Best dome port for the Olympus m4/3 9-18 lens

[Interceptor121]

is it really that simple? The focus distance changes with a diopter but not by zooming this lens. When I use the diopter i can get much closer to the subject improving a problem with the minimum focusing distance of this lens which is not so good.

I think the diopter changes the nodal point making it more aligned with the dome properties.

Actually the focus distance does change zooming the lens in most cases. However putting it simple I rather be closer than having to zoom to have sharp corners so the diopter is still an option
The whole issue is if the diopter will loose you focus at infinity if not there should not be any loss of field of view as the combined focal length is actually reduced

 

[ggibson]

It seems strange to me that it's necessary to add a +2 diopter to the 9-18mm in order to improve the performance of the Zen dome which was designed to work with that lens.

 

[Storker]

I don't know about the Zen dome. However, the performance improves noticeably behind the Nauticam 4" semidome (which also was designed to work with that lens).

According to discussions I've overheard, mushy corners aren't exactly uncommon for rectilinear WAs behind semidome ports. It's not limited to the Olympus 9-18. AFAIU, it's an issue with the curved virtual image that any dome projects, near the close focus limit of the lens. A well-designed rectilinear WA has a fairly flat focus plane, so it will naturally struggle with the curved virtual image from the dome. A moderate closeup lens will move the virtual image further away from the lens, making it easier for the lens to cope with the different distance to the center of the image and the corners of the image.

 

[Reefwalker]

My experience with the 9-18mm Oly lens behind the Zen dome port has been nothing but good. I never shoot wider than F8 and prefer F10/11, and just use higher ISO to compensate for less light. This gives sharp enough corners and plenty of depth of field. See some results from this lens/port combination at my Flickr site here.
You need to remember this is a rectilinear lens and so will never have perfect corners, a lot of the distortion present (underwater) behind the Zen dome is what I call "pulling", it is a distortion caused by the lens's rectilinear properties, a kind of stretching that occurs in the process of keeping vertical lines vertical in the image. This gets exaggerated in the corners when submbersed, as the optics of the lens are changed by the air water interface at the dome.
Also the corners of the image are mostly a lot further from the port than the centre of the image, so out of focus corners also comes down to depth of field issues, where the photographer should be focusing on something slightly further from the camera than the centre, then all of the image will be covered by the depth of field (out of focus corners are always exacerbated by using wide apertures). Personally I think Zen have done a great job with this port. I use it a lot for video and the results in the corners of the images are better than a lot of DSLR's I have shot with!
I have also tried using 'dry' diopters, but not to fix the corner sharpness, just so I could focus really close on large nudibranchs and get some background in focus as well. My experiments with +1,+2, +3, +4 dipoters all gave me incresingly shallower depth of field (what I expected and what optical science tells us, but I wanted to see for myself), and nothing further than 2-3 meters away was in focus. ALL diopters gave me softer overall images (all are Hoya diopters). All good fun, but not what I want for very photo with this lens. I even tried extension tubes.... but they don't allow the camera to focus at all with the 9-18mm lens.

 

[PHIL RUDIN]

Jeff is correct regarding closeup lenses like the +2 that everyone keeps advocating. You can't argue with physics, the job of these lenses is to magnify and as a result you need to be closer to the subject and the lens covers a smaller area. The same can be done by zooming the lens with better results regarding DOF and distance to subject giving a wider AOV. Jeff also points out the simple fact that many are overlooking, which is wide rectilinear lenses no matter how good they are ALL have soft corners behind a dome no matter how large the dome. In most cases the larger the sensor the greater the problem, that is why you at able to shoot at 9mm (Oly 9-18mm), a 100 degree angle of view lens behind a 100mm optical glass port while at 18mm also also with a 100 degree AOV on a 35mm size sensor needs a 220 to 230mm port to get the same results in the corners. Next is the cost issue, these ports were made for those using Olympus entry level cost wise housings. If a 1500.00 to 1800.00 wider ZEN port had been made instead in the hopes of getting better corners how many of you would have been willing to pay that price? If you truly need what you think are better corners you should try this lens behind the ZEN 170mm port, not only will the corners be a bit better but the AOV will also be wider. The laws of vertical images begin to change the closer the lens is the the inside of the port glass. The closer the lens is to the dome port the more it begins to act like a flat port rather than a dome. Unlike say the Panasonic 7-14 zoom or the 9-18 zoom with the ZEN 170mm port where the lens sits further back in the port the 100mm port is quite different in the the lens comes much closer to the glass.

The Olympus 9-18mm/ZEN 100 port is one of THE BEST values in underwater photography bar none. What other lens and optical glass port combo gives as good a value?

I have used this combo since the prototype with excellent results for its cost.

Attached is an image from 2010 taken with an Olympus E-PL1, 9-18 AT 9mm, ISO-200, 1/120th, F/9, two Inon Z-240 strobes in about 60 feet of water. 99.0% full frame and the corners don't look any worse than the 7-14 in the 170mm port or any of the dozens of wide lenses I have used over the past 50+ years.
Also a ZEN Underwater profile from 2010.

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