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For several years now I have used high speed sync (HSS) in order to light portraits in full sunlight at a wide aperture. If you’re unfamiliar with HSS, it allows you to shoot at shutter speeds that are higher than the native sync speed of your camera (usually 1/200 or 1/250 of a second, read your camera and flash manual to find yours) while still using speedlights. The reason this ability is so enticing is that you can shoot flash-lit images at wide open apertures in full sunlight, allowing for a shallow depth of field. Normally if you were using a flash, your maximum shutter speed would be at 1/200 or slower, meaning that you would need to close your aperture down in order to get a proper exposure in the sun.
How HSS works is that the flash will begin pulsing light, just before the shutter opens, since the exposure is so short. The problem with this is that much of the output of the light is lost in the pulsing process, meaning that you need more flash units to achieve a decent output. For example, when I am shooting at 1/8000th of a second, I need to combine four flashes, on one stand, in order to light a subject that is about five feet away. And that is without any modifiers, like an umbrella or soft box. The other issue with HSS is that not just any flash and trigger system will do the trick. You need to have gear that will communicate information from the camera to the flash.
A couple systems that can do that are the PocketWizard Flex TT5 and Mini TT1, or the RadioPopper PX system. Since most photographers don’t already own one of these triggers systems, this means starting from scratch, which isn’t cheap. I personally opted for the RadioPopper system, since the PocketWizard Flex system for Canon was super glitchy. The RadioPopper system wasn’t perfect either. Just the amount of batteries alone, for four Canon Speedlites with triggers, including a ST-E2 transmitter for the camera, required 27 batteries. Even though they were mostly all rechargeable (the ST-E2 required the hard to find 2CR5 battery), imagine trying to troubleshoot a misfire. Did the batteries need changed in one of the transceivers or was the speedlite misaligned, obscuring the sensor? Or imagine that one of the speedlites’ batteries may be slightly more drained than another, causing only three of four lights to fire. This made the overall exposure fluctuate with every frame.
I recently decided to compare HSS against using a variable neutral density (ND) filter. ND filters screw on to your lens and cut down the light that hits the sensor, thus allowing for a wider aperture in bright light. This allowed my shutter speed to stay at or below the sync speed cutoff, allowing the full strength of the Speedlite to light my subject. This meant that I wouldn’t need to transmit ETTL information (sell the RadioPoppers) and it meant that I would need fewer Speedlites (less batteries).
After setting my ISO as low as it would go (50), my shutter speed as high as was allowed (1/200th on the Canon 5D MarkII), and my Speedlites at their full output, I dialled down the variable ND until the ambient light perfectly balanced with the light from the flash.
Some people have pointed out that there could be the issue of a color cast with certain brands of ND filters. I have not experienced any issues with the ProMaster brand. However, it’s important to keep in mind that if you are shooting directly in to the sun, there will likely be glare in your image, causing a possible color cast or the image to appear washed out.
Note that this experiment was done using Canon 430EX Speedlites with RadioPopper PX triggers. I’ve since sold them all, opting for the cheaper, sturdier and more powerful LumoPro LP180 with PocketWizard PlusX triggers. Now with one bare bulb flash, and a variable ND filter, I can effectively cut the ambient light while fully lighting a subject at f/1.4 in full sunlight.