Ports, domes and lenses (Sony)

Hi All,

Piece by piece I'm building my first underwater rig. Sony a6500 (only because I couldn't bare taking my work cameras underwater). I read a few things about domes being positioned correctly etc. so I have this to ask.

How specific are domes to lenses? For instance, I have a Sony 10-18mm, Fantasea housing, dome port and 25mm extension (as per their suggestion). In addition to the 10-18 I have a 50mm just for walk around. Can you put a normal lens in a dome port?

If domes are super specific, should 3rd party lenses not be considered?

Thoughts?

Thanks

In general each manufacturer will have a number of domes of different sizes and the port charts will have recommendations on which dome will handle which lens and will have a number of extension rings available to fine tune position of the lens in relation to the port. Depending on the manufacturer they will have a limited number of combinations of rings available to fine tune this position and because of this they can only place the lens to within +/- 10mm in some cases. In general you don't have many options other than to take the offered combination or ports and extension rings.

The way to find out if the lens in the offered-port combination is good enough is to look for reviews and ask on forums such as this or buy and test the port/lens combinations which may be a good combination.

In general just because a lens performs well on land does not necessarily mean it will perform well in a dome, or if your manufacturer offers an optimised combination. For example rectilinear wide angles generally need large domes and this can be achieved with a hemisphere or a segment of a larger diameter dome, so the dome has the radius of curvature required but is smaller in diameter as it's not a full 180deg hemisphere.

there's no reason you couldn't place a 50mm lens in a dome, though you may need to use a different extension ring., though this seems to be less critical with the smaller angle of view. The question is more does the 50mm lens focus as close as you would like and what you would use it for underwater?

Hi Matt,

Since no one has answered so far, I’ll give this one a shot.

There’s no intrinsic reason why a 50mm lens on an APS-C camera can’t be used behind a dome port. For instance, the Sigma 17-70mm Macro that has been quite popular with underwater cropped sensor DSLR users as a “Jack of all trades” lens includes that focal length and is usually used behind a large dome port. But there are a few things you should consider before using this combination:

Firstly, the optical advantages of using dome ports are most significant in the more peripheral parts of images produced by wide-angle lenses. There is little optical advantage using a dome port compared with a flat port at the relatively narrow angle of view provided by 50mm lenses on APS-C.

Secondly, the lens needs to be able to focus on the quite close virtual image produced by the dome. Not all lenses have this close focussing ability. Lenses that don’t focus close may sometimes be used successfully behind a dome by adding a positive dioptre close-up lens on the filter thread.

Thirdly, the dome needs to be placed optimally in relation to the dome by use of an appropriate extension to achieve good optical correction. This usually means placing the nodal point of the lens close to the centre of radius of the dome port. For lenses not widely used underwater it can be difficult to establish the nodal point of the lens and sometimes the centre of curvature of the dome is also difficult to establish.

Fourthly, the uses underwater of an intermediate focal length lens such as 50mm are likely to be different from use on land. For scenic shots, shorter focal lengths than 50mm are preferred as they force a closer approach to the subject matter, reducing the murky intervening water column and making it possible to illuminate the subject with strobes. 50mm is a useful fish portrait focal length that works OK behind either a dome or flat port. 50-105mm is also a useful focal length range underwater on APS-C for macro shots as long as the lens has very close-focussing abilities (preferably able to achieve 1:1 reproduction). For close macro work with a 50mm lens, a flat port is far preferable to a dome as working distance will be very short and a dome port will eat into this distance. The diameter of the dome port may also make it difficult to get close enough to the subject as rectilinear lenses don’t work well behind mini-domes. Finally, it is nearly impossible to add wet close-up lenses to a dome port and even if it was physically possible to do so, the added distance between 50mm lens and wet lens wouldn’t be optically optimal.

I hope some of this helps. There are a lot of new technical considerations when moving from land to underwater photography.

Mark

Some great advice here!

Agreed. Some good info. I had just read about virtual images.

I am trying to get as much technical information before my first dive.

Chris and Mark,

Thanks for all good info. I bought the recommended extension tube and dome for the 10-18mm. Can I assume that the appropriate distance from the lens to the dome exists for only 1 specific focal length? I haven't found any information about this.

Thoughts?

Thanks for the info.

Hi Matt,

It is true that the nodal point of zoom lenses may vary according to the focal length. By how much is a feature of the particular lens design. The housing manufacturer's port extension recommendation is therefore likely to be a compromise. As Chris said above, extensions come in fixed increments so there's also likely to be an additional compromise there. Despite this, there is some leeway in optimising the port extension and you can expect reasonable performance from this lens behind the dome and recommended extension throughout the zoom range. I have experience with the Sony 10-18 and it works pretty well behind a medium sized (170mm) dome. It would probably work marginally better with a larger dome, but ~170mm is pragmatically a good solution for use on mirrorless APS-C CSCs. As a rectilinear lens, it is unlikely to perform very well at the periphery of the image with a mini-dome (~100mm). Should you at some time in the future want the advantages of using a very small dome port for close focus work, you would need to consider fisheye lenses. Currently there isn't a high quality native APS-C E mount fisheye lens in the Sony range. One workable alternative if your housing provider has an appropriate port/extension is the Metabones E mount to Canon adapter coupled with a Canon mount Tokina 10-17 lens. There is a Nauticam solution for this combination but I don't know about Fantasea.

Mark

Thank you Mark. It's good to know there is going to be a compromise in quality here. The dome is approx 155mm for the Fantasea housing. I am beginning to consider switching housings before I get underwater for the first time, while I can still return this stuff. I know you said there would be a lot of new technical considerations for me to learn but when it comes to IQ I am pretty sure I will be really disappointed if I am getting poor results from using suboptimal equipment. That being said, I thing the a6500 is a very nice camera and I think it has the ability to make great pictures... only if it has the support to do so underwater.

Given my experience with Nauticam, I'd swap to them in a blink, very good range of ports including the Zen dome options, you can use the excellent Zen 170mm dome for your 10-18:

http://www.jaredparsons.com/portchart/zen-only/by-combination-group/nauticam-n85/

or Nauticam 180mm dome: http://www.bluewaterphotostore.com/images/articles/NauticamPortChart/2017/N85%20Sony%20E-mount.pdf

If you went that way you could use the N85-N120 option and get multiple use out of the extension ring potentially as it is also used with some of the macro options in the Nauticam system.

Regarding the zoom optimization i suspect that widest setting is where it is optimised as it is the outer edges of the curved virtual image you are trying to bring into focus, when zoomed in you are looking at smaller segment and the edges are not so much closer I also expect the limited range of extensions may be the reason some lenses are not performing so well UW compared to land use.

The real downside of the Sony system is the limited range of UW lenses.

I agree with all Chris's observations.

Compromises are part and parcel of all photography, whether it be size and portability, flexibility, usability, durability, cost and of course IQ. Photography in the aqueous environment imposes an additional major layer of compromises, not the least of which being the need for refraction correction for wider angles of view. Understanding the compromises we are all making is important as apart from helping with equipment choice, it will determine subject selection and approach and best camera settings. Although there is currently a resurgence in interest in more sophisticated water contact optics, the simple dome port with all its optical compromises remains the most practical and cost-effective option for most of us at the moment. Just how much IQ compromise we are willing to accept, particularly at the edges and corners, ends up being a personal decision.

I am sure your 10-18 will perform adequately behind the Fantasea 155mm dome, but assuming that the different diameters are more or less proportionate to their radius of curvatures, I would expect better performance behind the Zen 170mm dome with which I have experience. To a degree it is possible to compensate for the poorer edge performance of smaller domes by using smaller apertures (perhaps no larger than f11 to 16).

Although I am primarily a DSLR shooter, together with my daughter we also use an A6000 (land and underwater) and A6500 (currently only land). I agree that both of these cameras are very technologically advanced and capable at their relative price points. Limited lens choice was long an Achilles heal for Sony E/FE mount cameras. Although this has been steadily improving for the full frame lenses, Sony don't seem to be investing in new lenses for their cropped sensor cameras. This forces Sony cropped sensor users to consider using the FE lenses, but their size, weight and cost negate one of the advantages of cropped sensor CSCs. For underwater macro use we use the Zeiss Touit 50mm f/2.8M Macro and the Sony FE 90mm f/2.8 Macro G OSS, both of which are quality lenses. For wide-angle, the Sony 10-18 is good. The Zeiss Touit 12mm f/2.8 is said to be even better but of course it doesn't deliver zoom flexibility. But to complete an underwater set, a fisheye is necessary if you want to really close in that water column distance and get the most striking perspectives. In my opinion it's there that the Sony system struggles with native options.

I hear good reports of Fantasea housings but have no personal experience. I have lots of Nauticam experience and although more expensive, Nauticam currently clearly provide a more mature and comprehensive system that is continuing to evolve and expand. That may be important now, but as your underwater photographic journey progresses, it may become even more important. For underwater photographic gear, post sales support is also important and you may wish to take availability of local support into consideration when making equipment purchase decisions.

Mark

Valuable comments from Mark, as always.

The FE 90 macro is indeed a bit of a giant, but I find it much more useable then the Touit 50, especially with diopters. There are very few creatures that allow one to approach closely enough to use the Touit's tiny focus distance.

For fisheye, he Tokina 10-17/Metabones combination works excellently with the a6500, although again it's quite a size. I have the Mark V adapter, and had no complaints about the autofocus at all (which can be a problem with adapted lenses): quick and consistent. It's true that the lack of a proper native fisheye is a downside to the Sony range underwater, but the Tokina works fine.

Sent from my iPad using Tapatalk

Or to put it another way Justin, regarding macro lenses underwater, Sony APS-C shooters face fairly similar choices to Nikon APS-C shooters (60mm or 105mm) and Canon cropped frame shooters (60mm or 100mm). For small stuff the longer focal length lenses offer much more useful working distances for a given magnification and diopters are more effective. For medium or larger sized subjects, longer focal length lenses force you to back off if you want the entire subject in frame. Apart from perspective differences, this introduces a longer water column that is liable to reduce image contrast and quality and at the larger subject end may require more power from your strobes.

As we often don't know what creatures we will find before committing to a dive, I often find that Sod's law dictates that the optimal lens for a particular subject is the one that's topside!

Mark

Thanks for all the useful comments and experiences. I have decided that its likely that within the first year or so I am going to switch up to a DSLR so Im going to just start out with the entry level gear I already have. The arms and strobes I bought are professional level so those can be salvaged as I move forward.