Angle of view comparison of wide lenses

I discovered today that my Nikkor 8-15 zoom fisheye is wider behind a 6" dome than an 8.5" dome. I think the lens is in about the right position within each dome. Anyone else noticed this?

My Sigma 8-16mm at 8mm behind an 8.5" Nauticam dome seems wider than the 16-50 Z lens behind the WWL-C. The WWL-C has 130 degree corner to corner angle of view and the 8-16 has one of 114.5 degrees. What am I missing here?

Using a crop sensor Nikon Z50 in Nauticam.

4 hours ago, PeteAtkinson said:

I discovered today that my Nikkor 8-15 zoom fisheye is wider behind a 6" dome than an 8.5" dome. I think the lens is in about the right position within each dome. Anyone else noticed this?

Curious. I've not noticed this, Pete, using the 8-15mm on a D500 with a 4" or 8" dome. But I'll check it out over the next couple of days.

This could be due to focus breathing since the lens is focused closer when using the smaller port for the same subject distance.

Thanks Tom and Tim.  One reason maybe is that the custom  6" is hemispherical whereas the Nauticam 8.5" is not; so the no parallax point of the lens sits in front of the centre of curvature of the bigger dome in order not to vignette. I suspect with hemispherical domes of different sizes there would be no difference in angle of view.

I think it is most likely to be the effect Tom mentions, assuming the ports are both correctly positioned. 

On 5/15/2022 at 2:05 PM, PeteAtkinson said:

I discovered today that my Nikkor 8-15 zoom fisheye is wider behind a 6" dome than an 8.5" dome. I think the lens is in about the right position within each dome. Anyone else noticed this?

My Sigma 8-16mm at 8mm behind an 8.5" Nauticam dome seems wider than the 16-50 Z lens behind the WWL-C. The WWL-C has 130 degree corner to corner angle of view and the 8-16 has one of 114.5 degrees. What am I missing here?

Using a crop sensor Nikon Z50 in Nauticam.

While the 140mm dome is actually a complete dome the 8.5 port is not which means it will have additional distortion that will make the angle of view drop. More specifically the lens is closer to the glass with the wider port so although the distance feels correct it is not

This is likely the reason why Nauticam puts a *on the 140mm dome

There is a misconception that larger dome better quality and wider view but that is only true if both ports are actually a complete semidome.

I have done extensive test against nauticam port charts and although they do not clearly communicate why the items with * are actually the best 

so the small 140mm dome will outperform the 8.5" acrylic and 230mm glass not because of size but because it is a dome while the others are just large ports in terms of field of view.

The fact that corners may look better on the 230mm is because the field of view is cropped...

Re your other query 8mm in 1.5 crop is 122 degrees diagonal the WWL lenses generally have a correction factor of 0.36x 16x1.5=24 24x0.36=8.64 max field of view 118.2 degrees not sure where the 130 degrees comes from perhaps one of the cameras with a smaller elements sitting closer to the glass can achieve that

The original WWL-1 had a 0.36x magnification so on a 28mm lens gave 10mm equivalent diagonal (but less horizontal and vertical as it is distorted) normally lenses that accept wider lenses have higher magnification (i.e. they look narrower)

I measured my Nauticam N120 140mm dome recently. I used a compass (the type you used in primary school to draw circles, not the one on your phone!) and ruler, so measurements are approximate. I confirmed the radius as 70mm, but the height (top of dome to port flange) came in at 64mm. So this dome is not a full hemisphere; or to put it another way, the optical centre for this dome is 6mm behind the port flange.

I also measured my N85 #36125 "4.33" dome (d = 110mm, so r = 55mm), which has a built-in extension I had previously assumed was about 30mm. Well, guess what - the dome height came in at 40mm to where the external housing changes direction. It is then another 30mm to the port flange. This gives a total port height of 70mm, and deducting the radius of 55mm, means the built-in extension is only 15mm... and the dome is not a full hemisphere either.

I also have the "8.5 inch" (216 cm) dome. I haven't bothered to measure it, but am confident its optical centre is behind the port flange...

Curious whether others have done the same and what results they got...

Very annoying when housing manufacturers don't quote useful measurements for their domes... and based on my experiences contacting Nauticam, no way to find out either, unless you can lay your hands on one and measure it yourself.

 And why do we put up with mixed imperial and metric measurements? !

Regarding the comments about the superiority of the 140mm dome, I imagine that would be more about use of optical glass vs acrylic, and (possibly) improved coatings on the glass port. This assumes both ports are correctly aligned with the lens. However, it may be that the optical centre of the 216mm dome is further behind the flange than for the 140mm, so all else being equal, if corner cut-off is to be avoided with a 180 degree fisheye, then the 140mm might appear to perform better because the optical centre of the dome can be positioned so as to be less out of alignment with the EP of the lens cf. the 216mm dome.

Regarding the original post, I wonder if Pete is right; i.e. one result of positioning a lens 'too close' to the dome will be to reduce the FOV slightly.

If only we could measure these variations - can't see myself measuring FOV underwater any time soon!