Basics of Flash Photography
Four Fundamentals we must know

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HSS flash reduced power level

HSS flash has advantage of allowing flash fill with a fast shutter and a wide aperture in bright sun. This is its purpose. Its purpose is NOT speed, continuous light has no speed capability at all.

HSS flash power also can be a disadvantage. It is continuous light at much lower power level. It is also decimated by fast shutter speed (like in sunlight), but aperture can compensate that with Equivalent Exposures. But continuous light has no motion stopping ability like flash does, just like sunlight in that regard. And basically, its maximum power level is around 2.3 stops less power than regular flash mode.

Is it actually 2.3 stops loss? Here are four ways to verify it.

But at the HSS shift point above, the ratio of 52 feet changing to 22 feet on the SB-800 LCD computes 2.48 stops loss (some round off in the numbers shown). Note that HSS mode works same as continuous sunlight - each faster shutter stop is half as bright (Unless also equivalently compensated by a one stop aperture change). For flash, double power is 1.4x distance range. 14 feet is one stop more than 10 feet, and 20 feet is one stop more than 14 feet. Very important to realize that we did not compensate by opening aperture one stop each step, to create the customary equivalent exposures we deal with continuous light. People are surprised that the HSS flash falls off this way with shutter speed, because they are used to regular flash which is not affected by shutter speed. But HSS is not regular flash, and the sun and other continuous lights are decimated the same by shutter speed, in exactly the same way, so no reason to get excited. :)

A second set of numbers, at ISO 800 and 105 mm zoom, and same 1/250 second Auto FP mode at f/4:

The first value is limited in the display at 66+ feet. But this is from Guide Number, which for ISO 800, computes GN 184 x 2.8 / f4 = 129 feet.

66+ feet (which is 129 feet) - For ANY shutter speed not exceeding maximum shutter sync speed (full power)
56 feet - 1/320 second (continuous HSS mode kicks in above 1/250 second - reduced power)
50 feet - 1/400 second
35 feet - 1/800 second
25 feet - 1/1600 second
18 feet - 1/3200 second (shutter speed decimates continuous light)
12 feet - 1/6400 second

Again, all at f/4, which are NOT equivalent exposures (but all HSS equivalent exposures are equal range).

A -1 EV stop of Flash Compensation (for fill) increases distance range by 41%, in either regular or HSS mode.

These methods all introduce an additional 1/3 stop shutter speed increase, as the only way to enable HSS mode, which are not quite equal situations. But 2.3 stops is the ballpark loss, about 20% power level.

Some HSS power examples

Pictures below show a garage door, looking west at 11:20 AM, in partial shade from a roof shadow at top, and a tree shadow lower. Unfortunately some minor clouds, some minor variance, but I tried. The fill flash is illuminating the dark shadow on the garage door of course (concept works same as a dark shadow on a human face). Nikon D300 in 1/250 second Auto FP mode with hot shoe SB-800. ISO 200 and Center metering, Aperture priority. 24-70 mm lens at 24 mm. Subject distance (garage door) was carefully measured to be at 12 feet (3.66 meters), which is about the limit for SB-800 HSS fill flash to help much.

Garage door in shade, at 12 feet, ISO 200, hot shoe flash

Above: No flash. 1/250 second f/16 (dark shadows are the problem).

Above: Regular TTL flash 1/250 second f/16. Fill flash, notice the top right corner.
The flash LCD range says 8.8 feet (SB-800 Guide Number at 24 mm is 98 x 1.414 = 138 ISO 200, divided by f/16 is 8.7 feet range). If we had been at 8.8 feet, the flash power would have lighted the shadow to full expected exposure. At 12 feet, it is fill (41% farther is -1EV), and we still have shadow, but a lighter shadow, an appropriate fill level.

Above: Regular TTL flash -1 EV flash compensation, 1/200 second f/16. The LCD range says 12 feet (range for -1EV fill).

Above: TTL HSS flash, 1/400 f/11, 0EV flash compensation.

Above: TTL HSS flash, 1/6400 f/2.8, 0EV flash compensation. The LCD range says 4.6 feet, yet the flash seems helpful for fill at 12 feet. Note that all equivalent exposures show the same flash range in HSS flash mode. It is just not much range. But fill does not need as much power as a sole light source would need, as fill is expected to be down about a stop (real fill range is 40% more range... or we could instead use -1EV flash compensation). And note that the regular flash mode LCD above reported only 8.7 feet range, and was still usable as fill at 12 feet (but at f/16, which allowed maximum sync speed to be honored).

Above: TTL HSS flash, -1 EV flash compensation, 1/6400 f/2.8, The LCD range says 6.6 feet (range for -1EV fill).

So, is HSS fill flash usable in bright sun? Yes, for the purpose of a wider aperture. Is it powerful? No. And the regular speedlight is not so strong either (not at the necessary f/16). So perhaps HSS mode may not be optimum power for fill at 12 feet, but we still get considerable helpful flash fill, often usable for f/2.8 in bright sun if desired (within these range limits).

Note: I gotta say, generally, the ONLY goal of any of this HSS flash business is just to be able to use fill flash in bright sun, at wide apertures like f/2.8, if we crave that. Otherwise, HSS flash is rather weak and its range is limited. It is the full opposite of a fast flash. It conceivably could allow flash with fast shutter for fast action in sun, but the range may be too short for action. IMO, we'd be dumb to use HSS mode indoors, where regular flash will run circles around it. But... HSS can allow fast shutter so we can use f/2.8 in bright sun.

Above: Again, no flash. 1/250 second f/16. Notice the bricks in upper right. Fill level was significant at 12 feet in bright sun.

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