. The Metz 52 AF-1 speedlight mentioned above gave almost as good results at full power (allowing for the longer recycle time), while at low power the results were less consistent. At 1/128 (i.e. minimum power), readings varied between f/5.63 and f/5.69. This is still good enough to consider the short-term output stability as excellent, with no fluctuations visible in images. Cheap speedlights and studio strobes often continue to recharge for several seconds after the recycle LED lights up, and their output fluctuates substantially if fired when recharging is not complete. Using the AD200 in the field
The AD200 is not shaped like a speedlight. It is just an oblong box with flash head at one end, and attaches to other equipment via a 1/4-20 socket at its bottom. There is a second socket on one side, but this surface is not flat and the socket is close to ventilation slits, so I would not use this socket for attaching the flash. The AD200 has no shoe mount, and the head does not tilt or rotate. This means you cannot mount the AD200 on the hot shoe of the camera or on an ordinary flash bracket. If you need any kind of articulation, you must add it. A tilting mount for lighting stands is included in the AD200 kit. A few owners have remarked about its inadequate strength. The head of the 1/4-20 stud, in particular, is too narrow and the AD200 easily unscrews when tilted. 
In the field, I would rather use a custom-made flash bracket that attaches under the camera body and carries a short articulated arm attached to the AD200. This setup can be made easy to assemble by adding an Arca-compatible clamp and plate between the arm and the AD200, and another clamp with multiple 1/4-20 sockets that attaches to a bracket I keep permanently attached at the bottom of the camera. This allows enough flexibility in the positioning of the AD200 for use in close-up and macrophotography, while still handling the camera with strobe as a single unit instead of requiring three or four hands. Stacking multiple AD200 unitsThe external casing of the AD200 has grooves and ridges that allow stacking two or more units together. Stacked units do not snap together, and need some type of tying (e.g., Velcro strips, rather than duct tape). Still, stacking multiple units may impair their cooling and subject the 1/4-20 socket at the bottom of the stack to excessive mechanical load. Lastly, stacking is only possible if all units carry the speedlight head. It is not possible to stack units with the bare bulb head, unless the stacked units are used without modifiers. As a whole, although the casing seems to be made with stacking in mind, it would not be a good idea to operate multiple units while stacked together. Kit bag
The AD200 comes in a semi-rigid padded bag. I have a few gripes about this otherwise well-made bag:
It is a good thing, instead, that the battery has its own compartment. The AD200 with the battery inside can be stored in the large compartment, and the battery compartment can hold a spare battery. Potential durability problemsA few posts and reviews on different sites have highlighted a couple of potential problems that may affect the durability of some AD200 units.
It is a fact of life that electronics sometimes fail without apparent cause. When this happens in a speedlight or studio strobe, a return for repair under warranty, or a new purchase, are the usual solutions. It is also a fact of life that rechargeable batteries have a limited useful life, and sometimes go bad faster than average. The electronics that protect a modern battery from overcharging and overheating may also fail. I am unable to say whether the problems reported for the AD200 are more common than those found in other flash units, but the reported incidents I am aware of are not numerous. I do not know for sure what type/size of cells are used inside the AD200 battery. If they are four 18650 as suggested above, perhaps Godox or a third-party manufacturer could come out with a battery case that accepts loose batteries. Using low-discharge batteries in such a case would solve the problem of discharging when not in use (although perhaps not if the battery is left in the unit). The modular construction of the AD200 involves both advantages and disadvantages with respect to durability: electrical connectors may wear out, become soiled, bend or break, thus lowering the overall reliability with respect to an all-in-one unit without removable parts. The latter type of unit, however, in principle requires replacing the whole unit when something goes wrong, while a modular unit may require replacing only one of the modules. AccessoriesThe availability of accessories, especially light modifiers, is essential for an electronic flash with bare tube. This is all the more important for a unit, like the AD200, which is equipped with a proprietary attachment for modifiers. Luckily, Godox decided to use the same attachment for light modifiers already used on the AD360 models, which have been around for a few years. A range of accessories is therefore already available. 
The Godox 2017 catalog lists two parabolic aluminium reflectors for the bare-bulb head. One of them comes with diffuser (above), the other has a hole for the rod of an umbrella and comes with a custom umbrella holder that screws into the 1/4-20 socket at the side of the head. This umbrella holder is apparently not sold as a separate accessory. There are also colored gels and a grid for the 12 cm reflector, a 30 cm metal beauty dish with grid and cloth diffuser, a snoot with grid, a 12 cm hemispherical diffuser of plastic (see the following figure), a metal protector sleeve that protects the bare tube when mounted in its head, and a very small metal reflector that covers one half of the bare tube. 
The hemispheric diffuser spreads light in an angle of more than 180°. It is one of few modifiers that allows the use of flash in interiors with a fisheye lens. Because of the broad diffusion, power may be an issue even in ordinary rooms, forcing a low aperture and/or increased ISO. The knob on the side of the head tightens a modifier inserted around the bulb, and prevents it from accidentally falling off. Do not tighten this knob without a modifier on the head, because in this case a plastic pad presses directly against the side of the glass envelope. The 1/4-20 socket on the opposite side of the head is for attaching umbrellas (via a proprietary rod mount that comes with Godox umbrellas), not for fastening the unit to a stand. A translucent umbrella and a silvered umbrella soft-box are available. For the speedlight head, there are a combination of barn doors and magnetic gel holder/grid holder, one grid, and a set of colored gels. I am not aware of any Godox diffuser to increase the size of the speedlight head of the AD200. This type of light modifier is often used in field macrophotography with speedlights. Perhaps Godox wants us to use the bare bulb head for this application, instead. The problem is that the 12 cm metal reflector with diffuser for this head (above figures) is about as easy to pack in a camera bag as a small cooking pot. Over the years, I equipped most of my speedlights with one turn of Velcro tape around their head, to attach small light modifiers in front of the flash window. It may not be a good idea to do the same with the AD200, since the higher power of this units warms up the head and the plastic window of the flash has ventilation slots along its top and bottom (in addition to more ventilation slots near the back of the head). A Velcro tape may impair cooling, or the heat may melt the glue of the tape. 
The LED head for continuous illumination is not included in ordinary kits. It carries 60 5-mm white LEDs, rated at a total power of 3.6 W, roughly the same as an ordinary LED flashlight. The dial of the AD 200 allows the regulation of the LED intensity in four steps (including off). The emission is diffuse, but has a visibly brighter center, giving a detectable central hotspot in images taken with lenses shorter than about 50 mm (on Micro 4/3, i.e. 100 mm on full frame). Together with the lack of a diffuser for this head, this limits the usefulness of the LED head in photography and cinematography. With the AD200 already in the bag, it may still be useful to carry the LED head in the field, as a focusing help and as an emergency flashlight. This head is much lighter and smaller than an equivalent flashlight, Godox markets an S-type bracket for using light modifiers with Bowens attachment on the AD200. According to reviews, the AD200 fits in this bracket with difficulty. Some manual filing of the edges of this bracket may be necessary. The bracket also has a tilt attachment for a light stand. Radio master X1T-O
So far, there seems to be only one model of master for Olympus/Panasonic, i.e., the X1T-O. It has a sufficient number of physical controls to avoid the "multifunction-button syndrome", at least for the most common settings. This is where the same physical switch is used in different ways for different functions, easily confusing the user:
Changing the most common settings is intuitive, and for these I usually do not need to consult the manual. The screen is large enough to show all important settings at once, on three lines of text. Four symbols are lined up along the right edge of the screen
The main exceptions to the "one function per button" principle is the CH/OK button:
Also, pressing any of the rear buttons on the master when it is asleep turns on the LEDs in the speedlight head. After the first press, the master wakes up and the buttons just do what they are expected to do. Double click the CH/OK button on the master, or click the bottom right button on the AD200, to switch off the LEDs. Better yet, get into the habit of waking up the master by half-pressing the shutter button on the camera instead, which has no effect on the LEDs in the speedlight head. The LCD screen is not touch-sensitive, which is good news for me (I prefer "real" switches and dials to virtual ones). The master is powered by two AA batteries, slightly more fiddly than normal to insert properly in the battery compartment. Opening and closing the cover of the battery compartment is also fiddlier than necessary. In addition to the controls and screen, the following features are present:
Some kits sold by Godox contain both AD200 and X1T-O master, and may cost slightly less than a separate purchase. The master has a range of up to 100 m with a clear line-of-sight and no interference from other radio sources. It can be configured on one of 32 available channels. 5 slave groups are configurable (A to D). Radio master X1T-S
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