Search KnowledgeBase

 

Main
Minimize Text   Default    Enlarge Text
 

Flat ports? Dome ports? Diopters? What does it all mean?


Print
Save to list
Subscribe
Created 2006-09-29
Modified 2006-09-29
Views 6141
Author Richard Perry
Let's try to clear the waters a bit.

You most likely learned in your open water scuba course that objects underwater appear about 25% larger than they do in air. This is the result of the refraction of light rays as they pass thru the flat plane of your dive mask which forms a barrier between the water and the air trapped inside your mask.

The same rules apply to a camera lens behind the flat port of a housing. Light beams are travelling from water into air, just like your dive mask. For macro photography, this is a good thing. You gain free (as opposed to paying for a more powerful lens) magnification of your subject.

But what about wide angle photography? For wide angle, we're trying to capture a wider area. We want the subject to appear smaller to encompass more area in the image. This is where a dome port comes to the rescue. A dome port, when used under water, creates a "virtual image", which is what the camera records. This virtual image is located close to the  camera, usually at a distance of about twice the diameter of the dome from the sensor (or film plane). Using this formula, a 6inch dome will have a virtual image 12 inches from the recording media, and a nine-inch dome at about 18 inches.  If your camera/lens combination cannot focus on the virtual image, you're in trouble. Out of focus images are the one thing that can't be fixed in Photoshop.

This is where diopters come into the picture. A diopter when added to a lens, decreases the working distance of a that lens, meaning that the lens now can focus on objects closer to the camera. It also generally means that the lens can no longer focus on distant objects, but for this application that is not a concern, as our only concern when using the dome port is focusing on the virtual image.

That's all there is to it. The rule of thumb is flat ports for macro lenses - dome ports for wide angle and general use zoom lenses.




Related Articles
How much magnification will the INON UCL-165 lens yield?
The INON UCL-165AD close-up lens (or it's elder sibling, the INON UCL-165 M67 )is an excellent option for extreme macro photography using compact digital cameras. It...
Subal Port Mount Identification
As SLR camera manufacturers trend toward larger and larger diameter lenses housing manufacturers have been required to address the issue of fitting ports over these...
I'm adding a strobe to my rig. Which arms should I get?
The default configuration in our Deluxe Lighting Packages (3" + 5") is by far the most popular, particularly for those just starting out with strobe rigs. It is...
 

[ + ]
 

Powered by: SupportTrio