Hi Simon,
I also have an MDX-D300 and several lenses and ports, and been using Sea&Sea (amazing maker) for the last 10 years.
I will try to give you my humble opinion on the matters you rose:
"I currently own 2 ports for my SEA&SEA MDX D300 housing, the NX fisheye dome port and the Macro port 50 including base. What I'm interested in is how to evaluate port compatibility for future lenses."
I believe it is the NEW DX Macro Port Base and DX Macro Port 50 for you NEW AF-S 105mm/2.8 VR right?
Sea&Sea does extensive tests and has an extremely high standard for image quality in it's previous tests before releasing an "official port configuration".
But it is also OK to say that due to market driven reasons the choice of ports/lenses can be improved upon with some tests.
One good parameter to check that is the Athena compatibility table.
Their big dome can be compared to the NX Fisheye Dome, and the smaller one is about the same size as the Compact and Optical Dome Ports.
Other than that, the min. focus distance + lens length are the best parameters, but I would double check it in a pool with a couple of ERs.
I recently bought a simple Nikon 18-55mm DX VR lens after reading through Ken Rockwells recommendations and since this is the focal range I had nothing for under water so far, my other existing lenses being the 60, 105mm macro Nikons, the 10.5mm fisheye Nikon and the 11-16mm Tokina wide angle.
I also love Ken and access his site almost daily, but you gotta filter some of his thoughts because he is definitely not an uwphotographer.
That said you are pretty well equipped for uwphoto with your quiver, and your 18-55 (great value lens!!!) is probably a better addition for your topside work.
But it is OK to use it underwater if you want, on some dives that I know very well I like to dive with a mid-zoom sometimes, specially on some wrecks to register the details.
Now if you are to believe the Sea&Sea port chart, there is no less than 3 items I need for this kit: custom zoom gear (makes sense), another port base and yet another port, the compact wide port. I am a bit put off by the thought of buying yet another port and simply tried the lens at 18mm inside the dome port which works fine on land, at least I couldn't see any distortion. I had previously done the same for the Tokina lens and it also worked fine from all ranges 11-16mm inside the dome port.
You really CAN believe the Port Chart, but there are a few things to remember:
- Zoom lenses in dome ports (not on the port charts) should be checked underwater, a pool is OK to do that
Why? Zoom lenses "normally" have some problems to be used inside domes:
1) Minimum Focus distance might be too far to use it inside a dome underwater, and you might need a diopter (close-up lens) to use it (the less powerful the better).
Domes produce a "virtual image" very close to the dome (see this) in which your lens focuses (if you focus on infinte underwater with a lens on a dome and you look at the lens focus distance window, you will see it is actually focusing a few (2-3) feet away!!!).
2) You would like to try to place the lens at the best spot in the dome, but as the lens zooms if it extends too much it might get out of this spot and presents some distortion (out of focus areas) on the borders. So you might need one kind of ER (only testing will tell which one, but you might get a visual clue on which is best).
3) If you used it behind a flat port (as S&S suggests) you don't need a diopter but loose a lot of it's wide angle capability (33% more) and your 18-55 becomes a 24-73 (or 36-110) with very much of its WA capability lost (but still good for several sizes of fish).
So do I really need another port? Obviously if I don't have the zoom gear, I miss out on adjusting for a particular focal range which could be helpful underwater (keeping in mind to not use it as a zoom and still staying within 1m of the subject), especially when it comes to framing bigger objects.
Only you can tell which you would prefer, I think that for the Tokina the dome is a better option as not to limit its WA characteristics, and you might make a zoom ring in plastic or metal (Fun-In makes zoom rings for Sea&Sea for extra lenses, you might give them a call).
I have tried the lens at 55mm on land inside the dome port and it looks fine, however is there any distortions to be expected from the dome port with such a lens if you dive? I obviously haven't just tried it yet, so I was curious what you guys think?
Do you always believe the port charts or do you just put your gear in the housing, see if it fits and go diving?
I do believe the Port Chart. But it is very helpful to understand the physics of UW Optics.
Buying ports can be a bit of a money grave, on the other hand I don't want any optical disadvantage either.
Yeah, I know... I have the Optical Dome, NX Compact Macro Port Base, 3 Compact Macro Ports, and 2 ERs (40 & SX)!!!
Are there any hard requirements as to the shape of the port and the distance the lens has to have from the port end (other than obviously the lens being short enough to fit in and not scratch the port). Does the curved surface of the dome port have much influence on the image quality and at what fov? (does this only come into play at ultra wide (weitwinkel) (weitwinkel) (weitwinkel) angle?). I do realize that the wider the field of view, the wider the port opening needs to be, else black edges. Is the opposite true as well? Can the curved dome port have neg. effect on the image?
Yeah, lots. Focus distance and distortion at the borders (check the link I gave you above).
Many questions, I know.
Hope I helped on some, if you have any further you can return here or check my website, I have some info on Sea&Sea Ports on my "News" sections.
Will be renamed to "Articles".
Seeya
