Underwater Ports: Size Does Matter

I've decided that I have a love/hate relationship with my compact Zen super wide angle dome port.  

First, why I love this port.

It's solid glass with a metal hood so it protects my Canon 8-15mm fisheye lens quite well.  In addition, it is super small and very easy to tuck into my always overweight check-in.

It allows the minimum distance between subject and lens with no fear of damage.  In other words, it isn't a large bubble dome which has the potential to bump into things, be bumped by things, get scratched or worse yet, crack if something hard gets too close too quickly.  

While the image below isn't my best of the trip, it illustrates just how close this whale shark fin was to my Zen dome port.  This big boy didn't bump or graze me; it was amazing that my port avoided impact and that this gentle giant was graceful enough to steer clear of both of us.  From the vantage point of this image, it looks like we would have gotten solidly whacked after the frame!

Also, if a bit of distortion is harmless to the composition as in the above and below examples, there are no worries.  Seems like the perfect solution right?  Not!

For those glorious lush full frame wall scenes, I want sharpness throughout the image including edges and corners.  And I want to avoid cropping in order to take advantage of the full frame sensor resolution with the entire composition that I see through the viewfinder.

Here are two coral reef scenes below.  Try clicking on them and look around the edges of the photos compared to dead center.  Can you see the blur and distortion compared to the crispness of the subjects in the center?

This is one disadvantage of having the port glass so close to the lens itself; there is an obvious loss of optic quality.  My full size dome port seems to have less edge blur than the Zen compact.

Another downside is related to over/under or split shots when the dome is raised partway above the surface so that the topside and underwater scenes are captured.  Here is an over/under using a full size dome port.

Since the diameter of the Zen port is so small (4 inches compared with 8+ inches for standard ports), it is much more challenging to get a clean water line across the middle of the image.  I would only attempt over/under shots with a Zen port if the seas are completely flat and I can firmly stand on sand or dead coral but as all divers know, these are not typical ocean conditions and situations!

So to sum up, the Zen compact is a great travel partner that handles close encounters and scenes where soft corners are acceptable but it can't compete with standard dome ports for edge-to-edge sharpness and split shot versatility.

Which one will I bring on my next dive trip?  I may need to flip a coin or bring both . . !