Separating Fact from Fiction:
The purpose of this article is to acquaint you, the photographer, with a working knowledge of the mechanics of HSS (high speed sync); what it is, what it isn't, and how it can be confidently incorporated into a functional workflow, when photographing an outdoor environmental portrait, in a brightly lit setting.
Within this article, you will be introduced to two new terms; Exposure Value EV and Guide Number GN. These represent the ambient brightness of the scene EV and the output of the flash unit-modifier combination at a specific distance from the subject. GN is a function of the flash power setting, the light modifier and the distance from the subject.
GN - For example, let’s say that for a particular flash unit and modifier combination, the measured value of light at full power at a distance of 10 feet is f-5.6 - The Guide Number then, will be 10 x 5.6 or 56. The guide number is the numerical product of the measured light in f-stops and the distance from the subject. Once the guide number is defined for a specific flash unit and modifier, changing the distance from 10 feet to let’s say 5 feet, therefore dividing GN-56 by 5, provides the new aperture value, f-11 for a correct flash exposure. Note: The GN value may be expressed in either feet or meters.
Therefore, what will be presented in this article, is a practical way to determine the exposure contribution of a specific flash unit and light modifier combination, once the X-Sync GN (guide number) is defined by measurement. This will then be followed by examining the resulting shift in light value output, when incorporating the high speed sync mode, at various camera shutter speeds.
The Beauty of High Speed Sync:
Beyond the obvious scenario of allowing a full measure of control over flash and ambient lighting ratios, the ability to manage a wide dynamic range of light values, opens the door to creative direction and the ability to render otherwise distracting background elements into soft shapes of light and shadow, through the control of the depth of field.
In the setting for the photos below, there was a 5 stop exposure difference in the open shade of the parking lot overhang and the brightly lit palm tree in full sun. The measured light value in the open shade was 1/200th at f-2.0, EV-10. The background area, being lit by full sun, measured in at 1/200th at f-13, for an EV-15. That is a 5 stop difference.
I wanted to be able to tame the background and illuminate my model, Keely with a 32in Octa-Box, positioned directly above the camera, at a 3 foot distance from Keely, for Paramount Beauty lighting. To accomplish this, I would need to pull my ambient exposure down by 5 stops, from 1/200th at f-13, (to correctly expose the background), to the equivalent exposure of 1/3200th second shutter speed, to allow for an aperture of f-2.0 for the portrait.
The Setting for the Photograph
Ambient Exposure f-2.0 1/200th
The ambient light from open shade, while exposed correctly lacks vitality and with a five stop difference in the open shade and the brightly lit area, directly behind Keely, the background is totally blown out.
Exposing for the Background
Adjusting the exposure to lightly under-expose the background, 1/200th at f-13, as metered, renders our model as a near silhouette and the background elements are a bit distracting. Though the background was selected to provide shapes of soft focus color for the following image.
X-Sync vs. High Speed Sync
As observed in the animation to the left, in X-Sync Mode, the shutter curtains are fully open when the flash is triggered, allowing the flash to evenly illuminate the frame. Animation, courtesy of fstoppers.com
Some Technical Info:
In order to achieve faster shutter speeds with a focal plane shutter, the second curtain activates early in the shutter cycle and chases the first curtain across the sensor. This traveling slit, diminishing in height as the shutter speed is increased, effectively reduces the exposure. However, the mechanical time constant of the shutter open-close cycle remains at 1/200th second, or approximately 5 milli-seconds. In order to illuminate the frame evenly at faster shutter speeds, the high speed sync mode, stretches the flash pulse to remain on during the full 5 milli-second shutter open-close interval. This is accomplished by electronically switching the flash on and off at a very high rate, causing it to behave as if it were a continuous light source during the period the shutter mechanism is traversing across the sensor or film plane. As far as the sensor is concerned, this stretched flash pulse, now, 6 milli-seconds in duration, is equivalent to a continuous light source, over the full period of the shutter open-close cycle. As such, in high speed flash mode, the flash illumination is now subject to the effective exposure reduction produced by the narrowing traveling slit of the shutter, in the same manner as the ambient light of the scene. Therefore, the light value of high speed sync flash, diminishes proportionally with an increase in the effective shutter speed.
The Magic of High Speed Sync:
Given the two extremes shown above, high speed sync, allowed me to create a much more dynamic image, by returning my lens aperture to f-2.0, and providing flash illumination across the full sensor frame at a shutter speed of 1/3200th of a second. A pulse of continuous light, while the second curtain chased the first curtain, as a traveling slit, exposing the camera sensor with ambient light and the high speed sync flash pulse, while combining and blending the two exposures for a dramatic portrait.
CL-600X at 1/2 power in HSS mode with 32" Octa-Box - f-2.0 and 1/3200th. To see the delicacy of light achievable with high speed sync, click on the photo above to enlarge it.
Free PDF Download:
In the effort to keep the content of this blog post efficient, I am providing a link to download an excerpt from my book, "Creative OCF Lighting Techniques for Photographers", covering the essential basics of Flash Synchronization. You may wish to read the excerpt along with the content of this blog post for a full understanding of synchronizing electronic flash and an accompanying intro to the subject of high speed sync. Download your free copy here.
Notes:
The term, X-Sync., denotes the camera shutter synchronization with a Xenon gas discharge or flash tube, where the shutter is completely open when the flash unit fires.
The term Guide Number, denotes the measured light output of a specific flash unit and light modifier combination at a specified distance. Once the Guide Number is known, the variables of distance and aperture setting may be easily computed. GN = Aperture X distance-feet. (see download below)
The term EV, denotes Exposure Value and is a relative measurement of Ambient Light Value in terms of shutter speed and aperture value combinations at ISO 100. (see below)
The Basics:
Exposure is the integration of light over time and can be defined by the following formula.
Where the variable N represents the Aperture value in f-stops and t represents the time value in shutter speeds. Essentially, the camera sensor is presented with light during the period of time that the shutter transitions across the sensor or film plane.
This article will explore the technique of blending flash illumination and ambient light to produce a well lit portrait in an outdoor environment. An EV table will be utilized to assist in defining the lighting ratios for combining X-Sync based flash and High Speed Sync. with the ambient light value of the portrait setting.
What HSS is:
Essentially, high speed sync, or HSS, provides today's photographer with the ability to synchronize electronic flash with shutter speeds above the normal X-Sync limit of 1/200 - 1/250th of a second, and thereby greatly extend the range of ambient lighting conditions, wherein electronic flash can be creatively used.
The synchronization with higher shutter speeds is accomplished by electronically stretching the flash pulse to effectively present a burst of continuous light to the camera sensor. That is, during the period of time where the focal plane shutter curtains are transitioning through the open-close shutter cycle as a variable width slit.
What HSS isn't:
High Speed Sync is not a method to freeze action. The flash pulse is actually extended to present an even and continuous light source during the time the shutter is traversing across the sensor.
HSS Calibration:
With my recent purchase of the Godox manufactured AD-600X and AD-200X, from Cheetah-Light in Dallas, TX, I was eager to incorporate HSS into my location portraiture and Lighting Workshops. As an engineer and instructor, I would need to design a useful way to measure the HSS performance of these two new lights.
In the photo below, I have set up the Cheetah-Light CL-600X with the 36" Rice Bowl soft-box. Placing a black-gray-white exposure target five feet in front of the Rice Bowl and setting my Sony A99 to a successive progression of shutter speeds from 1/320 to 1/8000, I began the measurement process, by examination of the resulting exposure distribution histogram, then charted the results.
The first step was to measure the X-Sync light output at a specified distance of 10 feet, using a Sekonic L-358 light meter. This was followed by moving the light five feet from the exposure target and viewing the resulting peaks on the camera histogram. (see exposure table below)
Note: the setup below was repeated indoors to remove any ambient light contribution.
Fig. 1. Setup method for measuring HSS performance CL-600X with 36" Rice Bowl
Fig. 2. Sample camera histogram for HSS output measurement.
CL-600X HSS Exposure Chart
Fig. 3. - Resulting HSS Exposure Chart with X-Sync Reference (click to magnify)
The two setups, were alternately set up to measure the CL-600X and CL-200X in HSS mode, while recording the measurement data to be compiled into the following spread sheets.
CL-200X HSS Exposure Chart
Fig. 4. - X-Sync and HSS Exposure Chart for CL-200X
How-To Measure and Use HSS (high speed sync):
The light output of a flash is based on; the input storage power in Watt-sec, the flash electronics efficiency, the flash power setting, (1/1, 1/2, 1/4, etc), the light modifier being used, the beam projection angle, and the distance of the flash to the subject.
The unit of measurement for light output is GN, Guide Number. For example, the CL-200X has a guide number of 160 ft. with the stock Fresnel lens. That means at 10 feet, the flash will produce f-16 light with the camera set for ISO 100.
Attaching the bare bulb accessory with the 5" reflector and disc diffuser, produces a similar guide number.
Adding a 24 in soft box with the bare bulb attachment, the guide number measured was 63, or f-6.3 at 10 feet. That is about two and a 2/3rd stops less light output.
Note: The above measurements provide the baseline for calculating the light output in the HSS mode, at various shutter speeds. Ex: 1/320, 1/400, 1/800, etc.
Note: The high speed sync mode reduces the effective light output, as the flash pulse is being lengthened or stretched to provide, what appears to the camera sensor, to be a pulse of continuous light during the shutter open-close cycle.
When switching from X-sync (1/200 sec) to HSS (1/320 sec) and faster shutter speeds, there is initially about a two and a half to three stop loss of light output.
As the flash takes on the characteristic of a pulse of continuous light in the HSS mode, each full stop increase in shutter speed corresponds to a full stop decrease in flash output power. While the flash output is actually not decreasing with the increase in shutter speed, the increase in shutter speed, decreases the light contribution that the sensor sees, just as in the ambient light level.
What this means: In HSS mode at a shutter speed of 1/320th of a second, with a 24 in light modifier, the CL-200X will produce a guide number of about 20, or f-2.0 light at 10 feet.
Typically, however, the flash when used with a diffuser will be placed closer to the subject. Ex: at 5 feet, the light output will be f-4.0 with the 24 in modifier and the flash power set at 1/1. (See histogram below)
Fig. 5. Inverse Square Law
The rule states that the power intensity per unit area from a point source, if the rays strike the surface at a right angle, varies inversely according to the square of the distance from the source.
Note: The HSS light values indicated in the two spreadsheets above, was measured by projecting the flash onto a black, gray and white exposure target and observing the position of the three corresponding histogram peaks. (see resulting histogram Fig. 2. above)
As standard flash meters do not measure flash output accurately in the HSS mode, using the exposure target provided the needed measurement tool to determine the effective guide number, as depicted in Fig. 1., above.
Estimating HSS Performance:
Once the X-Sync guide number of a flash/modifier combination is known, given the approximate 3-stop light output loss when switching to HSS mode, that is switching the shutter speed from 1/200th second to a shutter speed of 1/320th second, the flash output in HSS mode can easily be estimated.
Example:
CL-200X X-Sync GN with 24 in modifier equals 63, or f-6.3 at 10 feet.
Then at a shutter speed of 1/320 in HSS mode, the guide number becomes GN = 20, or f-4.0 at 5 feet, about a 3-stop loss. (from f-6.3 - f-5.6 - f-4.0 - f-2.8 - f-2.0) three and 1/3rd stops.
From step 2. above, estimate a 1/3 stop decrease in flash contribution to the exposure with every 1/3 stop increase of the shutter speed.
Of course, manually decreasing the flash output, will also result in a corresponding decrease in flash exposure. (See CL-200X Exposure Chart Fig. 4, above.)
The above flash exposure chart of Fig. 4, identifies the flash output in both X-Sync and HSS modes for the CL-200X with various light modifiers and distances from the subject. For the HSS mode, a wide selection of shutter speeds are calculated. Note: For the 24" Beauty Dish, the highest practical shutter speed is 1/3200, producing f-1.4 light at 3 feet.
Putting the Measurements to the Test:
Putting the Measurements to the Test
With the setup depicted in the above photo, it was time to test the theory behind the measurements.
1. The CL-600X was set up as the main light, about three feet from our model, Kimberly. I wanted to have a shallow depth of field. So using my Carl Zeiss 135mm f-1.8 lens, I measured the ambient light of open shade. Measuring f-8.0 at 1/125th gave me an EV number of 13. Looking across the EV chart, (red tiles) for EV 13, to get a shallow depth of field would require a shutter speed of 1/2000 sec for an aperture of f-2.0.
2. With the HSS GN Chart of Fig. 1, above, I could easily achieve f-2.0 light at full power, five feet - light to subject distance. Taking into account the inverse square law, I brought the face of the 32" soft-box to approximately three feet from our subject and set the power for 1/4 on the CL-600X.
3. To bring up the shadow values, I set the CL-200X flash directly under the camera lens with the five inch reflector and diffuser disc. 1/32nd power, brought the shadow values up for the look I was wanting. The following photos were achieved in a similar manner.
Notice how the catch lights in the eyes reflect the main and fill lights.
Sample Photographs taken in HSS mode, using the flash and EV charts. Once the ambient exposure value was defined by use of the camera or hand held light meter and the aperture and shutter speed combination was selected to be in HSS range. See Ambient Light EV Chart below.
Use this EV Chart to select the desired shutter speed and aperture combination.
The term EV, stands for Exposure Value and represents scene brightness. The chart shows that there are a number of combinations of shutter speeds and aperture values that will produce a correct exposure for a given EV.
For example, EV-15, is represented by the set of dark blue squares. Showing that f-2.8 at 1/4000 produces the same exposure as f-16 at 1/125. By the way, Sunny Sixteen is equivalent to EV 15. ISO = 100, S = 1/125, A = f-16.
1. Using a hand held or in-camera light meter, measure the ambient light of the scene you wish to incorporate as the background or setting for your environmental portrait.
2. Now find the metered shutter speed and aperture value on the EV chart above.
3. Follow the corresponding diagonal set of colored squares to see the various shutter speed and aperture value combinations that will produce the same exposure. That is, having the same EV number as that just measured.
4. Based on your lens focal length and setting, select an aperture value for the desired DOF "depth of field". The EV chart will then provide you with the corresponding shutter speed for the setting to be correctly exposed for the EV measured, no guess work.
Note: You may wish to under-expose the background setting, just raise your shutter speed.
5. If your suggested shutter speed is below the camera X-Sync speed, (1/200th or slower) refer to the X-Sync portion of the flash GN chart above to set the power and distance for your flash.
6. If the shutter speed is 1/320th or faster, you will be using the HSS portion of the flash GN chart to set your flash power and distance.
Following the above steps will provide you with an accurate baseline from which to determine the useful range of flash settings and aperture values that will best correspond with the needed flash output to balance and set the desired ambient to flash ratio.
Ref: EV = log2*(N^2/t) Where, N is f/stop Number, t is shutter speed (duration Time). The idea is that settings with a constant EV ratio are the same Equivalent Exposure. And that each full EV increment is a 2x exposure difference.