Essential factors when imaging through viewfinder with digital SLR housings:
No vignetting of finder image, even when viewing through a mask.
Size of finder image (bigger is better)
Brightness of finder image (brighter is better)
No vignetting of finder image, even when viewing through a mask. When using a housed camera underwater, often only the center part of the camera’s finder is visible through the housing and diver’s mask because the distance between the eye and camera’s viewfinder is longer than the camera’s “eyepoint”*.
Optical systems to extend eyepoint without changing size of finder image. "45 degree Viewfinder Unit” and “Straight Viewfinder Unit”
“Eyepoint”
The appropriate position of the eye to see the full circumference of the image through the camera’s viewfinder. Sometimes also used to refer to the distance between the eyepiece and eyepoint (= eye relief).
The Canon EOS 10D/20D/30D viewfinder eyepoint is 20mm (according to Canon specifications) and vignetting will increase when eye position becomes farther away than the eyepoint.
Size of finder image (bigger is better)
With a DSLR cameras having a smaller image sensor size compared to traditional 35mm film, the actual angle of view is appro ximately equivalent to x1.5 (Nikon DX format sensor size), or x1.6 (Canon APS-C format sensor size) of the focal length displayed on the lens.
The size of the finder image is also calculated the same way, and will be approximately x1/1.5 (Nikon DX format), or x1/1.6 (Canon APS-C format) considering “finder magnification”*.
Finder magnification
Ratio between size seen with naked eye and size through viewfinder. In DSLR camera specification charts, this ratio is described based on a 50mm lens at infinity, regardless of image sensor size.
This ratio does not reflect “finder image size” which changes depending on image sensor size or coverage of viewfinder.
Brightness of finder image (brighter is better)
Compared to shooting on land where there is more ambient light, a bright viewfinder is important underwater, especially for effective manual focusing.
Generally, the more optical elements used in the optical system, the dimmer the viewfinder. To compens ate, the Straight and 45 degree viewfinders use a wide entrance pupil diameter and total reflecting high refraction index prisms with anti-reflection coatings to maximize viewfinder brightness.