Underwater Photography: Are Glass Domes Better Than Acrylic Domes?

Underwater photographers seem to be using (and are being sold) glass domes more and more.  I've been stunned at the rising price, bulk, and weight of these glass domes.  I have shot behind acrylic domes for most of my 28-year career, and I've always preferred acrylic ports.  I've always thought that images shot using acrylic domes were just fine in terms of sharpness. 

Here are some pros and cons of glass domes versus acrylic domes:

Glass domes are heavy and expensive compared to acrylic domes. 

If a glass dome is scratched, then it's difficult, if not impossible, to get that scratch out.  Glass domes are more resistant to being scratched.  On the other hand, I've been able to polish out scratches in my acrylic domes for years.  In Bali once, I forgot my polishing kit, and I was able to polish out a huge scratch in a surf housing dome with progressively finer sands from different beaches. 

I've had glass domes over the years, and one of the earlier problems was etching of the glass.  There are ways to prevent this, but it is a hassle.  Once a glass port is etched, then there's no way to remove the etching, as far as I know.  I had this happen with a glass dome recently, even though using newer coatings on the glass has supposedly solved the problem. 

A recent glass dome port I bought came with a neoprene cover.  My glass dome got etched after two weeks of whale photography, where we were jumping in and out of the water constantly.  The boat crew put the wet neoprene cover on the dome (as they should have).  The coating on the glass dome became etched.  As it turns out, the worst thing you can do to a glass dome is put a wet neoprene cover on it for hours and hours.  With glass domes, you should rinse them with freshwater as soon as possible (hopefully immediately after coming up from a dive) and then dry them with a microfiber cloth. 

I had been told that the etching problem had been solved with new glass ports -- but this was not the case.  I therefore still don't trust glass domes.  You have to wash them with fresh water after every dive?  You should not use the neoprene cover with the domes in the field?  OK, now I know better, but they require a lot of care, are heavy, and a lot of the benefits being touted for glass domes don't hold water.  Excuse the pun.  My acrylic domes all work great, some after 20 years.  

One final reason some folks prefer glass is that there is supposedly less flare when using acrylic.  I have not been able to test this. 


Are glass domes  sharper than acrylic domes?  I decided to test this. 

For this test, I used a Zen 170mm glass dome for Nauticam mirrorless housings (zen-dp-170-n85), which retails for $999.  It weighs 2.5 lb. and the amount of glass across its face measured 7 inches. 

The Nauticam 7-14mm acrylic dome port for Nauticam mirrorless housings (36133) retails for $480.  It weighs 1.5 lb. and the amount of acrylic across its face measures 6 inches. 

To produce these comparison images, I used a single Panasonic GH-4 camera body in a Nauticam GH4 housing (a superb housing, by the way).  The housing was mounted on a tripod, and the camera was set to a 4:3 aspect ratio to use the full sensor area.  I took a series of shots with a Panasonic 7-14mm lens with a Nauticam acrylic dome port first, and then replaced the port with a Zen glass dome port.  When taking the housing off the tripod and back on again, it was jilted slightly, or not put on in exactly the same spot again... this was a mistake. 

All images in the comparisons were shot at 1/250th second, at f8, in manual mode.  I focused on my foot (about 4 feet away) before putting the housing on the tripod for each “swap”, and then changed the camera to manual focus. 

Here are the comparisons and explanation of the images in the folders:

Compare sharpness of a Panasonic 7-14mm lens behind an acrylic dome and a Zen glass dome.

Acrylic dome on the left; glass dome on the right for all images

Screenshot 1: comparison of angle of view:
The 7-14mm lens behind the acrylic dome is the photo on the left.  The same lens behind the Zen glass dome is on right. 
The image taken with the glass dome seems a bit wider.  The alignment of the images is off; I must have shifted the tripod a bit, or mounted the housing on the tripod shoe off a bit.  Forgive me.  It was cold and wintry in the pool, and I was using a tripod that was not firmly mounted.  Still, the image taken with the glass dome seems a bit wider. 

Screenshot 2:
Looking at corner sharpness: the image from the glass dome seems a bit better the corner, but it is hard to tell.  The only points of comparison are the leaves in the lower left of both images, and the leaves got shifted around.  Neither dome is that great in the corners in terms of sharpness.

Screenshots 3 and 4:
Comparison of images of Ikelite strobe about 3 feet away: Both images at various magnifications look similar in terms of sharpness. 

Screenshots 5 and 6:
Zooming into the pony bottle in the background: both images seem very similar.




Conclusion: There is not a significant difference in sharpness between acrylic and glass domes.