Port quality

After deciding on a DSLR I waited to make the purchase until a housing became available for it. Price was the one, but not the only, motivating factor. The camera is now on its way but I'm thinking twice about the housing. I've realized that the port functions as a lens element so it's optical quality has to be considered.

Is there much variation in the optical quality of ports from the name-brand suppliers? Am I too concerned about this?

-Irwin

Hi Irwin,

yes there are differences. Planports for Macro do not cause problems in terms of optical/image qualitiy in general.

It is more an item when it comes to dome ports.

I don't know what camera/housing you plan to buy?

Julian

We discussed this at length recently in another thread, although I don't think we answered the question. Dome port and diopter are both items to consider in the optical quality equation.

Hi,

I have written some basic stuff on domes and diopters.

Maybe this can give just an idea.

http://hometown.aol.de/_ht_a/calypsophot/dome1.htm

Julian

Julian,

I think that your image of the entrance pupil of the lens is excellent. It shows clearly why this should be at the centre of the (imaginary) sphere perscribed by the dome. It is very useful.

I have linked your image below. Please let me know if you would like me to remove it, for any reason. But I am sure many people will appreciate seeing this image.

Alex

Hi Alex,

I am pleased that you find the drawing useful. I had some doubt that the words/image would be to theoretical and therefore just posted the link.

So no reason to remove. I am happy if it is of use for someone.

Julian

When you talk about 'apparent pupil', is this the same as the nodal point?

No, the nodal point is far behind the entrance pupil.

Pano shooters often try to find the "nodal point". The point they are looking for is correct. It is just not called nodal point. It is one of the most popular misunderstandings I did not realize that a while ago becuase I did not know the correct translation of nodal point into german.

The nowadays SLR wideangle lenses are built retrofocus.

You can see a retrofocus lens as a base lens with an integrated converter.

Otherwise there would be no chance to built a 15mm wideangle, the lens would hit the SLR mirror. This is also the reason why some viewfinder camera wideangles were better than SLR ones in the past, because the rear end can be more close to the film - no mirror (my loved Nikonos :roll: )

The center of perspective is located with the lens entrance pupil (in the "converter part" of the lens). This is where the radius of a perfect dome has to point to. The nodal points (front and rear one) are located with the "base" lens.

Thanks to all that provided a lot of in depth information.

My dilemma goes unanswered (and completely ignored on uw-photo and a couple of honest "don't really know"'s on Digital Diver) though.

The intended housing is an Ike DSLR. The first port will be an easy choice - for the 100mm macro. I'll also have a 17-40 mm L USM that Ike's web site says a diopter isn't required for it but I wonder if the image quality, dof, etc, would be enhanced with it.

To round out the lens selection I expect to later get a fisheye but I'm undecided on which. Opinions ?

Probably because there is no straightforward answer.

To a degree you get what you pay for. Good quality optical glass ports are always going to be optically better than plastic ones, but whether you can tell the difference when looking at your shots is another issue entirely.

More important perhaps is whether the port you use happens to adequately match the lens/camera combination as alluded to by Julian above. One of the reasons that U/W photographers raved about the sharpness of the Nikonos 15mm lens was because the port is a perfect optical match for the lens element inside. So it depends on the luck of the draw to a degree. I have always used Subal and I've had some port/lens combinations that were excellent and some that were not so.

Which housing (and therefore ports) you choose will be affected by a lot of other factors such as availablility of spares and service in your area, your own personal taste, and to a significant extent what you are are prepared to spend.

At the end of the day whatever you buy will be a compromise in one area or another and will have good and bad features. We all experience this and either learn to work around the faults or change for something else.

Good luck.

The intended housing is an Ike DSLR.  The first port will be an easy choice - for the 100mm macro.  I'll also have a 17-40 mm L USM that Ike's web site says a diopter isn't required for it but I wonder if the image quality, dof, etc, would be enhanced with it.

 

To round out the lens selection I expect to later get a fisheye but I'm undecided on which.  Opinions ?

I have the Ike 10D housing. My main lens will be the 17-40L lens you mention, for which I finally received my 330550 dome port yesterday. I've previously asked exactly the same question as you re: 1/port quality and 2/what does diopter do to image quality. I've also bought a NL4 B&W diopter (+4), so I'll test it with and without this.

Ike says his port quality is the equal of anyone elses, and I think this may be correct if you're lucky enough to get the 'apparent pupil' (I'm still not sure what this is on the camera. OK, it's not the 'nodal point', is it the aperture iris (you know the bit I mean), which is about 3cm back from the front element??) or whatever in the centre of the dome port radius. The problem is that one port is made for several lenses, so this isn't necessarily so. The 17-40L is an extremely sharp piece of optics, so with a well matched dome port it should produce great photos.

I've also got the fisheye port, but not the fisheye lens yet. I'm tending towards the Canon 15mm due to possibly better AF. They both use the same dome, but I don't know which one matches it better.

If you want to use a full frame fisheye (either a 15/16mm on a FF camera, or a 10.5mm on a 1.5x crop Nikon/S2) this is the most crucial lens to get the right port for. One you have a suitable FE port, then you can use it for other wide angle lenses (with appropriate extension rings). Most manufacturer's ports made specifically for the 17-40mm would not work well with a FF fisheye (not that one is available for 300D or 10D).

In terms of port optical quality I think this is most important for dome ports and much less so for flat ports. Like Andrew, I have always felt that manufacturer's that offer optical glass dome ports are superior to ones who offer plastic ones. My main evidence for this feeling is that my lenses are made of optical glass not plastic. That said I have seen my images beaten in competitions (many times) by pictures taken through plastic domes. Plastic domes are lighter for travel and scratches can be polished out easily.

But as Andrew says, if you have already decided on a housing manufacturer then you are stuck using only their ports. Each manufacturer has their own port system that is not interchangable with others.

I'd also add that seawater is a part of the optical setup for taking pictures underwater - and learning good technique (so as to remove its degrading influence) is far more important that small differences in port quality between manufacturers. That is why you will see great images taken with all sorts of UW kit (and also loads of poor ones).

In general zoom lens are more difficult to get setup for UW use that prime lenses. Indeed it has only been in the last couple of years that wide angle zooms have become popular. I bought a 17-35mm in 2000 and everyone told me not to bother housing it. It was such a novel thing to do that I even wrote an article about it for UWP Magazine!

Alex

It is difficult to compare dome optics of different manufactures before buying. I agree that normally the more expensive Domes designed for just a few lenses are the better ones. But no manufacture wants to tell how the dome is designed and optically adapted to the lenses (we may guess why ). Every housing builder just tells that their setups are optically superb and the best on the market. But that is not true in many cases.

There are two points giving you a good optical dome setup:

1: proper dome design

That means that the radius is pointing to the lens’s centre of perspective. And the dome curvature has an angle equal or wider than the lens field of view.

2: A big dome

The entrance pupil moves a little when focus is travelling (almost 1cm with Sigma 24mm WA). The difference will grow when a zoom lens is zooming.

A big dome will reduce those errors because the mentioned differences are relatively small compared to a big dome radius.

A big dome will also equalize design imperfections as stated above in point 1.

Also close focusing is improved with a great dome radius. And eventually avoids a dioptre.

Optical glass vs. acrylic is another question.

Acrylic domes have a refraction index which is more close to water. But I don’t think this will gain anything in real world. On the other hand they can suffer from reflections inside the dome. As Alex mentioned they scratch more easily than glass.

Personally the dome size and proper design is more important to me than the glass/acrylic issue. For example the big acrylic Sea&Sea Dome with 211mm diameter makes a good impression to me.

As Alex said, the domes are unfortunately bound to the manufactures housings.

Rob, the centre of perspective is at a different position with every lens. The almost “3cm” is the value Sigma Japan told me for this certain lens (in wide angle position and focus infinity).

Just think of the two elements your wideangle combines.

The base lens and the built in converter on top of it.

The base lens:

If you have a single peace of glass (like in school time for demonstration) there is one nodal point, the one peace itself. Camera lenses of course have several elements and are longer than just one peace of glass. So you have the front and rear nodal points (entry and exit of the unit). The space in between where the several elements are is some kind of no man's land. This includes the aperture.

But now you have the converter part of your lens put on top of the base lens. And this is where your entrance pupil and centre of perspective is.

Julian

I've also bought a NL4 B&W diopter (+4), so I'll test it with and without this.

Ike does say the diopter isn't necessary so my order didn't include one. But, I suppose, the images will have to be different somehow. After you do your test, would you please post a with/without comparison shot of the same subject?

I've never heard of "NL4" - what does that mean?

How did you decide on +4 ?

-Irwin

If you want to use a full frame fisheye (either a 15/16mm on a FF camera, or a 10.5mm on a 1.5x crop Nikon/S2) this is the most crucial lens to get the right port for. One you have a suitable FE port, then you can use it for other wide angle lenses (with appropriate extension rings). Most manufacturer's ports made specifically for the 17-40mm would not work well with a FF fisheye (not that one is available for 300D or 10D).

I don't think I need to won't have to worry about which fisheye I want as Canon only has one, a 15mm. Prehaps someone else manufactures a ~10mm EF mount lens?

Do I assume correctly that the 1.6x factor for the 10D affects the minimum focal length, not the angle of view which is listed as HxVxD 141°54' x 91°44' x 180°. But the angle might be affected by port and water ? I guess I'm wondering how much better the 15 FE would be over the 17-40 (assuming each has the appropriate, different port).

Also I would think the 17-40 may be more versatile than the 15. I don't really know, I know my wreck pictures with a wide angle (INON on a Ike housed 4040) were much better than without it.

The 1.6x crop factor will crop the angle of view by 1.6x. So yes, it does affect it.

Sigma also makes fisheye lenses 15mm and they are reported to be excellent.

Cheers

James

Hi!

Do I assume correctly that the 1.6x factor for the 10D affects the minimum focal length, not the angle of view which is listed as HxVxD 141°54' x 91°44' x 180°.

Actually, the crop affects the angle. You end up with an expensive piece of glass that's 1.6 times less fun than it was on an SLR. ;-( You may compare it with a crop done in PhotoShop.

But the angle might be affected by port and water ?

A proper dome should prevent this. A flat port (as used with long focal length lenses) is easy to build but distortion happens towards the edge. You wouldn't mind this with a 50, 60 or 105 macro lens but it'll turn your 28mm wide angle into a 35mm (approx.) compared to dry land. Oh no, not another crop factor! The dome prevents distortions and keeps the angle. Huh, that was close.

Couldn't they flood the DSLR bodies and build a dome in front of the chip to get rid of that crop factor?

(serious again?) Ideally, you'd have a dome port for every single lens, designed to the used focal length. Macro lenses might turn out to be tricky as they move (at least) the front lens elements pretty far during focusing.

I think that some advantages of acrylic ports are the weight and the fact that a scratch (which most probably will occur on the outside) will be filled by water of a similar optical behaviour and so may have nearly no effect on the image at all.

Helge ;-)=)