James is in the ball park but I don't find this a very compelling argument for 35 mm sensors over smaller ones.
To me what is more relevant in U/W photography would be shooting at a base ISO, shutter speed and F-stop to compare results.
First let me say that I think the Nikon D3 and D300 are both outstanding cameras which can produce great results.
The D3 with a 14 mm lens has an angle of view of 114 degrees.
At ISO 100, f/5.6 & 1/60th for a proper exposure at 2.5 feet, DOF is from 1.52 to 7.1 feet for a total of 5.58 feet
The D300 with a 10.5 lens (non-fisheye, if one was made) would have an AOV of 114 degrees and at ISO100, F/5.6 & 1/60th at 2.5 feet would have a DOF of 1.09 to 10.0 ft. or a total DOF of 9.51 ft.
The Olympus E-3 with the 7 to 14 zoom at 7 mm has an AOV of 114 degrees and at ISO 100, F/5.6 & 1/60th at 2.5 feet would have a DOF of 1.08 ft. to infinity.
With macro ISO 100 at 1/250th and F/22 is common at say ten inches.
D3 with a 100 mm macro lens AOV 24 degrees (105 mm 23 degrees) DOF 9.9 to 10.1 total 0.21 inches.
D300 with 75 mm macro (if one was made) AOV 24 degrees, DOF 9.86 to 10.1 total DOF 0.29 inches.
E-3 with 50 mm marco AOV 24 degrees, DOF 9.73 to 10.3 total DOF 0.55 inches.
For macro keep in mind that at life size, 1:1 the full size image would be 36 mm X 24 mm for the D3
1:1 for the D300 is an image/subject 23.6 x 15.8 mm and for the E-3 the 1:1/subject size would be 18 X 13.5 mm.
Note, DOF Master was used for these results and compared to other DOF calculators, http://www.dofmaster.com/dofjs.html
If I wanted very shallow DOF for underwater macro and W/A or to shoot at ISO 400 rather than 100 the larger sensor would make more sense to me. But greater DOF is more often than not a good thing for underwater photographers and then their is still the issue of soft corners that show-up the larger the sensor gets.
Images #1 life size for 35 sensor, #2 life size for 4:3 sensor.
Edited by tropical1, 04 May 2008 - 03:44 PM.