Shooting CFWA with the mini dome
A mini dome is a small dome with a spherical diameter of 4 or 5 inch. CFWA stands for Close Focus Wide angle. This is an UW photography technique that combines two advantages of the mini dome. First, the possibility to get very close to a small target object like a small fish, anemone, or soft coral. The distance to the front of the dome can be 10 cm or even less. Only a fish eye lense is capable to focus so close. Secondly, a very wide angle of view that enables awesome and sharp images of the background environment, like a boat or a diver(s) near the surface in blue water (see the inset picture as an example). See also the strobe positions section: its important to pull back your strobes close to the housing and behind the plane of the dome to avoid hot spots showing up in the corners of your image.
CFWA works nicely on a Olympus 4/3 camera (like the EPL5) with the Panasonic F 3.5 8mm fish eye lense and 4 inch mini dome. I use a half sphere Precision acrylic dome that is specifically designed for the Panasonic 8mm F3.5 in a PEN housing. But I found that it also gives nice results with the new Olympus 8mm F 1.8 and a Zen 25 mm extension ring, on the same camera/housing combo. Zen now also sells a 4 inch dome wth the extension already built in for the same lense. CFWA works also very well with a mini dome (or the slightly larger 5 inch dome) and the Tokina 10-17 fish eye lense on a DX (or APS-C) camera.
see for 4 inch domes designed for 8mm FE:
Some more points:
-A dome port (a mini dome in particular) should de aligned with nodal point of your lense. That is: the centre of the base of the dome hemisphere. The nodal point of the lense is normally located in the more frontal segment of the lense, and is also called entrance pupil or zero parallax point. The designer of the mini dome has usually taken these meaures for you, using your FE lense as a reference. So the mini dome is customized for your specific lense. There are also ways to estimate the location of the nodal point yourself. See:
-The strong curvature of mini domes may present corner sharpness problems, which can be minimized by having a) a dome that is correctly aligned with your fishy eye’s nodal point, b) a smaller aperture and c) blue water in the corners. This is because most lenses (even a fish-eye) are made for flat planes, and in the virtual image of the mini dome details in the corners are much closer to the lense than those in the center.
-The virtual image of a mini dome for an object at infinity lies at a distance of 3 times the radius of the sphere to the front of the dome. That is at 15 cm for a 4 inch dome and 18 cm for a 5 inch dome. This means that with the fish eye lense larger objects like a wreck, a large fish or mammal at greater distances will also be in focus and fill your frame. For a wreck natural light and a filter seems the best shooting option. With a shark or manatee, a distance of 1-2 meters should still give a nice frame filling picture.
-With a fish eye lense and mini dome a close up shot of a large fish (like the head of a hammerhead) will show perspective distortion, for example at 50 cm distance the head will look enormous. see for example: https://www.flickr.com/photos/fttpwwwflickrcomalbert/26608855536/in/dateposted-public/ The positive side is that details of the head like eyes, teeth, pores etc. will show up razor sharp. At a larger distance of 1 m distortion will be much less. If you crop that picture in Photoshop, the details will still look fine although somewhat less 'crip' than at the picture taken at 50 cm distance (given that visibility is good). If you posses a fish zoom lense (e.g. Tokina 10-17 on a DX camera) zooming in at the longer end of the lense (e.g. 15mm in stead of 10 mm) will give approximately give the same result. See: https://www.flickr.com/photos/fttpwwwflickrcomalbert/26660962605/in/dateposted-public/
see also for more details: CFWA article by Alex Mustard