1 minute ago, Paul Kay said:
Sort of, in some ways but your statements are imprecise.
Larger dome ports position the virtual image at a greater distance from the lens which has various benefits. I'm not so sure that positioning error is one of them if you are after optimum available performance.
All camera lenses seem to be 'optimised' for infinity unless they are specialised macro lenses. I can't think of any which the manufacturer provides MTF data for closer distances, but perhaps there are some? Wide-angle rectilinear often, but probably not always, make use of a shifting rear group to increase close performance. Such optical adjustments have been around for decades but may well shift the focal length slightly (it is already increasing due to the need to focus on a subject closer than infinity) and will provide better performance in closer use than a similar lens optimised for infinity without ary close range correction. Many current Leica M lenses use such floating groups to enhance closer range performance even though they don't focus particularly close (most 0.7~1m minimum). The tolerance and complexity of such groups can be problematic though.
Fisheye lenses probably don't work as well close up as at infinity, but some may have a close focus correcting group perhaps. I haven't checked. Edge performance close up can be a bit iffy at times though in my experience.
I don't profess to understand lens design as I'm not a lens designer, but I can appreciate the end results. If you know what you are trying to achieve then its perfectly possible to optimise whatever system you use to achieve as good results as possible. Unfortunately you can't beat physics so you do have to appreciate the limitations equipment imposes and change it to produce better if it is not acceptable. Up to a point at least.
I am not sure that having infinity further away is in itself a benefit
In practical terms the way it works is that I have a finite number of ports avaialble and most of them do not have a 180 degrees field of view
If we look at fisheye lenses they focus very close. A dome with a small radius may contain the lens working distance
Example Canon 8-15mm it focuses at 2cm from the front of the lens. A dome as small as 4 cm in radius may be sufficient however it can't physically contain the lens. So in practical terms I have 4 choices that can contain the lens field of view
1. Zen 100mm
2. Nauticam 110mm
3. Nauticam 140mm
4. Nauticam 8.5" acrylic dome
All other Nauticam ports do not have 180 degrees field of view so I will ignore them.
All lenses above can focus right on the surface of the dome, however as the radius goes up this means things will be further away and start looking smaller. On the other hand the infinity point will also move further away
Zen 100mm infinity is 20cm from the entrance pupil
Nauticam 110mm infinity is 22cm from the entrance pupil
Nauticam 140mm infinity is 28cm from the entrance pupil
Nauticam 8.5" infinity is 43 cm from the entrance pupil
In terms of working distance
Zen 100 mm I will be 16 cm from target
Nauticam 110mm I will be 16.5 cm from target
Nauticam 140mm I will be 18cm from target
Nauticam 8.5cm I will be 21.8 cm from target
In terms of infinity from the dome we have respectively
Zen 100mm infinity is 15cm from the dome
Nauticam 110mm infinity is 16.5cm from the dome
Nauticam 140mm infinity is 21cm from the dome
Nauticam 8.5" infinity is 32 cm from the dome
total range of focus from focal plane (11 cm behind entrance pupil)
Zen 100mm infinity is 31cm
Nauticam 110mm infinity is 33 cm
Nauticam 140mm infinity is 39 cm
Nauticam 8.5" infinity is 54 cm
I cam guarantee you that the lens is not going to become amazing when it goes from 31 to 39 cm but it may start to work in a more comfortable way when the target is 1 meter and this will never happen
So going all the way to get to 54 cm infinity did not really change the world in terms of performance but it did make your rig much more bulky
And this is the reason why Nauticam at the end recommends the 140mm and this is mostly because glass allows for antireflective coating not because of those 6 cm difference and at the same time if you want to get close because you want to you can
One use case where a massive dome helps are split shots and that is because the depth of field required by the land section not the underwater one.
Now if you go into a case where the lens can't focus inside the dome your objective is to have the biggest radius no matter what as you are wasting range due to the lens not being able to focus at all within the dome
Or alternatively choose a lens that DOES focus close and is built to focus in the dome in which case one or two cm radius more are not going to make a dramatic difference
Every system tends to have this lens, in MFT it was the Panasonic 8-18mm in the Sony E-Mount is the Tamron 17-28mm I do not know much about other systems but if I was planning to use them I would do the related research
Those lenses tend to be not too wide, focus very close but not physically that small which means the difference between Minimum focus distance and entrance pupil is not small which means even a small radius dome won't affect the lens to focus on the glass in water
The vast majority of rectilinear lenses on the market have working distance of 25-28 and are sometimes very short which means the radius of your dome to contain the working distance is 16 cm. And nobody wants to lug around a 30cm wide angle port
I study all the lenses that I purchase and before taking them underwater or even investing in ports I take close distance shots on a tripod because that is what matters behind a dome. This system has given me results that i consider more than acceptable
How many underwater people even shoot a wide angle or fisheye lens topside? I have spoken to many people that have never used their fisheye lens topside and have no idea of what to expect before taking the lens down...
