Need help with Aqualens calibration

[radarguy]

hey guys,
Anyone an expert on the Aqualens? I just got mine from B&H. They are losing out their Aqualens stock if anyone is interested. Tell Steve that Russ Thornton sent you. He'll appreciate it.
Anyway to my question while I wait on my gears.
I am a fairly advanced amature above water photographer but this one has me confused. Can anyone explain to me better the creation of the 12" focus setting? It says in the instructions that this now represents infinity. I need some help here.
Thanks,
Russ

 

[blacknet]

Hello,

I wrote an article describing this, you can find it in the tip/techniques section or on my web site.

Ed

 

[DesertEagle]

I think that the 12" represents an aparent image, which would be double the 6" port. From my own experience, I can tell you this: take the rig into a swimming pool and test it! When I purchased my aqualens I didn't have time to test the setup before my trip to the Red Sea. I used a Vivitar 19mm f3.8 lens and thought I followed the directions. Most of my photos were soft, meaning that the lens was focused at a point further out than the dome port. Now I use the same lens and dome port in an Aquatica 80 housing for my Nikon F801. I have found that most of my underwater photos are taken with the lens set to a very close focusing distance- and now they are sharp. Using wide angle, I usually get 60cm or closer to whatever I am photographing.

If you use a 16mm fisheye, you should have very little problem focusing. But if you use a 20mm, check very carefuly.

Good luck!

 

[blacknet]

Hello,

This is a clip from my dome port article, to clear up some confusion.

Ed

Dome ports:
Dome ports correct the problems we see with a flat port but bring on their unique problems, in most cases they are easy to work around. Dome ports will add an additional element to your lens and change the focal distance, the object shape and the object size; this is referred to as the 'virtual image' or the secondary focal point.

The distance, size and shape of this virtual image are determined by several things; the magnification ratio and index of refraction of the dome port material, water and air.

We can use a simplified formula that states

AF" = 3R - 4t

Where R = (r1 + r2)/2 and the dome thickness t = r1 - r2

This is out viewable 'infinity' point. Our lens will need to have a close focus of AF inches from the front of the dome or less to take good sharp photos. However, many lenses can not focus this close, we must add a diopter to the lens and this will change the focal length and the minimum focus distance.

The downside to dome ports is the curved field of focus. If we look at the hyperfocal distance we learn as the f-stop changes so does the depth of field. If the depth of field is very shallow our curved focus point will begin to loose focus at the edges. If we stop down to count for this curvature our demand for light increases so we have a trade off, sharpness around the edges or improper exposure. The lesser of the two evils, in most cases, would be the edge sharpness. Coincidently this is another reason to not attempt to shoot full frame images. NOTE see my other article on composition and the cropping