Corner softness with Ikelite dome port?

The effect due to the curved image is exactly one of the main reasons for corner softness with rectilinear wideangles as Paul has explained. Corners are more close in terms of the lens's focus distance and have to be covered by DOF.

The effect is increased by:

1) close subject distance

2) open aperture

3) small dome radius

Comparisons have shown that a well chosen dioptre can help to increase corner performance as the focus range is extended to a greater range towards infinity.

Fisheyes are better in this respect. Fisheyes do focus on a curved image. Perfect lens to shoot a dome’s curved virtual image.

Julian

So what do the rumors say about the possibility of an Ikelite 8" dome? (Ike please feel free to jump in here)

Is Ikelite ever going to get an 8" dome port? At some point I will need a dome port for the 10.5. I have lots of other lenses to occupy me for the time being. I would have serious buyers remorse if I bought a 6" port and Ikelite announced an 8" version a year later. But I also can't wait to try this underwater.

What have you heard? An 8" dome would be sweet for over/unders with the 10.5 and "possibly" improve performance with the 12-24mm. Is this even in the works?

David Haas used the 15mm Sigma on following: http://www.ikelite.com/web_pages/0photos.html

We installed 8" Aquatica domes on our housing and have seen no conclusive improvement with the 12-24. Have driven Peter Schulz nuts with testing, and have several more combinations for him to do. We will have some 8" domes available in a few weeks............

Ike, can you please test the 17-40L +- the 10-22 EF-S with the 8 inch dome? These are really the best two wide angle zooms for Canon crop DSLR shooters, and it would be great to get the underwater optics perfected for these. Will it be possible to use extension rings between the housing and the port to get the entrance pupil in the correct spot?

Yolly, does stopping down actually improve corner perfromance with a dome? I shot at f/8 on my christmas dive trip just in case it did, but from memory it didn't seem to make a great difference.

Oh and Ike, once you have the 8 inch dome sorted, have you noticed that there's rather a lot of people buying Canon 1 series cameras around here? 8 inch dome + Ikelite E-TTL2 1-series housing would be a good thing... (perhaps it could be painted silver).

We will have some 8" domes available in a few weeks............

For real? I am keen to hear what you learn, if you will offer these widely, and what the cost would be.

Regards,

Can I quote from one of my ex-lecturer's books ("Scientific Photography and Appled Imaging" regarding Dome Ports? The author Sid Ray states "if of wide-angle type, [the lens] produces a sharp image at SMALL APERTURES. Lenses with FOV [Field of View] up to 100 degrees and more can be used successfully with a large diameter dome". The capitalised bit is my emphasis. Basically I have found, as this statement confirms, that it is not possible to get good corner sharpness with lenses which exceed about 90 degrees (20mm on 35mm format) at anything other than small apertures (f/11~22). The exception are fisheyes. No matter what you try, the basic optical problem is that you are trying to focus on a curved image, and worse you are focussing on the furthest part from the lens. To bring the curved image into reasonable focus requires depth of field and although I haven't calculated it, I expect that you could show that mathematically, that any lens of over 90 degrees field of view has insufficient depth of field at wide apertures to bring the curved image into focus throughout, at anything other than near its maximum focus (ie where infinity lies as a virtual image). Incorrect positioning of lens/port wil no doubt exacerbate this, but regardless of the specific lens and dome this will probably hold true.

Can I quote from one of my ex-lecturer's books ("Scientific Photography and Appled Imaging" regarding Dome Ports? The author Sid Ray states "if of wide-angle type, [the lens] produces a sharp image at SMALL APERTURES. Lenses with FOV [Field of View] up to 100 degrees and more can be used successfully with a large diameter dome". <snippage by CeeDave

Yeah, unfortunately this widely-praised book is over $150...I am queued up for interlibrary loan, though.

I am cogitating a FE behind a dome ... how much does dome size matter for a FE? This is what I'm trying to figure out. Main reason for the FE: it's the widest fixed focal length lens out there.

Yolly, does stopping down actually improve corner perfromance with a dome?

Oh yes Rob, go ahead, shoot some images at different apertures.

If you focus on the image centre - the corners are alyways out of focus behind a dome. The corners a closer than the centre (curved virtual image). Without DOF no chance to cover the corners of a rectilinear wideangle behind a dome. Small dome = even worse.

Stopping down is the only chance to cover the corners with DOF. Or get a fisheye - they focus in a quite similar manner as the virtual image is shaped. Rob, as a Canon glass only user, can you imagine that even a Sigma fisheye beats your rectilinear L glass's corner performance behind any dome?

It's decent Ike facing bigger domes.

Julian