Nauticam recommends a 40mm extension for this combination. They also recommend the 230mm dome with a 55mm extension as an optically better option. But since I don't have a 230mm dome, I thought I'd try and see what the best extension is.

 

Calculating theoretically from the lens entrance pupil location (and taking into account that the center of curvature of the 180mm dome is actually about 25cm behind the back flange, because it's not a true hemi-spherical dome, you get the following:

 

 

So, a calculated extension of 55mm is needed for optimal image quality. Let's test this with a split shot of a checker-board. From top to bottom, 30mm, then 40mm, then 50mm and finally 60mm extension:

 

 

30mm and 40mm are too short. (portion of the checkerboard in air appears smaller than portion in water) 60mm is just a touch too long (checkerboard in air looks bigger than in water). 50mm is.. just about right, maybe a few mm too short. But the problem is you get vignetting from the dome shade (and the inside of the dome too).

 

So.. basically 40 is your best option, as Nauticam recommends. Maybe 45 would work better, but it would probably vignette a bit. I don't have a 45mm extension to test.

 

Or you can use the 50mm extension and zoom in a bit to avoid the vignetting. It disappears at a focal length between 16 and 17mm, which is still reasonably wide.

 

 

Conclusion: 40mm is the go to. If you have a 45mm extension, give it a try. It might work without vignetting, or you might have to zoom in a bit to 15-16mm. 

How much image quality do you sacrifice if using the 180mm dome instead of a bigger dome? Here it is compared to the Matty Smith custom 430mm dome, in both cases using a 50mm extension at F14, ISO 100.

 

 

Well.. you lose microcontrast in the center, and a in the corner you lose both resolution and microcontrast. This lens really does perform better with a bigger dome.

 

You could test 45mm extension for vignetting alone on land opening the housing back and pulling the camera back 5mm.  It proves Massimo's statement that 16mm is the widest the 180mm dome accommodates and you need to put the EP ahead the centre of curvature to avoid vignetting at shorter focal lengths

Good idea Chris.. I'll try that to see if 45mm vignettes on land.

 

I guess I need to build a water-tank of some sort with a sealed n120 diameter hole on the side with the dome inside the water tank so I can keep the camera dry and experiment with different lens-to-dome distances. Probably there's a way to mount the camera on some sort of measured slide to easily move it back and forth precise amounts? Any thoughts on what could be used?

 

Some sort of rail like this (but ideally cheaper)

 

https://www.bhphotovideo.com/c/product/1301197-REG/really_right_stuff_192_precision_plus_pkg_rail_clamp_package_mpr_192.html/overview?ap=y&ap=y&smp=y&smp=y&lsft=BI%3A514&gad_source=1&gclid=CjwKCAjwhvi0BhA4EiwAX25uj-fWJH9ZRmO1Ldr3ww7QavTdyExSoD40J2Sn0boAWVM9TTFLPHpb-xoC3UQQAvD_BwE

 

Edited July 22, 20241 yr by DreiFish

RRS stuff gear is generally top notch but pricey, I have a Novoflex rail which is really nice but see that the price for that has increased considerably since I bought mine.  If you have an tripod with an Arca swiss style clamp you could even use a a long lens plate and slide it in the clamp.  Google will find you lots of options at all price ranges if you search for focus rail.

 

I believe what you outline is the method used by the manufacturers or at least some of them to come up with extension recommendations.  Should be simple enough to do you would need a plate with a 120mm dia hole cut in in it and smooth enough surface for your o-rings to make a seal and add to to the side of a tank and pop in you port from the inside.  Then of course working out a system for measuring camera flange to port flange distance and getting things squared and centred.

 

 

Thank you for doing this comparison!  I have been interested in this topic, as Nauticam originally had the 50mm as the correct extension for this lens and then moved it to 40 after a couple months.  Marelux has it at 50 + their 180 dome - which is what I have been shooting (it doesn't vignette at 14) but was wondering whether I would be better off with using a 40mm instead, following Nauticam's chart.  Sounds like 50 is the right way to go.

 

Appreciate the effort!