The 16mm can only be used with the fish-eye port. The 20mm can be used with both the
fish-eye port and the standard wide angle port, but the waterline on the standard port is
too small and this is difficult to use for split shots unless you are in a pond and there is not
a ripple on the water.
With the 16mm I keep the focus at 30cm and at smaller apertures everything from a few
cm to infinity remains in focus.  The main problem is balancing the exposure.
The u\w portion of the image is often underexposed compared to the surface portion.
Exposures are more even when the bottom is quite sandy and bright.
The 16mm does not accept a front filter element due to the extreme angle of view of this lens.
I am experimenting with a small semi-circular section of plastic neutral-density filter which I
attach to the small bayonet filter behind the 16mm. I have the manual focus 16mm.
The AF version I believe has a different filter arrangement
This split filter will darken the topside portion of the image.

I am also experimenting with the 20mm lens in the FE port, but this lens requires a special split
dioptre lens. My optician made up two of these lenses for me using some 72mm 4X and 2X
Vivitar neutral density filters. He used a larger diameter filter with 72-62 step down ring, to avoid
vignetting on the 20mm. He cut half of the neutral density section away and added a half
section of +4 dioptre lens. This allows the 20mm to focus correctly on the u\w portion of the
image, and it cuts the surface exposure by one or two stops [depending which density of filter
is used] thus balancing out the exposure. One drawback is that you are limited by the orientation
of the filter in composing either portrait or landscape.  I have yet to try these filters but if anyone
needs any more info on how to make them you are welcome to email .
They cost 1\10 of the price of the ones commercially available.
 

back to underwater photographic equipment