Examples of three types of ' ideal' domes aligned with the nodal point (blue dot: The term nodal point defines the optical centres of a lens where the incoming light is bundled in the optical axis.The first (front) nodal point is the position on the optical axis of a lens where the entering ray crosses the optical axis of the lense on the camera). A: mini half-sphere dome, B: larger dome with segment cut off the hemisphere and extended port (in red), C larger hemispheric dome ('super dome'). Thick black line: back of the dome. B; thin vertical line, back of hemisphere from which the dome was cut. Most large domes are 'multifunctional'; they will fit different wide angle lenses, which means that alignment with the nodal point is not perfect. But this is considered not a big problem, given the greater distance of the virtual image and smaller curvature of the domes.
Light behaves differently when it passes from water into air. Due to refraction, flat ports magnify the apparent image and reduce the angle of coverage of a lens. This makes this port ideal when using standard, macro or zoom lenses to shoot small subjects.
A dome port comes into play when you use a wide angle or fish eye lense Here the correction for refraction or other aberrations of the lense is a must.
Much has been said already on the merits and types of dome lenses in UW photography, which
I shall not reiterate here. To keep things simple, I show here three popular types of domes. The mini dome, usually a hemisphere cut out of a small sphere of 4 inch (100 mm) diameter, and the larger domes (normally 8 inch, 200
mm or even 9 inch, 230 mm). The mini dome is mostly used in combination with a fish-eye (FE) lense, that can focus on objects very close to the front of the dome (virtual image around 15 cm at infinity, see A). The larger domes have a virtual
image that is further away from the front the dome (around 30 cm at infinity for the 8 inch dome) and are often used in combination of with rectilineair lenses. Most of the larger domes only use a segment of the hemisphere (figure B),
a few might use a full (or almost full) hemisphere (C) which makes them rather bulky. In both cases, the virtual mage (and minimal focusing distance) remains the same. Figure B also shows how a port extension can be used to achieve a better alignment
of the dome (i.e. the base of its virtual hemisphere) with the lense. Here, the radius of the hemisphere is always larger than the actual distance from the front to the back of the dome segment. For example, in a 200 mm dome the radius is 10 cm but
the dome segment can only be 3 cm deep. Corner unsharpness can be a greater problem for the strongly curved mini domes than for the larger domes with less curvature. On the other hand, the mini domes are better suited for CFWA (Close Focus, Wide
Angle) shots. Various tests have been run with different dome lenses mounted on cameras with a cropped sensor (like DX and 4/3 bodies) and full frame sensors (FX). Here are some 'take home messages' derived from these
-small sensors Mini domes (100 mm) work best with cropped sensors (1.5 X or 2.0 X cropped), and will profit from a good alignment with the nodal point of a FE lense. Somewhat larger 5 inch (125mm) domes,
although not strictly 'mini', will also work well with the DX format cameras (1.5 crop). Suitable FE enses for the DX/mini dome combo are the Tokina 10-17 mm and Nikon 10.5mm FE. And for the 4/3 (2.0 cropped) format the
Panasonic 8mm F 3.5 or the new and (longer) Olympus 8mm F 1.8 with a port extension (see .http://reefphoto.com/shop/index.php?main_page=product_info&products_id=6584
-large sensors. With full frame (uncropped) sensors the mini dome often show parts of the shades of the lens in the image (not to be confused with vignetting, which means darker or less saturated corners resulting
from a lense limitation). Moreover, your FE lense (probably a 16 or 15 mm FE) will likely not be able to focus on the virtual image (15 cm or less) of the mini dome. So, forget (really close) CFWA with the full frame format.
(DOF), focal length and format *). Lenses with a shorth focal length are wider and have a larger DOF that lenses with a long focal length. In addition, cropped sensor cameras use shorter focal length lenses, with more depth
of field for the same angle of coverage. So an 8mm FE lense (180 X 180 deg corner to corner) on a 4/3 camera will have the same size of field as a 15mm FE on a full frame camara. But the DOF will be larger for the 8mm/cropped
sensor format than the 15 inch uncropped format.
-Split level images *) ar normally taken with the larger domes. Notice also that there is no virtual image of the dome of objects above water. So its important
to focus always on the closest subject underwater. With a smaller aperture the large DOF will ensure to get the above water view in focus.
-FE and rectilineair wide angle lenses (like the Canon 16-35 lense) mounted on full frame cameras need much larger domes
to get sharp corners and an undistorted image. That is: 200 mm or 230 mm domes. The smallest acceptable size should be around 170 mm. But some companies like http://uwcamerastuff.com/precision_5_dome.htm
make 5 inch Precision domes adjusted with the nodal point of FE lenses like the Sigma 15 mm and Nikkor 16mm, meant for full frame cameras. Rectilinear lenses will benefit from a close-up lens also called a diopter, like a +2 diopter to
focus on that virtual image (30-20 cm or less). But its all relative: when shooting in environments like blue water or dark corners (like in a cave with light in the center) unsharp corners usually dont give much of a problem.
fish eye on a full frame camera? So, a full frame format camera will profit most from a large dome and a fish eye lense like the Sigma 15mm made for large sensors. But some DX lenses like the Tokina 10-17mm will also
give good results on a full frame camera, given that you don't use the very short end (10-12mm) and a 'shaven' Tokina without lenshood. See these three links for more discussions and information.