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Trevor Rees

Dome ports - glass versus acrylic

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I've started to look in a bit more detail at the difference between certain housings.

 

One noticable difference is that likes of Nexus, Subal, Seacam and quite a few others sell glass dome ports whilst Sea Sea and Ikelite sell acrylic ones.

 

It is no surprise that glass is heavier, scratches can't be easily removed and of course cost more. Acrylic on the other hand is lighter, cheaper and some sratches can be polished out.

Another thread mentioned that for split level shots glass may attract less water droplets.

 

Is there anything else worth knowing to prefer one over the other?

 

The big question I suppose is - if the port is correctly sized and matched to a given lens, is glass optically superior?

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Hi Trevor,

 

Glass domes are pretty hard to scratch. You basically have to scratch it with something harder than glass - like silica sand...or a diamond...

 

Acrylic, all it takes is something harder than plastic - like butter, or a rubber knife. But you can easily polish the scratches out which only takes a couple of hours.

 

As for optical differences, I think the jury is still out. All of the domes commonly used on underwater housings are very good and will yield excellent results with most lenses.

 

Cheers

James

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One noticable difference is that likes of Nexus, Subal, Seacam and quite a few others sell glass dome ports whilst Sea Sea and Ikelite sell acrylic ones.

 

Just to add a small point of clarification - Ikelite's flat ports are glass.

 

Too many topics to list - do a search on this forum.

 

a rubber knife.

 

110142.jpg

 

?

 

:)

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Is it glass versus acrylic post time already ? good god how time fly :)

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Scratch resistance is important for parts that require any frequent cleaning. After cleaning or buffing out scratches, most plastics will become hazy, decreasing total light transmission. But the degree of haze is not so perceptible to the human eye.

 

Besides haze and light transmission, low homogeneity and internal stress can cause striation and birefringence. The raw material and secondary processing quality (casting, thermal forming, blow molding, tempering etc.) are important. Optical glass specifications are pretty tight, and secondary processing is fairly standard, but acrylics and polycarbonates come in many grades, often with colorant additives to reduce inherent yellowness. These will decrease transmission and may cause haze. Also secondary processing can have a great effect on the optical quality of plastics. So manufacturers of ports with plastic optics need to be careful of maintaining a consistent quality spec.

 

Common multilayer AR coatings can decrease surface reflection to about 1-2% per surface. So coating the inside surface of the port optic will give you 1-2% more light. Most AR coatings are not durable enough to put on the outside.

Glass (BK-7) light transmission: 93-94%; with one side AR coat: 94-96%

Glass (B270) light transmission: 91-92%; with one side AR coat: 92-94%

Acrylic (PMMA) light transmission: 91-92%; with one side AR coat: 92-94%(*1)

Polycarbonate light transmission: 85-90%, with one side AR coat: 86-92%(*1)

(*1) In new, pristine condition. Transmission will decrease with cleaning due to haze. Softer plastic substrate means AR coating durability not as strong as glass.

 

The AR coatings applied to both sides of glass camera lenses brings the total light transmission to over 99%.

 

Glass is heavier, but the weight can offset the air space of large domes, improving underwater balance.

 

Once scratched, glass optics cannot be repaired, practically. Must be replaced.

Scratches on inside (air) surface of plastic optics will not be filled in by water and can degrade image.

 

Glass optics, especially larger domes are much more expensive than plastic.

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Glass optics, especially larger domes are much more expensive than plastic.

 

I think this is the key consideration here. Ike and Aquatica dome ports are made of Acyrilic. 1-2% difference in light transmission is not enough to make any difference in the real world. Do you measure the light transmission of the water in front of the port before you take a picture?

 

Durability may be an issue to consider. If you are careful with your port and use a lens shade and or port cover, you can largely prevent scratches. Small scratches will be filled on the outside by the water and it's pretty hard to scratch the inside (esp on Ikelite ports where the extension is glued to the port). The only cases I've heard of people scratching the inside were in trying an improper lens/port combination. But given the magnitude of the cost differential you could easially buy your ports three times over for the cost of one glass port. Given the odds I think its worth the chance.

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It's very easy to scratch the indide of an acrylic port while cleaning (knows from experience). Sooner or later, you'll need to clean the inside (stuff DOES get in there) so be CAREFUL.

 

Scratches on the inside of a dome port will appear in photos, and they can also cause small flare or star points in your photos when shooting towards the sun.

 

Cheers

James

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I think that there are good ports and bad ports in both camps. The Sea and Sea dome is a good plastic port (although too buoyant). I think it is more complex than just the material.

 

Alex

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Glass domes are pretty hard to scratch.  You basically have to scratch it with something harder than glass - like silica sand...or a diamond...

 

Acrylic, all it takes is something harder than plastic - like butter, or a rubber knife.  But you can easily polish the scratches out which only takes a couple of hours.

 

There are some other things that scratch glass ports - tiger shark teeth come to mind. Not an issue for some people, but worth considering if you're the type to put your camera in their mouth.

 

As far as acrylic goes, most scratches can be removed in 30 minutes very easily. The nexus polish works extremely well - i've seen some pretty heinous scratches removed from ports and good results afterwards.

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Since we're on the topic...I have some ugly scratch on my GLASS flat port, not caused by diamonds (I wish THAT was a problem I had :blink: ) but by regular rocks while climbing out of water. How DO I get them out? A new port is about $140, so is it even worth trying or is it more economical to just buy new port?

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Sometimes you can just replace the glass front piece. This is possible with Aquatica ports - not sure what you are using.

 

Cheers

James

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I've probably said this before but DON'T try having scratches polished out of glass domes by anyone other than the manufacturer (who will probably suggest a replacement glass). I've had optical firms try this twice, and twice the ports have shattered due to differential heating in the glass. 'Plastic' ports CAN be repolished (there have been several queries about this on wetpixel before and you can do this yourself) and provided the scratch is not too deep, the resulting slight alteration of curvature won't show. Its pretty simple to state the difference in as much as 'plastic' ports scratch more easily but can have the scratches polished out, glass ports are harder to scratch but cannot be repolished so easily! ANY port can be scratched if not treated with some care.

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