WA distortion with 5D/Ikelite 8"

"it's impossible to get sharp corners with a dome port underwater" - No, you can but you will struggle to match the quality achievable from ultra-wide (weitwinkel) lenses above water! Have a good look through books showing wide-angle underwater shots and you will see that the majority which have sharp corners are shot with fisheyes. But it also depends on subject matter (blue water corners don't matter for example) and your own perception of what is acceptable. On a slight aside, if you look at some of the specialist lenses available for underwater video systems, you will see that this issue has been addressed but at a high cost. I've briefly looked into what might be available for stills (there are various designs) but cost would be prohibitive I suspect given the number of potential buyers of specialist high quality still ultra-wide (weitwinkel)-angles.

The diagonal fisheye, be it 15mm on Canon FF or 10.5 on Nikon DX cropped sensor, is a really powerful tool. Because its "focal surface" is curved it matches our dome ports better than any rectilinear WA lens can, preserving corner sharpness. If we want a rectilinear image, we can use PTLens or DXO to defish it.

What could be potentially very interesting for UW WA is the Tokina 10-17mm fisheye zoom, or in my case, a 14-24mm? Now I don't know precisely what shape focal surface the lens has, but it should certainly help out in the corners, at the wide end. I guess until Tokina or someone else do a full frame version, it'll be up to cropped sensor shooters to give it a try. Come on John, give it a bash!

Martyn

In VERY loose terms (and aimed at practicality rather than a purely theoretical standpoint): using a dome port produces a curved virtual image which images into the sensor plane as a curved image too. The bigger the dome, the further away the virtual image and depth of field will tend to 'take up' the curvature of image, so corners appear sharper. With smaller domes this is less likely so bigger domes will give better optical results.

This is interesting. Never thought of it that way, you have a hemispherical virtual image plane and if you have enough DoF, you can get it all in focus...

But I'm having trouble with that for two reasons:

1. doesn't the real world have a hemispherical virtual image plane? Aren't there rays of light coming from surfaces all around me into my lens?

2. "the further away the dome is, the further away the virtual image is". Consider two domes, a 3" radius and a 4" radius. Don't they both put the image at the same place, in the center of the dome sphere? does it matter how big the dome is? if the dome was in the right place, would it matter how big it was / could your lens "tell" how far away it was / could your lens tell how big a dome it was?

Still seems to me like there is a mismatch in radius of curvature and what the lens wants.

You cannot compare fisheye lenses and rectilinear wides as they don't operate in the same way and fisheyes will have sharper corners (inc the Canon on FF).

I hate to ask, don't want to make you write a tome, but what's the difference? convex front element, to match the curved wavefront coming from the dome?

What I come back to in this, is a point and shoot with an add-on wet lens like a wide angle inon, or video with an add-on lens, or even on land, when you add a "wide angle filter" or wide angle add-on to a compact camera or video. If you keep your camera's lens zoomed out, full wide, it works well. If you zoom in at all, the corners get distorted, the same kind of distortion we see behind a dome, for the same reason: The lens, in the corners, is expecting light to be hitting the front element at a certain angle. And it's not. It's too wide. You're too close to the dome, or you zoomed into the wet lens. To work at all, light needs to be hitting that front element at the right angle.

Roger

In answer:

A subject at infinity will produce a curved virtual image with its centre at a distance of 3 x the radius of the dome being used in front of the dome, so the bigger the dome the further away the virtual image - and on this point, the lens being used behind the dome must at least be able to focus to less than 4 x the radius of the dome (assuming it to be correctly centred in the dome) to focus at infinity underwater. The need for a diopter can be calculated from P (dioptric power) = 1000/4 x Radius.

Unlike conventional lenses which use rectilinear projection, fisheyes use different types. I think that I am correct insaying that the fiseyes we as underwater photographers use, utilise equidistant projection which essentially projects a hemispherica object image onto a flat (sensor) planr. However you can see for yourself by looking at published images that edge correction is not perfect - again probably as a result of using a simple dome in front of these lenses.

thanks.

I'll have to think about that.

I always think back to the simple ray trace diagrams, with the little vertical arrows as images.

http://www.glenbrook.k12.il.us/GBSSCI/PHYS...rn/u14l5da.html

What you are saying, in effect, is that if your subject is a little vetical arrow, the arrow in the virtual image will be bent over a spherical surface. Gotta think about that...

But if that's right, then I completely agree with your whole argument. There will always be distortion out in the edges. The problem is in the dome, not in the lens, and the best a lens can do is get the whole subject in focus. You need a special dome optic, as you said.

thanks.

Seems to me the gist of this discussion is that the following hypothesis is being put forward as being true:

Hypothesis: Using a dome you will always have soft edges when shooting wide.

Corollary: Choice of diopter, dome size or dome position cannot fix this.

Fortunately this hypothesis can be proved false by a single wide image taken using a dome.

However even finding hundreds of images with soft edges does not prove that the hypothesis is true, because other factors like diopter or dome position maybe to blame.

Surely not everyone with domes and shooting wide has been content with soft edges?

(Some posters here have I think claimed not to have any edge softness.)

So if anyone reading this has a wide image with sharp edges shot from a dome please post it.

Those with Ikelite housings may want to check with them on what port extension they are currently recommending for certain lens with their 8" dome.

I shot some pics with my Canon EF-S 10-22mm on a 30D last month and they had badly distorted edges at 10mm. After doing a little searching online I discovered I needed a 5011.22 port body. When I checked what my local distributor got for my 10-22 I saw it was model # 5510.81, which uses a 5510.16 port body! Brett at Ikelite also informed me that they have a new port body for the Canon 10-22mm the #0076.22. My dome is now currently at Ikelite for installation of the new port body and pressure testing.

Aloha!

JCharles

I'll accept some softness and color fringing on the corners. It's just the physics of wide angle photography. However, something (or a series of somethings) is clearly wrong with my system. I'm going to try again this weekend using higher f-stops and see if that improves my results.

"ortunately this hypothesis can be proved false by a single wide image taken using a dome."

By making specific choices when shooting it is possible to minimise or even negate soft corners (ie shooting planar subjects at a reasonable distance and using a small aperture, using optimised dome/lens placement and using an appropriate diopter) but you can't get around the basic optical problems of using ultra-wide (weitwinkel) rectilinear lenses with simple concentric dome ports - they exist and for many shots the results will be unacceptable by above water standards.

So in answer to your assertion above, a single image proves that you have minimised the problem for that image, not that the actuality (hypothesis?) is false I'm afraid.

However you can see for yourself by looking at published images that edge correction is not perfect - again probably as a result of using a simple dome in front of these lenses.

Hi Paul,

Having seen people's fisheye setups I think many of them put the entry pupil nearer the glass than the centre of curvature of the dome, largely to save bulk. I'd hazard a guess that it's often this misalignment that causes the softness and aberrations we sometimes see in print.

For example, doing "the toilet roll test" on my 8" dome shows that the entry pupil on my fisheye is about an inch forward of the centre of curvature of the dome. I should probably run it with a spacer, but then it wouldn't lock in place as securely, which is a problem on a big dome with a shade. Lots of good leverage there to help undo the baynet accidentally.

Martyn

Hi Martyn

I'm sure you're right - in the past there have been some amazingly small 'fisheye' ports in us including one which 'pushed' onto a Subal bayonet because its diameter was the same as the bayonet. Trouble is that fisheye's appear pretty forgiving if you don't look too closely, and dome port theory, and its associated problems, has only started to become more important as people demand the quality available from high MPixel sensors to be producable underwater.

In poorer visibility where CFWA has to be the norm, the softness is apparent more too. I'm starting to look into what remedies might exist for ultra-wide (weitwinkel) rectilinear correction - I suspect nothing cheap, but.........

A good thread, thank you all for you input. I'm not going to pretend, for even a nonosecond, that I'm

an expert here. I just need results and I'm not getting it from Canon when compared to what I've

seen others do in our Digital UW Workshops with a Nikor 10.5. We have the new Tokina 10-17mm

on order, but while it may prove to be better than the Canon 10-22mm I honestly don't think it

will compare to the Nikor 10.5. Wide angle is our "bread and butter" and I may have to switch

back to Nikon to get what I need.

My question: Why did I get sharp corners when shooting a Nikor 16mm on film with an Aquatica 8" dome

port? Yes, the corners were sharp as I've spent 30+ years with a Schneider 4X loupe looking at film on

a lightbox.

Someone at DEMA suggested the current problem had something to do with the corner edges of the

sensor??

Thanks,

Tom Stack

http://www.tomstackphoto.com

I did a few dives on an oil rig yesterday, using the 17-40mm and many of your tips, and I was happy with some of what I got.

f/22, 1/200, 17mm, no cropping

f/14, 1/160, 24mm, some cropping

I shot the entire day at ISO 800, so I could crank down the aperture, and you can barely see the grain. (I only see it when I upped the exposure 2 stops in the RAW converter.)

More Photos

So I guess this is a happy ending. I have more to learn, but at least I'm starting to get acceptable results. Thanks everyone and especially pgk for your help.

Seems to me the gist of this discussion is that the following hypothesis is being put forward as being true:

 

Hypothesis: Using a dome you will always have soft edges when shooting wide.

I'd change that.

I'd say I came into the thread thinking:

Using a dome you will always have soft edges when shooting wide, unless your lens can focus close enough to work behind the dome.

and Paul taught me that

Using a dome you will always have soft edges when shooting wide unless your lens can focus close enough to work behind the dome, and has enough depth of field to capture the whole image, which has some curvature to it.

Which is a different way of thinking about what happens when your lens can or can't focus close. Pretty interesting.

It's basically the same thing you're saying, but by adding the detail, you figure out how to fix it: focus close and have depth of field, or change the dome or some other optic to flatten the image.