Sunday, April 1, 2018

Globe Flash V2

Before getting into technical details, let me show three photos. All three are with the same camera and lens but were taken using different flashes:

The first uses a Sigma Ring Flash:

The second uses a Sony Twin Flash:

The third uses my latest DIY globe flash:

I chose ants because they move so fast they are really difficult to photograph without a flash you typically end up with a blurry mess. All three flashes do a reasonable job of stopping motion and at least making clear it is an ant. There is a real quality improvement from better diffusion though.

The Sigma flash always had a real problem with too little diffusion. Odd hot spots and dark spots really retract from the quality. It calls itself a ring flash, but due to lack of diffusion it is more of a twin flash with two fixed light sources.

The Sony flash was a solid flash, for $750 it better be! Still, even it ended up with some hot spots. I always wanted more diffusion when I worked with it.

The globe flash doesn't quite eliminate all specular reflections, some images still have hot spots when using it, but they are a much reduced level than the other two flashes. Typically they show up as a bright area rather than a totally over-exposed spot. Compared to any previous flash I have used this is great performance.

It isn't entirely a fair comparison since the photos are ordered chronologically and were taken a few months apart so I learned as I went. Also, the dedicated macro flashes give more control over shadows. Still, the DIY globe flash I am using is almost half the price of the Sigma flash, and and a quarter the price of the Sony flash.

The previous globe flash I used almost had this level of performance, but I found two somewhat annoying issues. Too little light got in meaning the flash took a long time to recharge and it had oddly shaped reflections on shiny surfaces. You can see the diffuser shaped reflections on the ants in the photo below:

So I upgraded from a 6" globe to an 8" globe. I chose 8" because that puts a subject at minimum focus distance right at the center of the light source. I obsessively keep subjects at minimum focus distance so I can add scale bars.

Going to an 8" globe created problems attaching the globe to the camera. The 8" globes do not come with smaller than 4" holes. That is larger than any cheap adapter I saw. So I had to purchase an acrylic disc to use to connect the step up ring to the globe.

The list of materials and tools for the project were:
First I used the dremel cut a hole in the acrylic disc matching the opening on the lens, then I cut off the flange attached to the globe and epoxied the step up ring, acrylic disc, and globe together. Once the epoxy dried I used the dremel to cut the globe into the shape I thought would work best:

As you can see the new flash diffuser is significantly larger than the old one. Just like last time, too little light makes it from the flash to the diffuser. This was easily solved by putting a paper towel above the two. In an attempt to ruggedize it, I covered the paper with packing tape.

If I used a 12" globe I could probably avoid the paper towel, and I will probably ultimately find a better way to trap the light than the paper towel. It does seem effective though.

This has been very successful. Unlike the old version I can fire off photos rapidly using this version. Also, while reflections aren't entirely gone, the smaller black area on the central hole and larger globe make the reflections less obviously the shape of my diffuser.

While the diffuser produces very good images, I have found some downsides:
  1. The shape makes it difficult to image insects in a tight space. 
  2. You look like a crazy person when you walk down the street carrying this camera. Honestly this is a problem with all macro flashes though.
  3. Diffusion could still be improved! This seems the best trade off of usefulness and image quality but I still often find myself wanting more diffusion. The only viable way I see to get more diffusion without making the system hopelessly bulky would be to use two small flashes, one on each side of the globe. 
  4. The paper towel is a pain to take on and off. Maybe a little aluminum foil hat for the flash? Maybe a white cloth with velcro?

This could be adapted to just about any macro lens. The only big change would be to use a different size step up ring to connect to whatever lens you want. I suspect it might even work on a superzoom camera with the on camera flash, but I have not tried.

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