DCL-20 focal length & size of field-of-view?

Hi, my name's Miles Hoskin. I'm a marine ecological consultant in Cornwall, UK.

I've recently purchased a Canon G5/Ikelite housing outfit with a Epoque DCL-20 wide-angle lense. I'll be using this outfit for underwater quantitative surveys of various benthic organisms (things living on the seabed). Surveys involve counting the abundances or %-cover of different animals or plants within 'photo-quadrats' of known dimensions. For instance the density of barnacles or the %-cover of sponges on a rock-face.

To make sure all the photo-quadrats are of the same dimensions, I need to mount the camera on a framing device that places the lense a fixed distance from the rock-face to give a pre-determined size of field-of-view. I may need to use different sized photo-quadrats for different types of survey work - anything from say 20 cm width to 1 m.

To build this framing device, I need to know the relationship between the distance from lense-face to subject and the dimensions of the area of seabed captured in the image (when using the Epoque DCL-20). Obviously I could work this out by practical experimentation, but before I do, I was wondering whether anyone out there already had this information or could point me to someone that does?

Cheers,

Miles

Hi Miles and welcome to Wetpixel.

I think the values you are looking for will vary depending on the EXACT camera/housing used. For example, that lens may be used by Olympus shooters, with stock Olympus housings, however, the native camera lens will be different, and the add-on optic may be positioned differently with respect to the front of the camera's native lens. (hope that didn't sound too confusing)

So with that said, your best bet would be to rig up a "tee bar" of yard sticks in a similar configuration to your survey quadrant and take some test shots (must be in water).

I know a few other academics here at Wetpixel who also use digital cameras for surveys and they have a lot of good things to say about it.

Good luck,

James Wiseman