Following from a previous article about not increasing bulk I have considered a few options for the Canon 8-15mm fisheye.

The 8-15mm is not a small lens and due to the different flange distance between Canon EF mount for DSLR (44mm) and Sony E-Mount (18mm) we have a chunky 35.5mm N100 to N120 adapter port that makes the whole set up not that compact.

Dome Options 140mm vs 4.33″

The Nauticam port chart recommends the 140mm glass fisheye dome for the 8.15mm, this port is 69mm radius and is made with anti reflective optical glass and weights 630 grams.

There is another dome from Nauticam the 4.33″ acrylic but this does not feature on the port chart for the Canon 8-15mm.

I did some calculations and this dome should require the same extension so I ordered one conscious that this would be lighter but not necessarily increase the underwater lift due to a reduced volume.

Although there is a difference of 362 grams the smaller volume will result in less buoyancy 348g lift vs 688g lift for the 140mm so overall the additional buoyancy is only 22 grams.

The primary benefit of this smaller dome is that it gets you closer this in turn means that things will look bigger and as consequence depth of field will drop. Depth of field depends on magnification and as you will get closer it will drop compared to other domes. So larger domes have more depth of field not because they are larger when you are at close range but simply because your camera focal plane is standing further back.

To give an idea this is a little miniature shot with the 140mm dome with the target touching the glass port.

This is the same target with the 4.33″ dome.

Side by side shows the difference in magnification.

If we look at the same detail we can see that the 140mm dome image detail is less blurred.

We are on land here there is no water involved and the 140mm image is sharper at the edge simply because it is smaller.

As depth of field must be compared at equal magnification we can also bust another myth of larger domes vs smaller domes there is no increased depth of field you are just standing further back if you compared the front of the port instead of the focal plane.

Building the Rig

The extension required is still 30mm as for the 140mm dome,

The overall size of this dome means it is flush with the extension ring.

This is the overall rig with the amount of flotation in this image it is around 600 grams negative in fresh water.

Now that we know what to expect is time to get in the pool and take some shots. I got some miniature aquarium fixtures to simulate a close focus wide angle situation.

Pool Session

Once in water I set up my artificial reef and got shooting.

I was at the point of touching the props so I had to stand back a little. As expected the issue is depth of field.

Shots at f/11

For starter we try to get as close as possible and focus in line with the chick.

Due to the extreme magnification the front details are quite soft. So from here I start moving backwards a little.

Still focussed on the chick the sharpness improves due to reduced magnification this is a simulation of a larger dome.

There still is severe blurring of the front detail at f/11. However due to the increased depth of field that the dome brings behind the focus point the rest looks pretty good.

Focussing on the middle of the frame at f/11 results in blurry details for the features in the front of the frame but much less blurry than before and the chick is still relatively sharp.

Focussing on the pink reef detail results in a better overall result in a counterintuitive way.

Shots at f/16

Stopping down the lens results in increased depth of field so more of the image is in focus however the overall sharpness drops. This is a good place to be if you don’t want to be too sophisticated with the choice of focus point and you are close.

You can get closer but the front detail is still a bit soft but acceptable.

If you move your focus point a bit further in front the situation improves.

At this point I decided to get into the picture with a white balance slate.

Although the front is quite blurry due to the extreme close range the result is acceptable for the non pixel peeper.

Shots at f/22

We are here hitting diffraction limit and the image looses sharpness but we are after depth of field so be it.

Now the depth of field is there although the detail in the centre is less sharp.

Moving the focus point makes the image a bit better.

Time to insert the diver in the frame.

Overall ok not amazing consider the dome is on the parts.

Conclusion

The small acrylic dome does quite well at close range, the limitations come from the depth of field and not from the water and the dome increases the depth of field behind the focus point. This is something that you can use to your advantage if you remember when you are in open water.

For shots that are further away you can shoot at f/11 and get excellent IQ there is no need to stop down further to improve the edges. Consider however that f/8 may be just too wide on full frame and introduce additional aberrations regardless of depth of field.

Some numbers:

Nauticam 140mm Glass dome: £911

Nauticam 4.33″ Acrylic dome: £550

Price difference £361 or 40% however bear in mind that the primary benefit of the glass dome is to resist reflections and ghosting due to the coating and the fact you can keep the 8-15mm hood on.