posted on Nov 23, 2009 at 7:25PM
Head-2-Head Lighting Review: Profoto Pro-8a 2400 Air vs. Broncolor Scoro A4S
Flash Duration PerformanceBy Ted Dillard
Let’s look at the sequence of events when you take a shot using a strobe system. The exposure of the image normally gets determined by the shutter speed combined with the lens opening, right? The lens opening controls how much light gets through, and the shutter speed controls how long the light is getting through - all adding up to a perfect amount of light, or a perfect exposure. When you’re shooting strobes, the flash duration determines how long the light gets through, so the shutter speed has no effect on the exposure. The shutter simply has to be open for the period of time that the flash is going off. Here’s a little diagram of how that happens.
The shutter opens, and stays open for 1/125th of a second. While it’s open, the flash fires, arcing through a tube with Xenon gas. The gas heats up, light is emitted, and then it disappears when the arc stops. The timing of the shutter coinciding with the strobe discharge is your flash sync. (Ever take a shot with your flash and get half the frame cut off with black? Flash sync is the culprit. Your camera’s shutter speed was set too high. Basically, the shutter is opening, the flash is going off, and then the shutter closes before the flash is done firing. The flash continues to go off, but the shutter is closed for part of the “exposure.”)
The bottom line is the time that the duration of the flash of light is what freezes movement in the exposure. Short flash durations make crisp exposures of moving subjects.
There’s only one thing that makes this a little more complicated - that’s the fact that the output of a strobe system is most often controlled not by, well, output. It’s controlled by that same flash duration. If you want to use a lot of power, you’re going to have a long duration. Short flash durations produce less light than long ones.
The design challenge here is pretty simple. Get as much power output as you can, so when you cut it down to a short duration you are still getting enough light out of it to make a good exposure. Oh, and make the color and output consistent. No problem.
The Test Setup
The first round of performance testing will be evaluating the flash duration. To do this, we’re concentrating on a real-world, repeatable scenario. We’re shooting a simple degree-wheel target spinning at a known speed. By looking at the sharpness of the hash-marks at low-to-high RPM, and comparing packs at low, medium and high flash duration, we’ll be able to see what the systems are capable of, and how that looks. Take a look; this reveals some pretty interesting results that get missed entirely when you’re just looking at the specs.
Flash Duration Test Results
Round 1: Minimum Duration
These tests show some very interesting results that tell a little different story than just the published numbers of these two systems. Even at 160 RPM the Profoto appears slightly softer, almost like the lens resolution is slightly less than the shots with the Broncolor (Both test series were shot with the identical lens, at exactly the same point of focus). This is apparent even up to the 300 RPM range, and it’s due to the differences between how Broncolor and Profoto cut off the output.
You can see in the Profoto samples a slight shadow before and after the hashmarks on the wheel. The line itself is approximately the same density, suggesting they received the same exposure, but where the Broncolor has a clean cutoff, it would seem the Profoto has more of a buildup and dropoff, for lack of better terms, that contribute to this density shading. Interestingly, this shading with Profoto is very close in appearance from 160 RPM up to 300 RPM, where the Broncolor gets progressively softer as it approaches 300 RPM. That said, the Broncolor starts and finishes with a sharper image, from 160 RPM to 300 RPM. Compared at 1000 RPM, the two images seem fairly similar.
This effect backs up the explanations that we’ve had from Broncolor about how they handle the light cutoff. Without lapsing into overly technical explanations, they claim that they “clip” the curve - only using a segment of the ramp-up and cool-down, giving a cleaner cut to the exposure. In this case, you see it as a cleaner edge, rather than simply the definition of the edge. This would make you suspect the Profoto system more of the curve, and possibly a more complete color spectrum since the ramp-up and cool-down produce a color shift in the arc, from cool to warm light.
To see what the Broncolor claimed was their ultra-short flash duration, you need to turn their color control system off. This is explained by Broncolor by the fact that you’re using an extremely small slice of the color curve - only the most intense peak- and you get a strong shift in color, to blue. We were able to estimate it at around 1500K, but we’ll show you more about that in the color tests.
Despite the obvious blue shift, the Broncolor shots were, at 1000 RPM, nothing short of remarkable. They start to get slightly soft as you move up to 2000 RPM, and at 3000 RPM there is the slightest shadow and loss of contrast. However, it is still simply amazing. Just for reference, 3000 RPM is about twice the safe operating speed of our test target, and to see this kind of clarity coming from that kind of speed just seems unbelievable.
The second round of duration testing was even more interesting. Testing a system at minimum duration is fine, but often you need to run more power. Shooting a set larger than tabletop and using a smaller aperture, you may find that the output at minimum is just not enough light. To get a feel for how the duration performed under that situation, we ran a set of tests at ½ power - 1600Ws on the Broncolor, 1200Ws on the Profoto. Here are the results.
Round 2: Half-Power
These results, frankly, are startling. Even at the slowest speed there is a noticeable difference in the two images, and a decided advantage to the Profoto. As we move up into the range we’re seeing a slow decline in the Profoto performance, but a significant and severe decline in the Broncolor images. Where the Broncolor has a slight edge at minimum duration, the Profoto, at half-power, is remarkably better, and has a significantly shorter duration.
This is a very important performance difference in any shooting that requires some horsepower. Sets, room shots, people, dancers, medium-scale explosions - you get the idea. If you’re looking for astoundingly crisp, frozen action of tabletop subjects where you can move the lights in and run them at minimum power and still retain decent depth of field, you’re probably going to want the Broncolor. If you have something bigger where you need more light, more depth of field and still need to try to freeze action, then the Profoto is going to give you noticeably better results. If you don’t know what the next job holds, and want the system that is the best compromise - well, that gets into a slippery slope, but we’d pick the Profoto.