How to set up umbrella to get even lighting?

pgriz

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I have a T1i, 580EX II, 430EX II, Yuongo radio triggers, variousumbrellas, stands, etc. As a hobbyist, I am using inexpensive equipment to get the effects I want. Most of my photography with flash is still life, with occasional family portraitures. I set up my light using manual settings on all flashes, and use the radio triggers to fire everything. I use a L358 flash meter to get me close to the ratios I want and then make adjustments from there. So far, so good.

However, when setting up my umbrella(s), I noticed that the evenness of the light depends on where the flash head is in relation to the axis and distance from the umbrella. I also noticed that the flash head setting also seems to affect the distribution of light. Which makes sense, since the umbrellas are really curved surfaces which can have a focal length, and in principle, positioning the flash head at the umbrella’s focal point should allow the light beam reflected to be more or less parallel. Putting the flash head too close will cause the reflected beam to spread, and putting it too far away will cause it to focus at a point closer in.

What I have been finding, is that with one umbrella I’ve been able to find a sweet spot which seems to give even illumination over the entire umbrella. With the others, I can’t find such a ”sweet spot” and the light pattern seen shows hot spots that vary from umbrella to umbrella. Since I’m not the first hobbyist dealing with this, I’m sure someone has already figured out a solution, and that’s what I am looking for.

Probably one recommendation is to get a set of proper lightboxes / softboxes, and perhaps that is where I will need to go if I can’t get the results I want, but I’d like to see if I can get good results with the equipment I already have.

So the question is, is there any procedure or formula that allow me to determine the distance between the flash head and the umbrella, the amount of “off-axis” placement, and the setting of the flash head zoom to get even light?
 
The old square peg in a round hole issue.

The umbrella is round. The flash head is rectangular.

You might investigate as a solution a device that is part umbrella and part softbox: PBL Photo Studio 40" Reflective Umbrella Softbox Steve Kaeser Photographic Lighting

Thank you, Keith. I have two of those. Part of my frustration is that when I use them, the light spread over the diffuser fabric is not even - the "effective" diameter of the lit surface is much small than the diameter of the umbrella/diffuser material. So I've been playing with the positioning of the light. There are three variables:
1) distance of the flash head to the umbrella.
2) Distance of the flash head off the axis of the umbrella,
3) degree of spread of light at the flash-head (ie, is it set to give 14mm equiv. coverage or 105mm equiv. coverage or something in between).

If the head is at the plane of the diffuser material, and the flash is held in place using a standard bracket (about 6 inches off-axis) then only the central portion of the umbrella is lit and the flash head zoom is set to 14mm equivalent, and therefore the light pattern on the diffuser is about half the effective diameter of the umbrella/diffuser, with the pattern somewhat brighter on the lower side.

If the head is left at the same distance from the back of the umbrella, but now positioned closer to the central axis, the pattern of light is about the same diameter, but is now more even, which is what I would expect when having the light source closer to the axis.

If the head zoom is changed to (say) 50mm equivalent, then the circle of illumination in the umbrella becomes very small, and the effective light source as projected on the diffuser material is also much smaller.

Finally, if I put the head about 6 inches outside the plane of the diffuser material, on axis, with the zoom head set at 14mm equivalent, then the circle of illuminated diffuser material becomes larger, but does not still create even light across the umbrella/diffuser.

So I am scratching my head and asking myself if I'm trying to do with this equipment what cannot be done, or perhaps there is something that I am missing...
 
I'm wondering if you have stumbled upon the only usefull thing a Fong Dong can do. Use it with un umbrella and flash?
 
Hi Christina. Yeah, I had that idea as well... Tried it, didn't get any better results that using the flash head in its widest configuration (ie 14mm zoom equivalent).

I'm putting together a set of images to show what I have found to date. will post them in about 1/2 hour.
 
Ok, so here's the results of some of the experiments.

Umbrella1.jpg


In this one, the umbrella is a 32" diameter simple white reflector. The flash was head was positioned so that it was at the plane of the opening of the umbrella, and the flash zoom was set to 24mm. As Keith has noted, the light pattern is more or less rectangular, and at the distance of the head from the umbrella, only a portion is lit.

The next image has the zoom head wide-angle diffuser flipped out, giving the equivalent of 14mm zoom coverage. Better, but still not every even.

The final image in this set has the umbrella moved to the maximum distance from the zoom. Now the umbrella is more or less evenly lit.

Conclusion: you have to match the flash head position, zoom angle, with the umbrella to get the best "even" light.

Umbrella2.jpg


This sequence explores the use of an umbrella with a diffuser material over the front (the portable soft-box). The conclusion at the end of this sequence is that the closer the flash-head is to the axis of the umbrella, the more even the light will be, and the wider the dispersion at the flash head, the more even the resulting light circle will be. However, with this design of umbrella, there is a limit to how far away you can position the flash head without pulling the diffuser fabric out of position or shape.

Therefore, if I want to really get a wide surface of light, I will have to think about real softboxes.
 
Well, you're kind of fighting a losing battle: the huge,huge majority of umbrellas will have a central, hotter spot, and then a fall-off at the edges...that is the way umbrellas tend to work.

A typical studio flash head with its umbrella reflector fitted delivers a 110 degree beam spread, which allows the flash to "fill the bowl" of the umbrella from the relatively close distance imposed by the umbrella's shaft. Speedotron makes 110 degree "flood" adapters to convert their M90 lights from 55 degree beam spread to the WIDE beam needed to fill an umbrella. SO, one begins with a round reflector, and a round flashtube, and widens the beam to fill the umbrella.

With a speedlight, one begins with a rectangular beam that is QUITE,QUITE narrow and restricted at 24 to 30 inches,and so...the bowl of the umbrella simply can NOT be filled--especially with larger umbrellas. I honestly think that speedlights work best with 32 inch, reflecting umbrellas...I do not honestly think that speedlights work well with 40,42,46 or larger umbrellas...they just are NOT the right tools. My favorite umbrellas for speedlights are Photoflex white 32-inch models.

Still--I do not expect an umbrella to be "even"...I expect a hot, central area of light, with fall-off at the edges.
 
I am thinking you are right, Derrel. I'm going to continue to experiment and see if the light quality can be improved. I taped a white cupcake holder to the flash and tried that inside the umbrella/diffuser, and while the light level was lower, it was more diffused, and the diffuser had a larger amount of surface lit and more evenly. But based on your comments, I'm extracting just about the maximum amount of diffusion from my setup.

@ Chistina: you have kinda steered me to the cupcake "solution" when you mentioned the Fong diffuser. Mine has a clear body and translucent cap. If it was the fully cloudy model, it could have worked better. And that turned me to thinking about what else I could use, which led me to the cupcake holder... which seems to work.

@ Keith: that book you recommended to me (Light: Science and Magic) continues to work on me. It has given me much in terms of analyzing how light is bouncing around, and the relative contributions of diffusion, direct reflection, and the interaction with the "family of angles".
 

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