Looking at getting a small dome port to shave off some pounds and make things a little easier.

I am currently toying with the idea of either the lesser known (to me) Saga 4" Dome port or the Zen SP-100.

Any comments, suggestions?

Nauticam N120 for D810 and a Sigma 15mm Fisheye.

If you like you current dome size, I would suggest thinking about glass versus acrylic. I have both. The weight difference is very noticeable. My wife, however, has a large acrylic (she protects at all times with a cover). that is 6" and light weight. Lastly, with a nice fisheye lens, I would go wide. Good luck!

Hi Julian

The mini dome port usually are 100mm diameter (or 4").

Full frame sensors (due to smaller deep of field) usually are more prone to present soft corner on small domes.

Mini Dome works well with the cropped sensor and the Tokina 10-17 FE (or other fish eyes for cooped sensor), specially with the extender 1.4.

To use it in a Full Frame you need to stop down the aperture to minimize the soft corners, and that might reduce the flexibility to find the right setting for the picture.

View archived post in: Canon 8-15mm on Sony

If you want to do split shots, with the mini dome is not possible (the virtual image inside the dome gets too close and you do not have enough deep of field to have in focus the underwater part and the over-the-water).

My first suggestion would be to look at the Saga and Zen port charts at zen underwater.com and sagadive.com. These 100mm ports are very lens specific and according too the charts neither brand supports the Sigma full frame 15mm lens. In fact the only full frame lenses they support are the 8-15mm fisheye zooms. 

The Nauticam N120 140mm optical glass fisheye ports support both the sigma 15 and the outstanding Nikon 8-15mm fisheye zoom. The fisheye zoom requires the port with the removable shade for the 8mm and and an N120 40mm extension. The port with the removable shade is a bit more expensive but if you ever intend to move up to the 8-15 zoom it will have been money well spent.

Photos are with the Nikon 8-15mm F/3.5-4.5 fisheye zoom at the circular 8mm and and the 15mm full frame end using a Nikon Z6 with Nikon lens adapter. The housing is Ikelite with the eight inch compact acrylic dome and Ikelite DS-161 flashes.

I agree with Phil, the "N120 140mm" is what I would use. To use a Sigma 15mm with a 100mm dome, you need to shave off the lens hood - if you leave it on, the lens cannot get close enough to the dome to not see the edges of the dome. 

Alex

Dear All

Many thanks for the good advice and lovely photos.

Having carted my 230mm dome port around for awhile, I really wanted something nice and small, as luggage space and weight is always such an issue. Still I am not sure I would want to shave down the lens hood, and I guess at some point it might be worth trying to invest in a 'better' lens, the circular effect does appeal.

I have 170mm domes from Seacam and Zen, which are kind of a half way house. It works fine with the Sigma 15mm, (without shaved lens hood) and is significantly lighter and smaller than the 230mm dome. It also works well with the Nikon 8-15mm, but the shade intrudes at 8mm, so it is not possible to shoot circular fisheye. 

It is possible to shoot splits with it, although the rule is that  the bigger the dome the easier/better it is to shoot them!

Just to be clear the N-120 170mm Zen Underwater port has the removable shade for the circular end of the 8-15 zoom. 

Saga domes all have a removable shade. It doesn't look like it on the larger domes, but it's a press fit with an oring holding it on. You have to leverage one side off at a time.

It's been a while since I've seen a Saga 170mm (actually 165mm) dome, but I'm quite sure that the shade comes off with a little effort.

Just sort of peal it off. A bit of grease on the oring will make it easier next time.

Then it should work fine with the 8-15 for circular FE shots.

Jack

I just looked again...and the shade on my Zen 170mm is removable! My apologies.

Adam

The Zen domes and other similar are a compromise solution for fisheye lenses though as they are the segment of a larger sphere with a 110mm radius as I recall.  This places the centre of curvature for the dome about 40-50mm behind the dome base.  

Most fisheye lenses have the entrance pupil right up towards the front of the lens which means the entrance pupil will be something like 40-50mm in front of the centre of the radius of curvature to avoid vignetting, meaning the performance at the edges is not as good.  How bad I don't know as I have not tried it out, but the performance will not be as good in the corners.

The Zen 100mm and Nauticam 140mm domes on the other hand are 180° hemispheres and you can place the entrance pupil at the centre of the radius of curvature and also not vignette. 

Fisheyes don't suffer from the depth of field issue due to the curved virtual image as they are designed to render a hemisphere onto the  flat surface like the sensor unlike a rectilinear lens that tries to take a flat plane and render that onto a flat sensor.  If you were rendering a flat plane the point at 90° to the lens centre line would have to be at infinity.   

That's not to say they may not in general benefit from a slightly larger dome, they may well do but the impact on image quality is nowhere near as significant as it is for an ultra wide (weitwinkel) rectilinear lens.  This may vary with the type of lens, the projection it utilities and the sensor size and also mechanical issues like lens shades.