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I've recently been testing out an 18-55 Nikkor behind a flat port on a Sea&Sea/D50 housing.

Results at the 55mm end look fine.

At the 18mm end there is a definite smudged look at the image edges - at least that's my impression.

Is this just a result of using a wide angle behind a flat port - and would a dome port correct this problem? Initially I had not thougt 18mm was wide enough on a cropped sensor camera to exhibit this effect.

The attatched image from a well known UK quarry shows what I am referring to - note the horrible blurred left and right hand sides of the image.

Anyone care to comment?

In my experience 18mm is definitely wide enough to experience problems behind a flat port. The smudging also looks about right for that. Usually you get cyan fringing as well, which I can't see much of, but my guess is a dome port would be a large improvement.

Lanier

18mm is defiantly too wide to work behind a flat port. Anything wider than about 50mm (35mm equivalent) needs to be behind a dome or it starts getting mushy.

A dome port will correct things, but not entirely as zooms change their optical configuration whilst zooming. So behind a dome port they can only be optimised at one focal length as precise alignment is only possible at one focal length (there has been plenty of discussion about dome port placement on wetpixel before). That said, if you want to dive with the lens at its widest setting then a dome should help provided it can be correctly set-up. You'd need to check with the housing manufacturer as to what they recommend, or wade through all the discussions here on wetpixel to see if anyone has posted about your set-up before and can give recommendations.

Looking at that photo it looks like the edges you speak of to me anyway, are out of focus rather than a slight blur, the left side of the image, top and middle are in focus as they look to be in line with the fish head, where the rest to the right and bottom seem to be much closer to the lens, I would have thought (but and probably wrong ) that the whole edge of the image should have been blurry and not just some of it. I have shot my 20D in the past almost always behind a flat and didn't get this blur except under the circumstances just said above, but that's just my view, I suppose what I would do would be to take a few pics onto a flat surface which is the same distance away from the lens and then see if it still happens.

This was a 17-85MM shot at 17MM behind a flat port.

Yes, out of focus was my first impression too. The edges of the frame are roughly the same distance away as the fishes eye. The eye is not exactly sharp either but I'd say it is in focus. The edges though are horrible and this is evident in other images too. I was using F5.6 @1/90sec so depth of field would not be that great close up.
Very interesting to hear you've not had this effect on your Canon equivalent set up on a flat port.
More experimentation is need I guess.

Trevor

Who told you to put a 18-55 behind a flat port !

COMPACT DOME PORT, that will sort it out, save the flats for MACRO

craig

No Arguments please from anyone, i'll have none of it.. me heads in the sand on this topic

Hi again Craig,

At the time of trying out this particular lens and port it was all I had to play with. I could not even use the zoom as no zoom gear for it has ever been brought in to the UK. I therfore made 2 seperate dives - one with the lens at 55mm and the other at 18mm. I have no idea if I will use it again underwater. I'm just experimenting at present.

If I go to the Sea&Sea web site the recommended options for this lens are stated as follows:

NX Compact Macro Port Base (P/N #56201) + NX Compact Wide Port (56280) or
NX Compact Macro Port Base (P/N #56201) + Compact Wide Port L (P/N #30103)
* Also NX Standard Flat Port (P/N #56101) is compatible.
* Also NX Zoom Port (P/N #51101) is compatible.

As I see it these are all flat port combinations.

This is why I thought it worth a forum post. Clearly wet pixel members seem to favour a dome rather that what is stated in the Sea&Sea information.

For your 18-70 Nikkor, Sea&Sea info also lists the same compact wide port as above which as far as know is flat. How did you come to select the 40mm extention ring together with a dome rather than say the 22mm extention or the even shorter SX ring?

Hi Trevor

FORGET what the Sea & Sea website says, Most of its correct, but most of its incorrect also.

Like all things there are many different solutions to problems.

not all will be equal , some inferior some superior.

The Sea & Sea suggestions of Flat port will work.

But , Whats the point in taking a 18mm and slapping it behind a flat port to have it magnified by 25%.

NUMBER 1 rule in this game, GET CLOSE. cut out as much water from between your subject and your camera.

we can't step back as on land to capture an image as anything above 3ft away and we have so much water that the image quality is compromised.

(except those natural light WA of course).

so we have a lens bag that can allow CFWA (close focus wide angle) for the best quality in our images.

if you use a 18-55 behind a flat, all you doing is having to step back to do any wide angle work thus compromising the quality of the final image. it ends up being 33mm - 70mm lens with crop and refraction factor. fine for fish, but it suck for a general WA to Close lens

as for the extension ring M8,

its all down to experience and knowledge of what works and a ruler

the 40 extension ring refers to 40mm extension and the 22 - 22mm.

Trust me. It works.

i'm sure the 18-55 will serve you we'll as my 18-70 does as a general shooter for WA and larger fish stuff.

i really enjoy using my 18-70mm. for ultra wide stuff i've a 10.5mm nikkor and i use a 12-24mm F4 Tokina ATX-PRO

then for macro 60mm & 105mm.

regards

Craig,

Trevor , i live in staffordshire, if you wish to check out my system, you more than welcome to pop up and take a look and play.