I have made this point in other threads before, although I cannot seem to find them now. As far as I remember, nobody ever seriously challenged my working thesis:

“You cannot improve a lens’s in-air optical performance underwater.”

What I mean by that is fairly simple: any optical system not originally designed specifically for underwater use — with the obvious exceptions of systems like the Nikonos 15mm and Nikonos RS — will suffer once taken underwater. Image quality will always degrade relative to that lens’s native performance in air.

The moment we take a full-frame lens underwater and place it behind a flat port, dome, or a purpose-built underwater correction optic such as OPP, Ivanov, WACP, FCP, or Marelux Aquista, we are introducing a new front glass element into a lens design originally created by the optical engineers at Nikon, Sony, Canon, Sigma, and others.

In other words, we are adding more “filter” to the light path.

We are not performing magic.

We are adding more glass.

We are adding more interfaces.

We are adding more opportunity for compromise.

That does not mean such correction optics are pointless — quite the opposite. A lens may become far more useful, more practical, and more enjoyable underwater when paired with the right optic. But it will not suddenly become sharper, cleaner, or more transmissive than what the best land-based lab tests already show it to be capable of in air.

What it can do is perform vastly better in its actual underwater use case when paired with the correct optic, especially when compared with the same lens behind a flat port or a poorly positioned dome.

So when people speak as though a port or correction optic somehow improves a lens beyond its native land performance, I think that is where the language becomes sloppy. Better underwater than with the wrong setup? Absolutely. Better than its in-air optical ceiling? I do not see how.

So I am putting this out here for discussion and, ideally, confirmation or informed contradiction.

My working assumption has long been that the best way to identify a strong underwater optical system is to start by comparing land-based tests of lenses that already fit key underwater criteria — for example, short minimum focusing distance and other relevant characteristics — and then distill the strongest in-air candidate before bringing it underwater and matching it with the most suitable port or correction optic.

That, to me, seems to be the real algorithm:

start with the best land candidate, then optimize the underwater setup as intelligently as possible.

What about lenses that are not well regarded above water such at the Tokina 10-17 which has quite a following for UW use?

I basically agree with the premise, any additional elements tend to degrade performance to varying degrees, it may degrade a lot or a little to the point it is hard to see, but it entirely reasonable to expect that it won't improve.

As for the statement :

start with the best land candidate, then optimize the underwater setup as intelligently as possible

yes but it still seems that some lenses play better underwater than others, so there is a need to be selective in what you try to take underwater. Just because it is an amazing lens above water doesn't automatically translate into good UW performance. Though it seems that with the latest crop of close focusing mirrorless zoom lenses many of them work quite well UW.

For the specific case of the Tokina 10-17 potentially what we are seeing is that this lens works exceptionally well with small domes and the degradation in performance when taken UW is minimal. But if you compare the UW results with some of the older rectilinear zooms that don't seem to work well UW or perhaps people are using them in too small a dome, the Tokina seems to really shine if you just compare UW shots taken with these older lenses with shots taken with the 10-17. It is also a very flexible wide angle option the ability to zoom in and take reef scenics and CFWA on the same dive can trump ultimate sharpness for many people.

I don't have any expertise with optical design, but I am going to question the assumption that "adding more glass" degrades performance.

Logically, if you consider the design of any advanced lens, there are many pieces of glass, elements, involved. Lots of elements used to optimize the performance of the lens. However almost none of them are optimized for underwater use cases. Therefore it is completely possible to further improve the lens with more elements, even if they are external rather than internal glass.

Would we criticize Nikon or Sony for bumping up the element count on a new version of an existing lens if it improved performance? Of course not.

More glass does not equate to worse performance.

I would say it might be a mistake to assume that a lens performing optically better on land will be superior/sharper UW. Not always the case. Nikon 14-24 anyone? You shoot a whole system.

And fisheye lenses are normally easier to get sharp results from. The tough ones are inme (and quite a few others experience too) the wide rectilinear lenses (like Nikon 14-24). And since the Tokina fisheye zoom 10-17 was mentioned, the reason it wasn’t that popular for land photography isn’t because it’s a ”bad” lens optically… I don’t think it is, it is because it’s an odd ball lens.

Edited Saturday at 04:46 PM4 days by Christian K

7 hours ago, Grantmac said:

What about lenses that are not well regarded above water such at the Tokina 10-17 which has quite a following for UW use?

What makes a good underwater optic is, in my view, a different question and one that should be discussed separately.

The Tokina 10–17 stands out for good reasons — mainly its fisheye design and extremely short minimum focusing distance. Those are exactly the kinds of criteria that should be used first to identify viable underwater candidates before weighing them against one another.

But once you do that, the relevant comparison for the Tokina 10–17 fisheye zoom is the Canon 8–15 fisheye zoom. And in that comparison, the Canon wins on resolution and sharpness. At least in my experience, that advantage carries over underwater as well.

I am suggesting this as a thought, not a position I am taking...

Could it be that some lenses, due to their optical design, degrade less than others and some lenses that are "superior" on land may be more more degraded underwater and thus the "lesser" lens on land is superior underwater. (If that word salad makes sense)

To me, shooting underwater is somewhat like shooting through a dirty-ish window on land. Even if we can correct for how light is transmitted underwater and deal with water-caused distortion, seawater is not clear, especially when we are more than inches away. But anyway, I wonder if a lesser lens might, in some cases hold up better underwater and thus be the better lens than alternatives, and the inherent detrimental effects of seawater also mask some of the lens' shortcomings?

5 hours ago, JohnD said:

I am suggesting this as a thought, not a position I am taking...

Could it be that some lenses, due to their optical design, degrade less than others and some lenses that are "superior" on land may be more more degraded underwater and thus the "lesser" lens on land is superior underwater. (If that word salad makes sense)

I think this can potentially be a cause for some otherwise good wide angle lenses not working so well underwater. One issue is field curvature - all wide angle lenses have it to some extent and the plane of sharpest focus can be concave or convex or even wavy with respect the sensor which is perfectly flat. Wide zooms tend to be more complicated and the curvature may change from concave to convex or vice versa as you zoom in. This link discusses in some detail:

https://www.dpreview.com/opinion/7031211310/roger-cicala-field-curvature-pt-2

Why is this important UW? For lenses behind a dome port they are imaging a curved virtual image which mean the image plane of sharpest focus is also curved. If the lens's field curvature works with the curvature of the virtual image it can tend to help bring the edges into focus, but if it works against it it can push the edges even more out of focus. Field curvature doesn't change shape as you stop down, the depth of field just increases so that the edges progressively improve as the edges come into focus as you stop down. UW we tend to shoot well stopped down in dome ports which may mask some of this but a lens with strong field curvature in the wrong direction could potentially be impacted. It is most important on full frame, and gets to be less on an issue with smaller formats.

On the topic of sea water, that and the air/water interfaces reduce image quality, ultimately limiting resolution that can be achieved. So it seems to me there are limiting returns when chasing sharper lenses. This can even happen in air, I was shooting across a fairly wide turbulent river in Ecuador in the mountains, the river was snow melt so very cold - I had noticeable image degradation shooting across a 20m wide river. bad enough I could see it in camera, I ended up raising ISO , opening the aperture to get the shutter speed a lot higher which helped a little.

I'm not sure I agree with the premise that because lens manufacturers make lenses with lots of elements that adding optics does not always degrade performance. First we are talking about external elements such as domes and wet optics not being able to improve performance, not internal elements. Any internal elements are custom designed to work with other all the other elements while external wet optics are all so far designed to be universal. Even looking at the premise of adding elements will produce some degradation with each air/glass interface losing some light transmission and not being perfectly manufactured, however it is a compromise to correct aberrations in the optics, you might for example add an element to correct field curvature accepting that it reduces light transmission by 0.5% and centre sharpness by 0.1%.

12 hours ago, Chris Ross said:

I think this can potentially be a cause for some otherwise good wide angle lenses not working so well underwater. One issue is field curvature - all wide angle lenses have it to some extent and the plane of sharpest focus can be concave or convex or even wavy with respect the sensor which is perfectly flat. Wide zooms tend to be more complicated and the curvature may change from concave to convex or vice versa as you zoom in. This link discusses in some detail:

https://www.dpreview.com/opinion/7031211310/roger-cicala-field-curvature-pt-2

Why is this important UW? For lenses behind a dome port they are imaging a curved virtual image which mean the image plane of sharpest focus is also curved. If the lens's field curvature works with the curvature of the virtual image it can tend to help bring the edges into focus, but if it works against it it can push the edges even more out of focus. Field curvature doesn't change shape as you stop down, the depth of field just increases so that the edges progressively improve as the edges come into focus as you stop down. UW we tend to shoot well stopped down in dome ports which may mask some of this but a lens with strong field curvature in the wrong direction could potentially be impacted. It is most important on full frame, and gets to be less on an issue with smaller formats.

On the topic of sea water, that and the air/water interfaces reduce image quality, ultimately limiting resolution that can be achieved. So it seems to me there are limiting returns when chasing sharper lenses. This can even happen in air, I was shooting across a fairly wide turbulent river in Ecuador in the mountains, the river was snow melt so very cold - I had noticeable image degradation shooting across a 20m wide river. bad enough I could see it in camera, I ended up raising ISO , opening the aperture to get the shutter speed a lot higher which helped a little.

I'm not sure I agree with the premise that because lens manufacturers make lenses with lots of elements that adding optics does not always degrade performance. First we are talking about external elements such as domes and wet optics not being able to improve performance, not internal elements. Any internal elements are custom designed to work with other all the other elements while external wet optics are all so far designed to be universal. Even looking at the premise of adding elements will produce some degradation with each air/glass interface losing some light transmission and not being perfectly manufactured, however it is a compromise to correct aberrations in the optics, you might for example add an element to correct field curvature accepting that it reduces light transmission by 0.5% and centre sharpness by 0.1%.

Makes sense. One other thought is that perhaps lenses with high undesirable distortion when used in air could benefit from mitigation of the that distortion, or simply the lessened visual impact of it, when used in water. Maybe not? I shoot pictures and know very little about designing lenses.

I think you need to judge the results of the system, not just the lens alone with pincushion charts. The sharpest underwater photos I've seen are WACP-1 with Canon EF 28-70 f/3.5-4.5 or the Nikon equivalent, a modest kit lens.

4 minutes ago, jlaity said:

I think you need to judge the results of the system, not just the lens alone with pincushion charts. The sharpest underwater photos I've seen are WACP-1 with Canon EF 28-70 f/3.5-4.5 or the Nikon equivalent, a modest kit lens.

I would also put the Sony 28-60 in that same group.

While I understand the theory that layering additional glass may not improve uw performance, I have not experienced that when using the uw specific optics, especially the WACP-1. I have been shocked at how well the ~$300 lens performs behind it, especially on a high MP camera such as the Sony a1 and a7rV.

Edited Sunday at 08:33 PM3 days by ChipBPhoto

7 hours ago, ChipBPhoto said:

I would also put the Sony 28-60 in that same group.

While I understand the theory that layering additional glass may not improve uw performance, I have not experienced that when using the uw specific optics, especially the WACP-1. I have been shocked at how well the ~$300 lens performs behind it, especially on a high MP camera such as the Sony a1 and a7rV.

This was the original premise behind the WWL - the degredation of the image by the a plain dome is such that it makes a kit lens look good UW. Whether this is still truly the case with all the new wide lenses coming on the market I don't know, I have seen several posts that I recall expressed the opinion that a really good lens behind a well positioned dome could out perform the kit lenses behind the WWL and also stating that the long ends were not as good. The Canon 8-15 also gives the performance of wet optics a run for their money.

The Sony 28-60 and the Canon 28-70 if you look up tests on these lenses are rated as surprisingly good for kit lenses or words to that effect. The Canon 8-15 is also a highly rated performer behind a small dome. I'm wondering if what these wet optics (WWL type) do is effectively make these lenses into moderate fisheye lenses, they do produce the barrel distortion after all. In a fisheye lens the zone of sharpest focus is not a flat plane but rather a partial hemisphere wrapping around the front element - the shape is similar to the shape of a virtual image with a dome port.

From all of the posts talking about this type of topic it seems that the Canon 8-15 is very hard to beat for sharpness - however the downside at least on full frame is that it lacks reach, but some posts indicate people are willing to compromise and use the 8-15 with the Sony 2.0x and are happy with the performance. This is where wet optics come into play giving something with similar quality to the 8-15 albeit a little narrower maximum field and the ability to zoom in for subjects you cant get close to.

On 4/13/2026 at 3:52 AM, Chris Ross said:

This was the original premise behind the WWL - the degredation of the image by the a plain dome is such that it makes a kit lens look good UW. Whether this is still truly the case with all the new wide lenses coming on the market I don't know, I have seen several posts that I recall expressed the opinion that a really good lens behind a well positioned dome could out perform the kit lenses behind the WWL and also stating that the long ends were not as good. The Canon 8-15 also gives the performance of wet optics a run for their money.

The Sony 28-60 and the Canon 28-70 if you look up tests on these lenses are rated as surprisingly good for kit lenses or words to that effect. The Canon 8-15 is also a highly rated performer behind a small dome. I'm wondering if what these wet optics (WWL type) do is effectively make these lenses into moderate fisheye lenses, they do produce the barrel distortion after all. In a fisheye lens the zone of sharpest focus is not a flat plane but rather a partial hemisphere wrapping around the front element - the shape is similar to the shape of a virtual image with a dome port.

From all of the posts talking about this type of topic it seems that the Canon 8-15 is very hard to beat for sharpness - however the downside at least on full frame is that it lacks reach, but some posts indicate people are willing to compromise and use the 8-15 with the Sony 2.0x and are happy with the performance. This is where wet optics come into play giving something with similar quality to the 8-15 albeit a little narrower maximum field and the ability to zoom in for subjects you cant get close to.

There are some surprisingly great kit lenses out there (and one really bad I hear). Personally I see rectilinear optics as a different breed to fisheyes. Sometimes you might prefer the fisheye distortion, sometimes not. What the ”new” development of wet optics has done, inmo, is pushing the IQ of rectilinear lenses to a new level, comparable to what we previously only have seen in fisheyes for UW. And it’s mostly pronounced in FF systems, that are less forgiving compared to smaller sensors.

Edited Tuesday at 04:47 PM1 day by Christian K

5 hours ago, Christian K said:

There are some surprisingly great kit lenses out there (and one really bad I hear). Personally I see rectilinear optics as a different breed to fisheyes. Sometimes you might prefer the fisheye distortion, sometimes not. What the ”new” development of wet optics has done, inmo, is pushing the IQ of rectilinear lenses to a new level, comparable to what we previously only have seen in fisheyes for UW. And it’s mostly pronounced in FF systems, that are less forgiving compared to smaller sensors.

The vast majority of these wet optics have barrel distortion and act like a zoomed in fisheye lens. If you do thefield of view calculations using the same formula that matches a fisheye like the 8-15 you find that at the 130° diagonal field of a WWL the horizontal field matches what you get with a WWL which is about a 13-14mm rectilinear lens in terms of horizontal field. Putting it another way, the WWL at maximum field is similar to putting a 1.3x converter on a 15mm fisheye lens.

The exception seems to be the new OPP from Seacam which is a different concept and it seems it produces a true rectilinear field.

I suppose a question might be if a very high quality fisheye like the 8-15 used with a teleconverter and "defished" can produce similar IQ in the corners to an equivalent FOV rectilinear?

3 minutes ago, Grantmac said:

I suppose a question might be if a very high quality fisheye like the 8-15 used with a teleconverter and "defished" can produce similar IQ in the corners to an equivalent FOV rectilinear?

I'll be shooting my Nikon 8-15 with a 1.4tc up in God's Pocket next week. I will share some photos.

12 hours ago, Christian K said:

There are some surprisingly great kit lenses out there (and one really bad I hear)

This is wrong. The highly raved and recommended Sony 28-60 will not project corners @ 28mm on the image sensor and fakes this with digital lens corrections. Hence it cannot be as sharp (in the corners) as a lens that is not pitch black in this area.

This was Initially complained about with the RF 24-50 Canon, but it’s not the only candidate suffering from black corners at the wide end.

The Nikon 24-50 is likely to behave similar, though I have not RAW checked that one, yet.

To me there seems to be a lot of bs out in the net, when underwater users of the Sony tell everybody how nicely sharp their 28-60 sony is paired with a water contact optic.

It‘s true for the image center (but that’s with any lens) but not the corners on the wide end.

1 hour ago, Adventurer said:

This is wrong. The highly raved and recommended Sony 28-60 will not project corners @ 28mm on the image sensor and fakes this with digital lens corrections. Hence it cannot be as sharp (in the corners) as a lens that is not pitch black in this area.

This was Initially complained about with the RF 24-50 Canon, but it’s not the only candidate suffering from black corners at the wide end.

The Nikon 24-50 is likely to behave similar, though I have not RAW checked that one, yet.

To me there seems to be a lot of bs out in the net, when underwater users of the Sony tell everybody how nicely sharp their 28-60 sony is paired with a water contact optic.

It‘s true for the image center (but that’s with any lens) but not the corners on the wide end.

I've posted on this before, I don't shoot Sony so really don't care about the lens, but many people happily shoot with it.

From what I understand the field of these lenses is wider than the stated focal length and the lenses have quite a bit of distortion and this allows room to correct the distortion and crop back to a rectangular image. Presumably the pixel dimensions of the image produced are the same as a lens that covers the full field of view so what happening is they are up-ressing the file and you are losing resolution across the entire image. I guess it's also possible that black part includes excess pixels that might be used for extra coverage when the sensor is moved about for in camera stabilisation where that is used, but you should see that in the pixel count.

What I'm not clear on is if the vignetted image is is actually the same width as a regular 24mm image from a lens that doesn't use these digital tricks or they are short changing you on field of view as well.

1 hour ago, Adventurer said:

This is wrong. The highly raved and recommended Sony 28-60 will not project corners @ 28mm on the image sensor and fakes this with digital lens corrections. Hence it cannot be as sharp (in the corners) as a lens that is not pitch black in this area.

This was Initially complained about with the RF 24-50 Canon, but it’s not the only candidate suffering from black corners at the wide end.

The Nikon 24-50 is likely to behave similar, though I have not RAW checked that one, yet.

To me there seems to be a lot of bs out in the net, when underwater users of the Sony tell everybody how nicely sharp their 28-60 sony is paired with a water contact optic.

It‘s true for the image center (but that’s with any lens) but not the corners on the wide end.

Center has never been a problem, it’s the overall image sharpness inme. One can (rightly so in many cases) argue that corners aren’t really that important, but in many cases they have been really awful even with super expensive top glass behind the biggest and nicest domes.

I don’t know if the Sony lens you refer to has this acclaimed issue you mention or what effect it has in the real world, haven’t used it. I know there’s one good Sony 28-70, might not be the kit lens. Nikons z24-50 is certainly surprisingly sharp. I’ve used it. My only complaint would be its plastic and cheap feel feel. Here’s a test:

https://www.cameralabs.com/nikon-z-24-50mm-f4-6-3-review/4/

8 hours ago, Chris Ross said:

The vast majority of these wet optics have barrel distortion and act like a zoomed in fisheye lens. If you do thefield of view calculations using the same formula that matches a fisheye like the 8-15 you find that at the 130° diagonal field of a WWL the horizontal field matches what you get with a WWL which is about a 13-14mm rectilinear lens in terms of horizontal field. Putting it another way, the WWL at maximum field is similar to putting a 1.3x converter on a 15mm fisheye lens.

The exception seems to be the new OPP from Seacam which is a different concept and it seems it produces a true rectilinear field.

Yeah, that sounds logical. Haven’t gone full scientist on this. In my short but sweet experience with Nauticam wet optics, comparing to 20+ years of domes with Tokina 10-17, Nikon 10.5, 10-24, Sigma 10-20 and a few others, it looks straighter (and obviously wider than a recti 10 with DX) to my eye at the wide end. But for sure, some curvature must be produced by the wet lens. Not sure if rectifying a fisheye with a diopter will be as good, maybe, but a little doubtful.

Inme the wwl:s works great and looks good in the real world, inmo. Lots sharper all across than the any FF rectilinears behind huge crystal domes, and very easy and versatile to use. At the end of the day we shoot a system containing several pieces. Some really great lenses that are sharp as hell has proven to be very, very difficult to use UW because of that. Just focusing on the lens and assume that it is always best to buy the sharpest lens and build your system from that might not be the best idea for UW photography, inme. It’s fun to try new things, I encourage that. But it’s sometimes frustrating and can get very expensive. With poor results. So the premise of this thread is certainly true, but perhaps many times irrelevant for UW photographers.

Edited 8 hours ago8 hr by Christian K