A tale of two studio strobes
I openly admit that I don't have an extensive experience with studio strobes. In fact, my practical experience as buyer and user of this type of equipment, in total, involves only two units and a span of less than two years. Nonetheless, I learned quite a bit from this hands-on experience.
Studio strobes, or studio flash units, are conceptually similar to the much smaller, battery-operated electronic flash units that are ubiquitous among amateur and mobile (non-studio) photographers. Both types of flash units produce a short and intense light burst by discharging a capacitor through a xenon plasma between two electrodes separated by a gap of a few centimeters in a sealed tube. The discharge is triggered by ionizing the xenon gas through a high-voltage electrical pulse applied to an electrode located on the outer surface of the tube. Modern flash units of both categories contain sophisticated electronics to regulate with precision the discharge voltage, current, duration and color temperature.
Beyond these generic specifications, modern studio strobes differ from modern battery-operated electronic flash units in several respects. The following table shows the main differences in qualitative terms.
The power of studio units is generally rated in watt second (1 Ws = 1 joule). However, some manufacturers (especially those that produce studio units for amateur users) rate their units in "effective watt second". When power ratings for a unit are specified in both real and effective Ws, the "effective" rating is usually quoted with figures 2-5 times higher than real Ws. I have yet to see a scientifically acceptable definition of effective Ws. The large majority of flash manufacturers simply do not define what they mean by effective Ws, and several manufacturers do not even state whether they are quoting real or effective Ws. Therefore, the "effective" power rating means nothing, and is only a way to confuse unprepared buyers and make them believe they are buying a powerful unit when, in fact, they are buying a much weaker one that produces a light intensity 2-4 stops lower at full power. Each stop less corresponds to halving the light output, so 4 stops less means that the unit is producing only 1/16th of the promised amount of light.
Portable flash units are rated in guide number (the product of distance and required f/number). A doubling of the guide number means a doubling of the flash-to-subject distance, which means the power output is four times higher (because light spreads on a surface), i.e., a 2-stop increase in power. The guide number rating, in practice, is only reliable with flash units equipped with built-in reflector and collimator lens (i.e., the plastic window that protects the flash tube). The guide number varies in units equipped with "zoom" heads that concentrate light on an area of variable size. There is a (sometimes intentional) confusion in the guide number specifications released by flash manufacturers: the guide number standard requires this measurement to be specified at a camera sensitivity of 100 ISO and a distance of 1 m, while several manufacturers release measurements taken at 200 ISO (i.e., their flash output is overspecified by one stop). Care must also be taken to distinguish, and convert between, guide numbers expressed in meters (m) versus feet (ft). To convert a guide number from ft to m, multiply it by 0.328.
There is no physically possible way for a flash unit to release more energy than the one stored in its capacitor (as the "effective" Ws rating seems to imply), so the power output measured in (real, not effective) Ws, or the guide number (which is based on the illumination received by the subject) remain the only objective ways to measure the output of an electronic flash. There is also no way to directly convert Ws to guide number, because the illumination produced by a studio unit varies with the reflector and light modifiers being used. In general, however, it can be said that a studio unit with a rating of 250 (real) Ws should produce substantially more light than even the most powerful portable units.
Studio units are built as monoblocs, or monolights (i.e., units that include both the electronics and tube) for powers up to approximately 1,500 Ws, and as physically separate generators and heads for higher powers.
The traditional way to use a studio flash is to set up the subject and illumination, and then to measure the required exposure with a flash meter. This instrument measures the light emitted by a flash and received by the subject (either by placing the meter within the subject area or by reading light reflected by the subject) and produces a reading of the required f/stop aperture at the given ISO setting. Flash meters are designed to ignore ambient (i.e., continuous) light. With digital cameras, a flash meter is largely unnecessary. It is sufficient to take a test picture and to verify its exposure (with the built-in exposure histogram displayed by virtually all DSLRs and many high-end compact cameras). If the exposure is wrong, change the flash power settings and/or lens f/setting and repeat. I can usually reach the correct exposure with 2-3 attempts, and once the correct settings are reached for a given scene and illumination setup, they can continue to be used as long as the scene does not change (albeit switching to unusually dark or light subjects may require a manual exposure adjustment).
According to my experience, when buying a studio flash unit, its price is a better indication of power and overall build quality than any other technical specification. Professional units like the Bowens Gemini series are quite expensive, while no-brand units made in China may go for one-fifth of the price of a Bowens of comparable power ratings. If you want to know why, you may continue to read.
This cheap unit was sold on eBay and shipped directly from its factory in China. It is rated at 300 Ws, but a more honest rating is probably around 100 real Ws.
My principal purpose with this flash was to test whether it could be used for UV photography. For this purpose, I disconnected the original flash tube and mounted a tube without UV coating (Lumedyne, modified by removing its external UV-coated shield) in place of the modelling light (above picture). Thus, this unit contains two flash tubes, but only one at a time is used. This modification did work in UV photography (the uncoated tube produces approximately 1 stops more UV emission than the original tube), but the problems outlined below, caused by the overall cheap construction of this unit, did not make the BL-300 a very usable solution.
The following characteristics make the BL-300 difficult and frustrating to use.
This unit is honestly rated at 500 Ws, and built like German tanks were once built (I cannot vouch for the ones built after the split and reunification). In addition, it is microprocessor-controlled, and a large number of settings are available, in addition to those obvious from the control labels. It has two manual power dials, both with click-stops (and therefore highly repeatable). One dial is graduated in exposure stops, the other in tenths of exposure stops (and really delivers on the promised accuracy). I did not open the casing to inspect the electronics, but power regulation in this unit is much more sophisticated than a mere voltage regulation. For instance, when decreasing the power, excess capacitor charge is automatically dumped, so every flash is at the set power. Multiple capacitors are likely used internally and switched in and out as needed, and the electronics also compensate for the small changes in color balance and exposure caused by very high or very low power settings. In total, power can be regulated within an interval of 6 exposure stops.
The settings of this unit can also be controlled via a wireless remote control (optional, but the unit itself has all electronics for this function already built in - just set the left power knob to the REM position). Thus, you can attach this unit to a ceiling rail system and never need a ladder to change its settings. Some of the available settings are quite sophisticated, while others are designed to make the life of a photographer easier. For instance, the unit can be programmed to ignore a given number of pre-flashes (usually produced by DSLR flash units for setting the automatic exposure) when using the built-in slave photocell. An example of the latter type of settings is that the digital power display can be electronically turned upside down to accommodate different flash orientations. This flash also has a slot for a plug-in radio slave module, as well as the commonplace built-in slave photocell and socket for cable jack. It has a proprietary connector at the rear for a battery-powered generator, so it can be used in the field.
The unit comes with a metal mounting bracket that allows an easy regulation of the flash inclination. Since the unit rotates about its center of mass on this bracket, it remains balanced and in most cases does not require mounting on a tripod head. The bracket can be rotated upward for hanging the unit from a ceiling rail, without needing to turn the unit upside down, and it is long enough to allow any orientation (including straight downward) without causing the flash unit to hit its pantograph or rail mount. This is problematic with the Boling unit.
Unlike the Boling unit, which has all its controls on the rear panel, Bowens units have a larger number of controls and indicators, spread onto the rear, as well as as left, sides. When the 500R is placed at the left of the subject, however, the controls on the left side of the unit either face away from the photographer, or their labels are upside down. There is no solution to this dilemma, which is really the only negative feature I can find in the Bowens Gemini series of studio units. In scientific photography, small subjects are almost always illuminated from a top-left direction, and in this orientation the above problem is a nuisance.
Bowens reflectors and light modifiers are expensive (albeit of superior quality). Aftermarket accessories with the Bowens attachment bayonet, of varying quality, are very common.
This model is the largest in the Gemini series without an internal cooling fan. This is particularly desirable in macrophotography and close-up photography, where air blown by a fan may disturb the subject. The light output turns out to be more than enough for all my current uses. In fact, this unit, used as the only light source outside a large diffusing cube (80 x 90 x 90 cm) is usually set 1-2 stops below maximum and still allows an adequate exposure at ISO 200 and f/22. In the photomacrography of delicate subjects, care must be taken not to use an excessive power, which may actually incinerate small subjects at close range.
Although the specified power ratings of this and the preceding unit are not extremely different, the physical size, and especially the diameter, of their flash tubes differ substantially. The tube of the Boling BL-300 is roughly half the diameter and apparently less than half the thickness of the Bowens. Incidentally, the non-coated tube that I mounted at the center of the Boling unit is even shorter and narrower than the original Boling one, in spite of being specified for 800 Ws units (if this is a honest rating, it must mean that this bulb needs frequent pauses for cooling during use at full power, since the hole in its base is small and does not allow an efficient forced air cooling). The tube size and diameter are not in direct relation to the output light. However, the current density in a narrow tube is higher and the tube temperature increases accordingly. This, in turn, causes two problems: the glass is mechanically stressed to a higher degree, possibly causing microfractures around the electrodes that, in the long run, cause the Xenon gas to leak out and shorten the life duration of the tube, and overheating is more likely to cause a catastrophic failure and bursting of the tube (more about this below).
In conclusion, this Bowens model is the studio flash unit that I should have bought first. Should I need another one, it probably will be a second Bowens Gemini.
In general, for advanced amateur or professional photographers who consider buying their first studio strobe, I would recommend a comparable model. Although clearly more expensive than no-brand units, it is better to buy a good unit at the onset, rather than wasting money on a cheap and unreliable unit as a fist step on the way toward better equipment.
Studio flash units are normally supplied with flash tubes that are coated to absorb UV emissions. This was particularly necessary with photographic films, which are strongly sensitive to ultraviolet. However, digital cameras are equipped with very effective UV and IR filters in front of their sensors. Solid state sensors are also intrinsically less sensitive to UV than photographic film. Therefore, UV-coated electronic flash tubes are less of a concern in digital photography, and in fact many fashion and product photographers prefer to use non-coated tubes, which are less likely to produce color shifts (UV-coated tubes are distinctly yellowish at visual observation). Non-coated tubes are also slightly less expensive than coated ones.
Non-coated tubes, of course, cause a higher eye and skin exposure to UV, which could be a concern for a photographer who uses non-coated tubes on a daily basis. I am unable to confirm whether this constitutes a significant health hazard. Given the short duration of electronic flash, this additional exposure to UV for a professional photographer may not exceed the equivalent of a few tens of seconds of natural sunlight per day of work. The use of UV-blocking glasses should greatly reduce the risk of eye damage.
Above are a few pairs of visible light and UV images taken with the Bowens Gemini 500R with uncoated tube and a UV-enabled Nikon D70s with 60 mm UV Rodagon lens. The B+H 486 UV- and IR-cut filter was used for the first picture, the Baader U UV-pass filter for the second in each pair.
In my experience, the non-coated flash tube (original Bowens replacement for the Gemini 500R) increases the near-UV emission by approximately 1.5 stops (i.e., the amount of near-UV is approximately 3 times higher. This is a significant and useful improvement, although not a dramatic one. Coupled with the abundant power of this flash unit, the modified flash is very usable in UV photography, and produces roughly 3-4 stops more UV than the modified Boling BL-300 and at least 5-6 stops more than a UV-enabled Vivitar 285 HV.
As a demonstration of the effects of overloading a flash tube, the above picture shows the tube from a Vivitar 285 HV (a cheap battery-powered electronic flash, known among UV photographers for using a non-coated tube), after connecting it to the Boling unit and flashing it just a couple of times. The substantial power overload caused the interior of the glass tube to instantly melt. A portion of the tube near one of the electrodes also cracked, causing the Xenon to escape and ending the useful life of the flash tube (had this not happened, a third discharge probably would have caused the tube to explode). Lesson learned: Do not overload a flash tube.
Additional readings: see my book on scientific photography for a more in-depth coverage of UV and IR flash photography.