I go into some detail below, so for those who don’t want to get too techie, the take home message is mini-domes are best for CFWA and WAM. The most suitable lenses are close focusing fisheye lenses (e.g. Tokina 10-17mm, Sigma 15mm), rectilinear wide angle zooms will not work well. A mini-dome and fisheye can be used for more standard wide angle, but the lens needs to be stopped down to at least F13 (approx) for decent corner sharpness with DX/APS-C cameras (more on FX/FF). They are unsuitable for split-level images. You can skip to the concluding comments if you want now!
The main negatives associated with mini-domes relate to how the optics affects the virtual image. Every dome port acts as a negative lens underwater, creating a virtual image of the subject (much closer to the camera than the subject really is) that the lens must focus on. A small, highly curved dome will produce a virtual image that is closer to the camera and more steeply curved than a large dome.
Optical theory tells how close the virtual image will be for a hemispherical dome. Underwater, the virtual image of a subject at infinity will be 4 times the (spherical) radius of the dome (from the centre of the dome), in other words 3 times the radius from the front of the dome.
So for a hemispherical mini-dome, with a diameter of 100mm (4”) and a radius of 50mm (2”), a subject at infinity will be seen as a virtual image (distance = 3 x radius) just 150mm (6”) from the dome. Of course, our underwater subjects are always closer than infinity, so will require the lens to focus closer still. Standard sized fisheye domes will produce a virtual image that is between 350mm and 450mm from the dome (14”-18”).
So mini-domes require lenses that can focus close. Most modern fisheyes fit the bill perfectly, but the Nikon 16mm (that I use on my FX sensor D700), for example, does not focus close enough to be useful with a mini-dome. It provides a valuable lesson about the restrictions of a mini-dome (see below). The Nikon 16mm has a stated minimum focus (note this is sensor to subject) of 250mm, compared with 140mm for a Tokina 10-17mm or Nikon 10.5mm. This translates to about 110mm from the front of the lens for the 16mm. Most rectilinear zoom lenses from Nikon and Canon have minimum focus distances between 280mm and 300mm, which is one limitation of using them with small domes.
However, things are very different with the right lens. The Tokina 10-17mm, which can focus very close achieves much larger subject magnification with the mini-dome. I was not able to get any closer to the rubber duck with the standard dome because the duck (on the left, below) was already pressed against the glass. The mini-dome does not stick out as far, I can therefore get the lens closer to the subject and achieve a larger magnification (right).
The other optical consequence of a mini-dome is that the virtual image is steeply curved. Lenses are built to focus on a flat plane and when we focus in the centre of the virtual image the corners are closer to the lens and therefore out of focus. The closer to the camera and the more curved the virtual image (the smaller and more curved the dome) the more of a problem this is. At any given aperture, the corners of our image will be less sharp with a mini-dome than with a large FE dome. Rectilinear wide angle zooms often struggle for corner sharpness behind standard domes, a problem made a lot worse with a mini-dome.
We can overcome corner sharpness by increasing depth of field, stopping the lens down (when possible). Using an APS-C or DX camera will also give us more depth of field for the same angle of view and aperture than on FF/FX, which will also improve the corners. We can also help matters by making sure we have open water in all four corners of the frame.
The example below compares corner sharpness at two apertures (2 stops apart) taken with 10-17mm @ 11mm on DX camera using the Zen mini-dome (since all the mini-domes are the same shape you can expect close to identical performance on this issue from other brands). Note that both look fine at screen resolution, but at 100% the difference in the corners is clear. Also note, that if you mainly show your images projected or online this shortcoming of the mini-dome will be less important to you.
I also did pool tests, where I was able to compare the mini and stand domes. From these I concluded that the corner sharpness difference between a big dome and small dome is about 1.5-2 stops (i.e. for the same corner sharpness as f/8 on a large dome, you need to be at f/14-f/16 on the small dome). I have deleted these files so I can’t show them. But Ryan Canon said on the Wetpixel forums that they found the same in their tests of the Zen mini-dome against the standard Zen dome.
An obvious limitation of the small size of a mini-dome is that it is unsuitable for split-level images, because the meniscus of the water takes up a much larger proportion of the frame and is harder to control.
Finally, mini-domes, because the dome is so close to the lens, can cause problems housing a variety of lenses. Fisheyes, in particular, with their wide view can be troublesome and the dome really needs to be set up for one particular lens. If the lens is too far forward the hood/shade will hit the inside of the glass. If the lens is too far back it will see the sides of the port/shade. The difference is very small and too small to be solves with port extension rings.
For a real world example, the Zen mini-dome I tested was setup for the Tokina 10-17mm on a DX Nikon in a Subal housing, and worked perfectly. However, when I tried to use this port with a Nikon 16mm and Sigma 15mm on a FX Nikon in a Subal housing I had problems. The Nikon 16mm, a slightly shorter lens than the Tokina 10-17mm would see the shade of the port, causing cut off in the image (see first duck images). (This should not be called vignetting, BTW, which is an optical effect of a lens causing darker corners of the frame. This is just blockage or cut off, no need for fancy words that mean something else!). The Sigma 15mm was even more of a problem. Its longer hood meant that it could only fit in the port focused on infinity, when it tried to focus closer its longer hood hit the glass. While as the same time also suffering cut off from the port shade. The Sigma 15mm requires either some of its shade or that of the port being machined away to work.
So, should you get one?
If you are a keen underwater photographer, who already has a large dome port then a mini-dome should be high on your shopping list. Like the best accessories it will expand your portfolio allowing you to take photos that you cannot at present (such as WAM). The case will be even stronger if you shoot with an APS-C or DX camera. A mini-dome will also provide a light-weight travel alternative to your main dome, such as when travelling to macro focused destinations such as Lembeh.
If you are new to SLRs and looking for a cheaper alternative to a standard sized fisheye dome then the mini-dome does fit the bill. With a very close focusing fisheye lens (such as the Tokina 10-17mm), it will allow you to take a wide range of images from WAM to CFWA to standard wide angle. Just remember to keep the lens stopped down whenever possible. A mini-dome will also lessen the inevitable baggage expansion that comes hand in hand with the upgrade to SLR.
But there are a few warnings. First, as discussed above, the mini-dome has some optical shortfalls. If it is your only dome you will not be able to shoot quality split level images and you will struggle to get good results from lenses other than your fisheye. If you already own or consider buying a rectilinear wide angle zoom, then you will be buying a standard sized dome in the future too. Buying both and mini-dome and standard dome is always more expensive than just buying a standard dome in the first place!
Many of my most popular images were produced using mini-domes and I will always remain a big fan. I consider it an essential piece of kit. Just less essential than a standard sized dome.
Alex Mustard – March 2010.
Many thanks to Adam Hanlon for lending me his Zen 100 port and for help and support from Richard and Ryan and Reef Photo and Video.