A focusable parabolic reflector is…
…a light modifier with a parabolic shape that can be focused, i.e. the light source can be moved within the reflector. This allows very different light characteristics ranging from focused to wide-open. Besides this unique flexibility, other advantages of focusable parabolic reflectors are their high energy efficiency and the relative absence of hotspots.
Why make one out of a softbox?
Such reflectors are commercially available in uncompromised quality, however, their prices exceed the budgets even of many professionals. My goal was to make my own DIY focusable deep parabolic reflector based on a softbox (where the light source would not be attached to the speedring as intended, but to a focusable tube). I examined three softboxes regarding their fitness for this purpose. This article sums up my results.
Meet the candidates:
RimeLite grand softbox 90cm
A deep, supposedly parabolic softbox with 16 rods
(also branded Mikrosat Grandbox or Illuminate Pro)
Aurora Tera-D 35
A deep, supposedly parabolic softbox with 16 rods.
Same diameter as the RimeLite (90cm) but even deeper
Elinchrom Rotalux Deep Octa 70cm
Elinchrom's popular softbox with folding speedring and 8 rods
Great for beauty lighting when used traditionally (as a softbox)
I considered taking parabolic umbrellas into this comparison as well, however I noticed that they are generally much more shallow than the softboxes, which means that the focal point is further away from the vertex (often far outside the umbrella). That's bad for spill and makes defocusing impractical. Furthermore, the mechanical parts of the umbrella are in the way which makes it hard to attach a focusing mechanism. That disqualifies these umbrellas for the purpose of my project. A fellow DIY maker, Detlef G. of Schwarzweissart has also reported stability issues with such umbrellas.
The final goal of the project was of course a proper focusing mechanism, however that would depend on the choice of softbox, thus for this experiment I manually held the light source in place. The light source was an Elinchrom Quadra A Head, attached to a hand-held telescope rod (the „Long grip“ by Lastolite).
I tested four different positions per softbox:
- The focused position (manually determined with the help of the modeling light as the point where the beam of light is most narrow)
- The half-focused position (half-way between focused and de-focused)
- The de-focused position (where the light furthest from the focus point while still on-axis and inside the reflector)
- Direct light (light source inside the speedring, facing outside. The normal intended use case of the softbox, but without any diffusors)
Test case 4 is very different from a focusable parabolic reflector since the light is facing the other direction. It is the normal use case of a softbox but without the softening diffusors. I just tested this as well to determine how the focusable reflector compares to a normal softbox without diffusors, and whether making such an extravagant light modifier is even worth the hassle.
For test cases 1 through 3 I closed the hole in the speedring with aluminum foil so that light would reflect back also from the center (where most of it hits when focused).
A focusable parabolic reflector is obviously an indirect light source, so direct light from the flash was not desired in positions 1-3. I tested two configurations per position and per softbox: one of a bare flash head flashing into the softbox, and one of the flash head equipped with a small metal disc shielding direct light (I used the Lastolite Quadra adaptor for Ezybox, which is basically just an aluminum disc with a bayonet):
The biggest white surface I had available to be lit was a ceiling in a high room, so I put the softbox between two chairs, facing upwards towards that ceiling. The softbox was about 50cm from the floor and the room height is 420cm, thus the distance of the ceiling was 370cm from the innermost point of the softbox.
There are two minor shortcomings of this approach:
The ceiling is not completely flat, but slightly diagonal and rounded on one side (the left side). This leads to a little distortion of the light spot and slightly brighter light on the left side due to the rounded part being a bit closer than a flat ceiling would have been. Having that in mind, it’s not a big deal, especially since the softboxes are symmetrical and the other side of the pictures is unaffected
There is a window front on the back wall (bottom of the frame) which reflects some of the light back onto the ceiling. The reflectiveness of window glass is supposedly 3%, thus the effect is not strong, but nevertheless it is visible on some of the test shots. Fortunately the windows have window frames that do not reflect and the reflection of the glass is hard (undiffused) so it can clearly be seen on the photos how bright the ceiling would be without the reflection from the glass, by simply looking at the parts where reflection is absent due to the window frames.
The camera was an Olympus E-M1 with a 14mm lens (corresponding to 28mm on a full-frame DSLR), f/8, 1/100s. The camera was placed on a small tripod on the floor, about 150cm distant from the softbox, facing the point on the ceiling right over the softbox. A test shot was made without flash to ensure that the frame at these settings is completely black and all the light on the test shots indeed results from the flash and not from ambient light.
Let's see pictures. By the way, the round thing in the top right corner is a stair-case from below and the funny guy in the bottom left corner is me holding the flash rod into the softboxes. Just the perspective is somewhat unusual.
All of the tested softboxes can be used with different positions of indirect light to somewhat achieve the desired result, however the quality of the results varies a lot.
It hardly makes a difference whether a disc is attached to the flash or not. Well, we see only brightness on plain surface here. I can imagine that it does make a difference in shadows. In any case it cannot be wrong to avoid direct light on the subject, so a little screen is probably a good idea but it need not be big.
Direct light through the speedring of any softbox without a diffuser is just plain ugly. It's simply not a good idea to do that, especially because of the hard falloff edges. The learning here: test case 4 was pointless. If at all, the indirect parabolic lighting should have been compared to a regular softbox with at least one diffusor.
The Elinchrom Deep Octa suffers from two conceptual weaknesses: first, it has only 8 rods while the other two candidates have twice at much and are thus much rounder. Second, it's not even a parabolic softbox by design. That makes it difficult to focus the light since the softbox simply has no single focal point. In the de-focused setting, the result is quite pleasing (although it has a hotspot). But while manually trying to find the point where the light is most focused, I noticed that the further I moved the light into the softbox, the more focused it looked but also the harder the edge of the spot became. The visible edge in the spot of light is a shadow thrown by the rim of the softbox which is blocking light from the opposite side. I also attribute that to the non-parabolic design. And as a side effect, the would-be focused light is considerably darker than with the other softboxes, because some of it gets reflected across the whole room. Just to get one thing clear: this is not a bad softbox. In fact, it's my favorite softbox and performs wonderfully when used as intended. But as this test shows, the Deep Octa 70 is not fit for the purpose of a focusable parabolic reflector, because it is not focusable.
While the Aurora performs better focused, it disappoints in the half-focused setting. I was quite surprised to see a dark halo. It's even visible in the focused setting, but strongest in the half-focused one. Obviously this softbox focuses some of the light in a middle hotspot and throws more light towards the sides, but the falloff is nothing like a nice poisson curve; it is not even continuous. From the middle towards the sides, the light gets darker, then brighter again and then again darker. And not just by a bit - the deviation from the expected steady decline is a whole stop of exposure and can hardly be ignored. That makes this softbox unfit for practically all applications where a half-focused setting is desired. Why does this happen? I would guess that its shape is a little too round, but honestly, I don't know. One would have to calculate the reflections. But the images speak for themselves. Quite a pity, since this softbox has the highest build quality of all three and is also the sturdiest. A bit heavy maybe, and hard to (dis)assemble, but still quite convincing as far as handling is concerned. Nevertheless my conclusion: not first choice for a DIY parabolic reflector.
That leaves us with the RimeLite Grandbox, which delivered the most convincing results. In the focused setting, the light comes closest to a hotspot with a nice steady falloff. This softbox also produced the brightest spot (hence its ability to focus is the highest). In the half- and de-focused positions, the brightness is subject to subtle local variation, probably resulting from the hexadecagonal shape of the softbox and wrinkles in the material. A little bit of a dark halo can also be seen in the half-focused setting, but much less so than with the Aurora. That makes the RimeLite the clear winner of this test. In terms of handling, the softbox is very light and quickly assembled, however it's also the most fragile of the three. Fine for outdoor applications, but just as long as there is not much wind. It doesn't take much to deform this softbox, especially since we're not using any diffusors that would add extra stability. Somewhat stronger rods would have been a good idea. And the reflective material is also the one throwing most wrinkles - considerably more than the Aurora or the Elinchrom. It's light though. For a DIY focusable parabolic reflector, the RimeLite is my clear favorite out of these three softboxes.
Now that a softbox has been chosen, the next step is the construction of a focusing mechanism. Read here how I built a focusable parabolic reflector.
While this has been a comparative analysis of three softboxes and we have merely looked at visual results, Detlef G. has chosen just one softbox (duh... the same one :), conducted extensive measurements with a light meter and graphed his results, outlining the different lighting characteristics of the three positions. I highly recommend his article on the subject.
Why are parabolas so hard to achieve?
After I attributed most of the above mentioned problems to the shape of the softboxes, I started wondering why the heck they're not simply made to be accurately parabolic. It's apparently harder than it sounds to build a true parabolic softbox. Think about bending a metal rod: the naturally resulting shape will be an arc, not a parabola. If you want to bend it parabolically, you need considerably more force near the vortex. Basically that could be achieved by the surrounding tissue, but that again would put a lot of pressure onto small parts of the tissue and especially the seams.
If you think about how to avoid these problems, a promising approach would be to separate the rods from the reflective material by adding a T-shaped section to each rod. The rods could then run outside the actual reflector in a more rounded arc shape while the reflector inside could be accurately parabolic. It seems that I'm not the first person to have this idea: some of the unaffordable high end parabolic reflectors look quite similar to this.
In any case, we can learn a few things from this thought experiment:
Accurate parabolic shape is an important quality criteria for a parabolic reflector as it directly affects the ability to focus and thus the quality of the light
Building accurate parabolic reflectors is a challenge. While it's straight-forward to construct a roughly (approximately) parabolic softbox, constructing a perfectly accurate parabolic softbox calls for a considerably more complex and thus more expensive design due to mechanical reasons.
Therefore we should not assume that anything labeled "parabolic" is actually parabolic of even comparable. First of all, it's marketing. That's why tests like this one really make sense.
Hungry for more? Read how I built a focusable parabolic reflector.