The subject of rectilinear wide angle lenses and underwater optical performance has been beaten to death.

Most recently some photographers and videographers have done without rectilinear lenses altogether citing the horrible performance at the edge of the frame as the primary reason to shoot lenses with barrel distortion being those fisheye or standard lenses with an added water contact optic.

Is it all justified? Should you stay away from rectilinear lenses altogether?

Of course not. Rectilinear lenses have a place in underwater imaging that is there to stay but…

There are many many buts so let’s dive into some demistification and general considerations.

Topside Performance of Wide angle lenses 17mm use case

The vast majority of underwater shooters do not perform any type of topside imaging being that photo or video and use their set up only underwater with the exception of the odd event or those that like to shoot macro above and below this is where we stand in most cases.

In order to ascertain if the performance of the wide angle lens in water deteriorates it is appropriate to determine the performance of said lens topside. This is something that not many people are in fact able to ascertain. You can read equipment review but it is never the same thing.

I have recently invested in a Sony A1 and underwater housing and I decided to purchase a rectilinear wide angle lens. I always try to buy lenses that are good topside and underwater and my choice has fallen on the Tamron 17-28mm F/2.8 Di III RXD.

Several reasons for buying this lens I will list the most important are:

1. Close minimum focus distance (19 cm wide – 26 cm tele)
2. Lens does not extend when zooming
3. Reasonably compact (99 mm and 420 grams)
4. Not too wide

Items 1,2 above are important underwater and 3,4 are also important topside when you use the lens on a gimbal or when you shoot interiors or close up and you do not want apparent perspective distortion.

I have been shooting the Tamron topside on trips and I have been very pleased with it. Before taking it underwater I wanted to understand what to expect.

I built my scene using my underwater props on a table top and started taking a series of shots from f/8 to f/22 to see the results. What follows are a set of images taken on land with the focus on the eye of the chick.

At f/8 the first row of props is blurry and also the leaves behind are not sharp. The edges are soft.

This is due to lack of depth of field not to the bad performance of the lens.

At f/11 we have more depth of field however the props at the edges and the nearest part is still soft.

By f/16 all the props are in focus, the edges of the math are a tad soft but overall this is the right place to be.

By f/22 pretty much everything is in focus but the image quality has dropped considerably.

In conclusion topside we need to close down to f/16 to have all the props in focus on this scene.

Just so we are clear there is not an issue of edges or else there is simply lack of depth of field as we are shooting a close up.

Underwater Performance of Wide angle lenses 17mm use case

After rigging my camera and lens with the Nauticam 180mm dome it was time to hit the pool and try underwater.

What follows are a series of shots from f/8 to f/16 with focus on the chick as the topside shot.

The image at f/8 looks very much like topside and lacks depth of field, however interestingly the bush behind the chick is relatively sharper to the topside scene. The dome port is increasing the depth of field behind the subject.

By f/11 there is a considerable improvement all across the frame the items at the edges are still soft pretty much like the topside image.

By f/16 we are where we should be. Note how the entire set of props is in focus exactly like the topside image and only a slight deterioration on the very left of the frame prop.

Move your focus…

Dome ports increase the depth of field of the lens as infinity becomes nearer however they also shift the depth of field of the lens because the virtual image is closer and most of the depth of field shifts behind.

With that in mind I focussed on the first prop to see what happens.

At f/8 the first line is sharper even the props at the edges are substantially better and I would say acceptable however the leaves behind are out of focus.

At f/11 the edges are good and so are the props behind the chick with the exception of those really further back where we are running out of depth of field.

By f/16 focussing on the near prop we have pretty much everything in focus.

As final example this is a shot at f/11 with focus just behind the fake coral red and yellow on the right.

With the exception of the very very edge of the right frame which is bordering the dome edge the image is sharp throughout.

Why are the images not blurry?!?

You are now starting to ask why some images you see on the web look horrible and mine don’t? This is a legitimate question to which there are at least five answers.

Lens working distance

The Tamron 17-18mm has a working distance of 19 cm and is 9.9 cm long. From my calculation the entrance pupil is around 26mm from the front of the lens. This means that a dome radius of 10cm is sufficient to be able to focus right on the port. All other lenses for the Sony E-mount have working distance of 25 to 28 cm and would need extremely large radius to be able to focus on the glass.

Wrong Extension Ring

The Nauticam 18809 wide angle dome port is not a classic dome but has design without a flat base. The port has 11cm radius of curvature and is 83mm deep 180mm wide this means the entrance pupil needs to be 27mm behind the extension.

Nauticam recommended extension is 40mm when combined with the 35.5mm N120 to N120 port adapter. Due to the shape of the lens this cannot be used with the N100 port as the zoom is close to the front of the lens.

Underwater housing manufactures unfortunately do not apply any science to the selection of domes and extension for a lens but out of pure coincidence the 40mm extension ring is what this lens requires.

Nauticam criteria for the 180mm port seems to be the lens needs to be more or less flush with the extension ring. In our case the entrance pupil is 26mm behind and the extension ring is a few mm over so we are more or less spot on. THIS IS ENTIRELY A COINCIDENCE!

With the 40mm extension the glass port will be exactly 11 cm from the entrance pupil and focus right on the surface.

Unfortunately the principles applied by manufacturers which are either to be flush with the ring or the dome mount or to extend until it vignettes generate loss of angle of view and distortion.

One of the worst offender is the Nauticam 230mm glass dome, where the rule of extend until you can generates pincushion distortion (edge pulling) effect.

Software Correction of Distortion

The lens correction has a warping effect on the edges even when the dome is correctly placed.

If you have a full frame camera or a camera where you can disable the correction make sure this is set that way.

For systems where lens correction is baked in the lens profile (micro four thirds) ensure to use a program that can disable the correction such as DxO Photolab for best results.

Constant Autofocus with subject tracking

As we have seen in most cases it is better not to focus bang on the subject but to focus closer or even at the side of the frame and work out the depth of field.

Many users use subject tracking that may seem a wonderful idea and works very well with a flat port however underwater results in blurred edges due to the depth of field distribution of the dome port unless you close the aperture until you can care less.

If you want to ensure the edges of your rectilinear wide angle shots are sharper use single autofocus and position the focus strategically in the frame the dome compression will do the rest.

Superwide lenses (<16mm)

When your lens is superwide you may have all effects on top of each other: pincushion distortion because your extension is calculated with the ‘go until vignette’ method, distortion correction in software, subject tracking, lens with long working distance and to make it worse perspective distortion and spherical aberration which occur when the lens is very very wide.

Choosing your rectilinear wide angle lens

I have computed all lenses avaialble for my A1 and calculated the ideal radius of a dome in this table. Unfortunately while the 230 port has a wider angle of view it has a radius of 12 cm which is only 1 cm more than the 180mm port. The choice of port is therefore driven by angle of view and not radius as 1 cm does not change much. Most lenses would need 15 or 16 cm radius to be able to focus close to the glass.

Brand	Model	Working Distance	Field of View	Radius Required	Port
Sony	SEL1224GM	280	122	152	230
Sony	SEL1224G	280	122	152	230
Sony	FE14 1.8 GM	250	114	157	230
Sony	SEL1635GM	280	107	169	180
Sony	SEL1635Z	280	107	169	180
Sony	PZ1635G	280	107	169	180
Sigma	1424DGDN	280	114	164	230
Tamron	1728RXDIII	190	104	99	180
Zeiss	18mm	250	100	177	250

You can easily see that the Tamron 17-28mm has much better design characteristics to go into a dome port and this is the lens to buy.

From my tests there is no need of any field flatteners and correction lenses it works fine out of the box as the lens has minimal field of curvature and zero spherical aberrations.

Pool Session

If you are not happy with the CFWA studio scene here are some images from a recent pool session. Many f/8 and f/11 images no need to close the aperture more if you don’t have anything close and shot in single autofocus.

Quite interesting to see the fins at the edges of the f/8 shots.

If there is one challenge of rectilinear lenses is that you need strobe power. The narrower field of view compared to a fisheye means that you are standing further back that in turn means more strobe power and more particles between you and the subject.

Should I buy a rectilinear wide angle lens for underwater use?

My answer is definitely yes but each system will have the special lens, the one that focuses close, does not require a large radius and is not too wide and not too narrow (16-18 is ideal) however examine carefully images of others and check the lens construction as your suggested extension may be wrong.

But when your lens is fit for purpose using the correct size dome and the proper extension you will get high quality images that match or beat fisheye like lenses and water contact optics.

As example here an image shot with a canon 8-15mm at close range f/11. Does it have much better edges?

And here a WWL-1 image at f/8

I cannot see any advantages of the fisheye like lenses at the edges at the same aperture in fact in both cases but judge for yourself.

Phil Rudin reviewed the Tamron 17-28mm on uwpmag you can find here some open water images https://www.uwpmag.com/?p=uwp-back-issues&issue=119

Unfortunately Phil used the Sea and Sea correction lens an expensive accessory that this lens does not require but as soon as I am in open water I will post images myself.