A softbrella is a hybrid between an umbrella and a softbox. It is constructed like an umbrella, but it has a panel to cover the underside of the umbrella. As with umbrellas, there are two main types of softbrellas: reflective and shoot-through. In the reflective softbrella design, the underside of the umbrella is reflective and the panel covering it is designed for diffusion. In the shoot-through softbrella design, the umbrella's top surface material is designed for diffusion, while the interior part of the underside panel is reflective and the exterior part of the panel is usually black.
In theory, a shoot-through softbrella offers greater efficiency than a shoot-through umbrella because at least some of the light that would otherwise be wasted on the non-shooting side of the umbrella is recovered by the underside panel and reflected back into the umbrella. A shoot-through softbrella also would minimize the spill on the non-shooting side (which can affect the background lighting).
On the other hand, a reflective softbrella should be less specular than a reflective umbrella, and would be more convenient to set up compared to a softbox.
SHOOT-THROUGH OR REFLECTIVE SOFTBRELLA?
A shoot-through's advantage is that you can bring it much closer to the subject, which is important because of the inverse square law. However, for handheld use, I chose a reflective softbrella because it makes the angle makes it possible to use it handheld for backlighting or rim lighting (on the other hand, rim lights and backlights usually use hard not soft light).
SOFTBRELLA EFFICIENCY VS. CONVERTIBLE UMBRELLA
Up to this point, I assumed that a softbrella was significantly more efficient than a convertible umbrella. I decided to test that assumption.
I tested the maximum light output from a handheld shoot-through umbrella vs. a handheld reflective softbrella. I don't have a light meter, so take my results with a grain of salt. Testing method: set flash at full manual power, zoom the flash at full (105mm), set camera to sync speed 1/250, ISO 100, then compare the apertures between the umbrella and softbrella that yield an exposure that is identical (as measured by the histogram). I took the shots in raw format, and used ViewNX as the raw converter. This is supposed to be a 'real world' test, so I wanted to factor in the shoot-through's 'real world' advantage of being able to be positioned closer to the subject, so in both cases I positioned the umbrella and softbrella as close to the subject as possible without obstructing the camera's view. As previously discussed, this is an advantage for the shoot-through because of the inverse square law.
I tested at a portrait focal length of 50mm (75mm in 35mm terms) and a normal focal length of 35mm (52.5mm in 35mm terms), with the camera on a tripod, and I used an approximately infant-sized stuffed toy as a subject.
AT 50MM (75MM) PORTRAIT DISTANCE
At 50mm, I was shooting about 4 feet from the subject. Judging from the highlights of each exposure, I found that a shoot-through umbrella was actually more efficient than a reflective softbrella. Specifically, with the reflective softbrella, the highlights were at the edge of the histogram at an aperture of f/7.1. With the shoot-through umbrella, the highlights were at the edge of the histogram at an aperture of f/9. This implied that the shoot-through had a 2/3 stop advantage.
Out of curiosity, I also tested the convertible umbrella in reflective mode. In reflective mode (without the black cover), the highlights were almost at the edge of the histogram at an aperture of f/7.1 as well, which implied that it had almost the same efficiency as the reflective softbrella.
Here are the images (converted into JPEG):
softbrella at f/7.1
shoot-through at f/9
reflective umbrella at f/7.1
Note that the light pattern is more evenly spread with the reflective softbrella and reflective umbrella than with the shoot-through.
AT 35MM (52.5MM) 'NORMAL' DISTANCE
At 35mm, I was shooting at about 6 feet from the subject. With the reflective softbrella, the highlights were almost at the edge of the histogram at an aperture of f/7.1 (note: I believe the aperture of the softbrella test shot is identical to that of the 50mm shot because the white wheels, which reflect light more efficiently, showed up in the wider shot). With the shoot-through umbrella, the highlights were at the edge of the histogram at an aperture of f/8. Therefore, even at a more typical shooting distance, the shoot-through was slightly more efficient than a reflective softbrella. Here are the images:
Softbrella at f/7.1
Shoot-through at f/8
For handheld use, a reflective softbrella does not appear to be more efficient than a convertible umbrella when taking into account the shoot-through's ability to be positioned closer to the subject. Factoring in the additional conveniences of the convertible shoot-through (which is collapsible -- even with the umbrella swivel and flash attached, and also usable as a reflective umbrella), it appears that for use as a handheld umbrella, a convertible umbrella is superior to a reflective softbrella. I didn't have an opportunity to test a shoot-through softbrella, which might be more efficient than a convertible umbrella.