THOUGHTS ON MINI DOMES
Alex Mustard

  A mini-dome is small dome port usually around 100mm or 4” in diameter and typically hemispherical in shape. They offer significant advantages to the underwater photographer and I have long considered them an essential piece of kit for certain images. They also have significant drawbacks compared with standard sized domes (200-220mm, 8”-9”). It is important to see both sides of the argument to understand if you should buy one, when you should use it, and how to produce images that you cannot make with a standard dome.

  This is not a review of any particular brand of mini-dome (although I used the new Zen 100 dome for the test shots). Instead this is a discussion of the advantages and disadvantages that come with using a dome port of this size and shape (irrespective of the quality of construction, glass etc).

  Mini-domes have always had a small, but loyal following in underwater photography. Several of my friends in BSoUP have been using them almost exclusively for the last 20 years or more. Which is how I was introduced to them. Up to now, most have been self-made and Barry Guimbellot recently shared some excellent tips on how to do it.

  However, that has changed. Right now, commercial mini-domes seem like London buses. You wait ages for one to turn up and then three come along at once. They are now available in a range of flavours from Aquatica to Zen!

  Mini domes offer some significant advantages. They are smaller, lighter and (usually) cheaper than standard domes. All music the ears of almost every underwater photographer!

  Photographically their real advantage is that their small size makes it much easier to position (small) strobes close to the port. This greatly improves the quality of lighting we can achieve in true close focus wide angle (CFWA) and wide angle macro (WAM) shots. However, this important advantage only becomes significant when camera to subject distance is less than about 100mm or 4”. Further away it is not significant.

  The other advantage for this type of photography is that the small physical size of the port allows us get the lens physically closer to the subject, therefore making it even larger in the frame. For example, I took the photo of the sea urchin (below), which was smaller than half a tennis ball, with the Tokina 10-17mm on its widest setting at 10mm, yet I was able to fill the frame with it. However, this advantage is only realised with a very close focusing lens (like the Tokina 10-17mm or Nikon 10.5mm). Most wide angle lenses will actually give you a large subject magnification with a standard dome than a mini-dome (this is quite a surprise to many people, see examples on page 2).

  Small domes can also be used for more typical wide angle photography, but will not perform as well as a standard sized dome. But before we get into the optics we should heed a historical lesson: all the early dome ports were mini-domes. Big domes were too expensive and difficult to make. And they weren’t called mini-domes back then, simply domed portholes (port is an abbreviation of porthole, by the way) as they were the only size available. The innovation photographers wanted back then was actually to make them bigger, to overcome some of their optical issues. Photographers getting over-excited about mini-domes as the latest invention should take note!

Continued on page 2…