REDISCOVERING THE ELINCHROM ELC PRO HD – Early in 2014 Elinchrom released their new flagship flash unit, the ELC Pro HD 500/1000, described in their press release as “the world’s most complete, feature rich compact studio flash unit”. Several reviews have been written, confirming its quality. With this review I would like to go deeper and explore these units from a more technical point of view and provide additional information from a practical photographer’s perspective. Discussed will be the features offered and also their limitations.
Equipment used for this study are a set of two ELC Pro HD 500 strobes with Skyport Plus remote trigger, a Nikon D810 and Nikon D7200, Pocketwizard MiniTT1 and FlexTT5, and finally an awesome little exposure and flash meter which I would like to briefly introduce here, the Optivelox SS04U sensor.
Figure 1: Optivelox SS04U exposure and flash meter
The Optivelox SS04U, an upgrade to the former SS04, is a great utility tool. It consists of a sensor, which plugs into the USB port of any Android phone, and measurements are controlled using the Lxmeter app, which is available from the Google Play Store. The SS04U is an incident light meter, flash and continuous light, a reflective light meter using the mobile’s camera, and it is to my knowledge the only existing device for Android phones offering measurement of flash durations, t0.5 and t0.1. I have used the device for verification of the Nikon SB-910 Speedlight, and the results are within the measurement accuracy (down to 2 µs, with option for 1 µs) consistent with the Nikon specifications, and for the whole power range 1:1 (1.1 ms) to 1:128 (26 µs). All flash waveform measurements in this review were done using the SS04U, as well as a verification of the Nikon shutter times.
In this paragraph, we want to jump directly to one of the more important marketing points for this flash head, a “super-fast flash duration”. The meaning of “fast” has to be considered relative to its application. What action do you want to freeze? Sports events, water, a bullet?
The numbers of 1/5000 (ELC Pro HD 500) and 1/5260 (ELC Pro HD 1000) for flash duration sound awesome. What is however specified is not the t0.1 (time above 10% of maximum light intensity), which is the number important for freezing motion, but it is the t0.5, the time that the light intensity is above its 50% level. This topic has been discussed in many publications, but before looking at some practical results and measurements, I would like to open up the physical meaning of these two-time durations.
To do that, let’s take a look at a flash pulse as recorded by the previously introduced SS04U sensor, and for the ELC Pro HD 500 set to the minimum flash duration of 200 µs (1/5000 s).
The curve can be considered as a representation of the power distribution of the light pulse, as a function of time. Similar to the strength of a light bulb, expressed in Watt, the height of the pulse tells us something about the strength of the light being emitted… the light intensity.
What generates a signal in your camera’s image sensor is however not the power, but it is the energy of the light being absorbed by individual pixels that will return an image. This energy is expressed in Joule (J), or Watt.second (W.s), and equals the power multiplied with the time that this power is applied, or in other words, it corresponds to the response to a certain power-hitting the sensor during the flash duration.
If now we calculate for the above measurement the energy corresponding to the t0.5, which is the area underneath the t0.5 curve segment (green), and we do similar for the remaining (t0.1 – t0.5) curve segment (red), then we can derive a ratio for the signals returned during the time that a subject is well highlighted, the true signal, and the signal resulting from the remainder of the curve, originating from a less lit scene and responsible for motion blur. For the above example, the ratio indicates that 60% of the flash pulse is used for freezing the moving object and 40% of the flash pulse causes a signal spread over the leading trail of the object.
This leading trail will only be visible if the environment light is not overpowering the light intensity corresponding to the tail of the flash curve.
To consider the freezing capability, we next consider 3 cases for an ELC Pro HD set to a flash duration of 1/5000, a water drop, a ball that is being thrown and a dart being shot from a kid’s dart gun.
Figure 4: Above: water drop falling at about 9 ft/s or 3 m/s. Below: Water splash
The image clearly shows the leading trail caused by the tail of the power curve. The measurement was done in the dark, with a 1/100 shutter time. The speed of the water drop can be estimated to be about 9 ft/sec or 3 m/s. The conclusion is clear. The strobe is unable to freeze water.
Ball (Measured speed 18 ft/s – 6 m/s)
Figure 5: Ball being thrown at 18ft/s or 6 m/s
A 4 inch (10 cm) dog ball being thrown at a measured speed of 18 ft/s or 6 m/s. The image was taken in diffused sun light at the maximum sync speed of 1/250. Notice the trailing ghost and the leading ghost. The leading ghost can again be attributed to the tail of the flash’s power curve. The trailing ghost may be explained by the assumption that the flash doesn’t fire at the moment that the first shutter curtain exposes the sensor, but for example 1 ms later, such that the natural illumination causes a sensor imprint (a 4 ms shutter time, and an approximate 2.5 ms time for the first curtain to fully open, means that the flash has a ‘1.5 ms minus flash duration’ time gap to fire, before the second curtain starts closing). The assumption does make sense when taking into account that the functioning of the camera mounted Elinchrom transmitter is camera brand independent, meaning that it must have been configured for a worst case shutter curtain travel time. Figure 6 demonstrates this theory graphically. The red area represents lighting by the environment.
Dart (Measured speed 50 mph – 80 km/h)
Figure 7: Dart moving at 50 mph / 80 km/h
An extreme case of a dart flying at 50 mph (80 km/h). Notice again the leading trail caused by the fall-off of the light pulse.
The above images lead to the conclusion that the ELC Pro HD has a limited ability to stop motion. It is, however, wrong to consider this as a negative feature. All is relative and has to be considered in relation to the application for which this strobe, a studio strobe, is meant. It is the task of the photographer to be creative and to use what he has available, good or bad, in his advantage to create his work of art.
The ELC Pro HDs are studio strobes, for situations with controlled lighting and controlled background. This is confirmed by the lack of support for sync speeds higher than 1/250 s, as will be discussed a bit further under the subject of High-Speed Sync. Given the 4 ms minimum camera shutter time allowed, and flash durations with a maximum value of 1/2020 or 495 µs, the environment light, if set too strong, will take part in illuminating the subject in the resulting photograph.
The flash duration, both t0.5 and t0.1, was evaluated using the SS04U sensor. Roughly we can say that the true flash duration is within the specification given by Elinchrom, but an important remark has to be added. For low and high power settings, the form of the flash illumination pulse is clean, and the result fully matches the specification. For intermediate power levels, however, the pulse shape is very irreproducible, hereby leading to a degree of uncertainty regarding the exact value of t0.5. The indicated value for t0.1 is, on the other hand, less sensitive to the exact pulse shape and can be accepted as accurate.
As a side note, it might be interesting to look briefly at the operation basics of voltage controlled strobe heads, such as the ELC Pro HD, vs strobe heads using IGBP (Insulated Gate Bipolar Transistor) technology.
The essential difference may be recognised from the fact that voltage controlled flashes vary the voltage applied to the flash tube, in order to change the energy (J or W.s) emitted. IGBP based flashes on the other hand use in principle a constant voltage (in practice the voltage is modified in order to limit colour temperature variations), and the energy is changed by cutting off the light pulse when the configured energy level has been reached. It is clear that this technology allows for the shortest flash duration, which is reached at a minimum energy level. For a maximum power setting, there is no difference between voltage controlled and IGBP.
A more detailed description can be found in the Lencarta article “Flash duration, what’s it all about”.
The ELC Pro HD model comes with a build-in Skyport receiver. As a transmitter, the user has the option of purchasing a Skyport Plus (included when purchasing a To Go set) or the Skyport Plus HS, which for this particular strobe only offers the additional advantage of displaying the flash settings, in addition to full control of connected strobe units. The HS option cannot be used for the ELC Pro HD, as sync speeds higher than 1/250 are not supported.
This leads us to the alternative of connecting a PocketWizard FlexTT5 to the ELC Pro HD trigger port, combined with an on-camera MiniTT1, in order to obtain a High-Speed Sync (HSS) option. Using the standard PocketWizard configuration, the strobe can be triggered, but only when using sync speeds up to 1/250, a clean image is obtained. At higher sync speeds, clipping and/or image distortion (gradation) occurs.
As reason for the HSS incompatibility, the short flash duration has to be considered, and this can be understood by focusing on the general operation of PW products (reference PocketWizard support). The sequence of operations running down when taking an image is as follows:
The exact time needed for a curtain to cover the image sensor height depends on the camera, and can for a Nikon D810, which I will use here, be estimated at around 2.5 ms, which means that the minimum flash duration needed to obtain a full and equal illumination of the whole sensor is equal to about 2.5 ms or 1/400 seconds.
After much trial and error, the best PW MiniTT1 configuration was found to be the use of a “Manual Hypersync for Standard Channels”, and to set an appropriate “Manual Hypersync Offset”. The exact value to be set however depends on personal preferences. Using this configuration, the sync speed can without any clipping be increased to 1/800, however not without a graduated filter effect being present in the resulting image. Note however that this option only provides a solution for the lower flash durations. For a flash duration of 1/5000, no acceptable solution was found.
As explained by the PocketWizard support, in the case of manual channels and an HS offset, the time at which the first curtain reaches the bottom is used as a reference, and the offset sets the time before or after when the flash is being fired.
Offset: – 500 µs (equal to flash t0.5) + 200 µs
Figure 10: Flash duration 1/2020, shutter 1/800
Figure 11: Flash duration 1/5000 s, shutter 1/1000
Resuming we can say that although the fast flash duration is a nice feature to have, the price to be paid can be steep for those applications requiring a super-fast sync speed. This not only applies to the ELC Pro HD but is a general issue for short flash durations. The recommendation for the buyer requiring high shutter speeds, e.g. 1/8000, is to get a strobe head with longer flash duration or alternatively a strobe with an explicit option for HSS.
An important point in evaluating a studio strobe is the stability of its colour temperature as a function of power settings, as this is often a weak point for strobes with fast flash duration, more so however for IGBT based flashes than for voltage controlled flashes. The colour temperature was measured using the standard photographic way, a series of grey card images for varying power settings, and a white balance correction in a reliable photo editor, in my case Adobe Lightroom 6 and Adobe Camera Raw.
For each of both flash units, 3 measurements were done, and the average was taken. The variation for the same unit in the successive measurements is found to be practically zero. There is, however, a variation between the 2 flash heads, with a maximum difference of 100 K. We can assume this to be a measurement error. The average for each flash head, ELC1av and ELC2av, and a global average ELCav, are shown in Figure 12. Added to the same graph is the flash duration, hereby allowing to derive the synergy between flash duration and colour temperature.
The colour temperature measurement result indicates a value of 6060 +/- 260 K as compared to the by Elinchrom specified of 5500 +/- 150 K. When however adding a Rotalux softbox with reflective inside metal coating, the values are according to the specification.
One can say that for a given flash tube, there exist three characteristics that can be controlled, namely the flash duration, the colour temperature, and the power. A flash tube allows for 2 degrees of freedom, or in other words, two parameters can be chosen, and they define the third one. The higher the voltage in a given tube, the more brightness will be generated, and the bluer it will be. As the voltage is reduced, the brightness goes down, and the colour shifts towards red/yellow. In general, flashes are designed such that the bluer parts and the redder parts average to be daylight (5000K to 6000K, depending on manufacturer).
Voltage controlled flash units reduce the peak voltage when reducing power, thereby reducing the amount of blue present in the average light being emitted, and leading to red coloured temps.
IGBT based flashes cut the (lower voltage) tail in the flash curve, thereby reducing the amount of red in the average, and leading to bluer colour temperatures. The reduction in voltage can be used to compensate for this colour shift.
As can be derived from Figure 12, Elinchrom uses voltage control combined with control of the flash duration in order to stay stable across the full power range.
Another excerpt from the Elinchrom press release, which may stun photographers, concerns the recovery time of the flash units, “the ELC Pro-HD 500 and 1000 boast recycle times of 0.6s and 1.2s to full power”.
These recovery times are easily verified using the built-in strobe function, allowing to fire a set number of flashes per second. The parameters to be set for this function are i) the number of flash strokes (between 1 and 20) per second in Herz (Hz), and ii) the duration of the strobe sequence, from 1 to 10 seconds. If the unit is unable to fully recharge, the LED on the control panel will turn red during or after the strobe function has finished execution. And indeed, capacitors fully charged or not, the flash will fire and give what it has available.
The maximum strobe frequency was measured as a function of the power setting, and with a duration of 3 seconds. The results are given in Table 1.
What can be derived from the maximum frequency for a certain power setting, is an approximate value for the recycling time. This result is depicted in Figure 13. Note that the approximation is the most accurate at the right end of each horizontal segment (maximum energy for set frequency), and this value is indicated by the orange dashed line. Towards the left of each horizontal segment, the value is overestimated.
Figure 13: Approximate recovery time for each power setting
A small improvement that could have been made by Elinchrom, is a repetition of the flash after an interval equal to the recovery time + flash duration, till the set duration has been reached. Take for example maximum power, with a recovery time of 0.6 s. For this power level, only 1 Hz can be applied, meaning that a 3 s time of operation only results in 3 flashes, whereas theoretically 3/0.6 or 5 flash pulses could be applied. In some situations, these 2 extra shots might give the ultimate image.
And finally an example of the strobos mode, using a 1s shutter time and a 5 Hz strobe, and as subject little Joe having Christmas LEDs wrapped around his body.
Note for beginners why a dark/black background is required:
An image sensor pixel can be compared to a water bucket, being filled by the photons (in reality the electrons generated by photons hitting the silicon) in the available light, as comparable to drops of rain. The pixels are empty when aiming at black and dark, and become filled and further on saturated when being hit by light, reflected from an object. If the stroboscope function is used to capture a moving image in front of a white background, then the white, reflecting the flash light pulse, willfully blank out the image. All pixels are filled to the limit, saturated, and there is no room left for storing photons that are reflected by the object. If the background is darker, not black, then the reflection from the background will partly fill all pixels, and the reflection from the object will be superimposed. For getting a clear image, it is important that the part reflecting from the object is dominant, if not, then the object will be transparent to a certain degree, and the object will disappear in an imprint of the background.
Figure 14: Example of stroboscopic mode using a 5 Hz flash rate during 1 second
In order to accomplish a rear or second curtain sync, a delayed flash mode is provided, in which the flash stroke can be delayed with a value in between 1 ms and 10 s. As the ELC Pro HD user guide only provides values for the Canon EOS 5D full frame camera, I verified the numbers for the full frame Nikon D810 and the cropped frame Nikon D7200. The results are given in Table 2.
Notice the discrepancies which are marked in red. For a shutter time of 1/1.3, 3 and 6 s, a maximum delay larger than the theoretical shutter time can be set. An immediate question is, who is correct, Elinchrom or Nikon? The Nikon support told me that the exact shutter times are proprietary information and cannot be revealed to the public. The answer is given in Table 3, again obtained by using the SS04U sensor, and by assuming a shutter lag of 73.8 ms (74 ms – 0.2 ms).
Nikon D810 measured shutter times
Table 3: Verification Nikon D810 shutter time
To conclude this paragraph a simple application of the delayed flash mode on our swinging and LED’ed friend Joe.
The last option to be briefly discussed is the sequence mode, which allows up to 20 strobes to be fired in sequence, thereby reducing, even more, the effective system recycling time, and allowing a firing at a rate equal to the camera’s maximum picture frame rate. An example, a camera having a maximum of 5 fps, and a required high power setting, having a 0.5 s recycling time. The sequence of events after pressing and holding the shutter button:
Time 0 s: Flash 1 fires
Time 0.2 s: Flash 2 fires
Time 0.4 s: Flash 3 fires
Time 0.6 s: Flash 1 has fully recycled and fires
Time 0.8 s: Flash 2 has fully recycled and fires
As such a quasi-stroboscopic effect is implemented in which the camera’s frame rate becomes the speed limiting factor, and not the recycling time of a strobe. To get the true stroboscopic effect the resulting images can then be combined in post-processing.
The Elinchrom ELC Pro HD series flash is a true studio flash, for optimum benefit to be used in a fully controlled environment. The fast flash duration is nice to have but has its limits. In use, it will be fast enough to freeze a person jumping up, but it won’t be fast enough to freeze for example water drops. The biggest hurdle is the tail of the flash pulse, which will create a leading trail. It is up to the photographer to use this weakness to his or her advantage.
To conclude I want to state the, in my eyes, main positive and negative aspects of these studio flash units.
George Vittman is a French Alps based electronic engineer and image sensor specialist, who identifies job and hobby as one, and who expresses his passion for technology and nature through his photography. Describing subjects as Creatio Ex Nihilo, created from nothing and simply being there, George tries to replicate and immortalize what his eyes see and enjoy by squeezing out the edge of technology.
George is in his past author of countless educational/expository articles in international magazines, discussing numerous subjects, going from electronics to equestrian and photography. He loves the country life and likes to relax within nature. For him, living is learning, as also expressed 2500 years ago by Socrates “I know one thing, and that is that I know nothing”.
The author would like to sincerely thank Mr Marco Baroncelli, owner of Optivelox, for providing his novel SS04U sensor free of charge and for his patience and continuous support in solving unusual measurement issues, the PocketWizard tech support for their assistance in implementing HSS, and last but not least, the Paul C. Buff engineering department and customer support, for sharing their studio flash expertise.
DISCLAIMER NOTICE: The author is not affiliated with any of the brand names mentioned in this write-up, nor is he being compensated for using their name.