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Paul Kay

Member Since 27 Nov 2003
Offline Last Active Today, 07:46 AM
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Posts I've Made

In Topic: Wetpixel's Nikon FX wide-angle lens review

07 May 2018 - 10:12 AM

The virtual image is only spherical for an infinite object distance. It is otherwise aspherical. The source for this fact is Chapter 8 in Optical Fundamentals of Underwater Photography by Gomer T. McNeil, 2nd Ed., 1972.

And for a subject at infinity the virtual images lies at a point which is 4 x the radius of the dome from the centre of the dome which is where the principal point of the lens should be positioned. This means that the centre of the mage is at 4R but because the virtual image is spherical, depending on the lens's angle of view, the edge of the image will be somewhere closer - exactly where will depend on its position and the dome's radius (radii) and thickness and refractive index - its complicated. But the camera lens is not much of a factor here because it can only image what it 'sees'. Its ability to do so will be dictated by the virtual image produced by the dome port and subject and any inconsistencies of the camera lens such as field curvature at closer focus. Its all a bit messy.

 

The 'less curved' dome idea is in effect the suggestion of positioning the camera lens closer towards the dome as opposed to ensuring it is aligned at the centre of the dome. You may want to try doing just this (should be easy enough). If I remember correctly, I think that you will find that it results in the trade off of reducing the field of view as opposed to doing what your diagram illustrates so is counter productive.

 

Sadly I suspect the 'filmdays' rule of thumb of 90 degrees being the maximum viably/easily correctable field of view still applies with dome ports though bigger does help .....

 

I still disagree about the unpredictability of lens performance underwater behind dome ports. Its lack of technical information which hampers prediction nothing else.


In Topic: Seacam S6 -> Nikonos bulkheads (WTB or any advice...)

03 May 2018 - 02:35 AM

Just one question (sorry, maybe its basic, but to confirm), if I connect Nikon D800 through S6 bulkhead with Seacam S6-S&S cable and Inon Z330 strobes, will I have TTL available? Or do I need to purchase separate TTL circutry eg turtle...?

 

Sorry Marco but I simply don't know. First you will need to check if you have any circuitry in your Seacam housing - it may be fitted already - it will be under the metal cover in the 'prism housing' part of the housing and is accessed by one screw. If so then the housing will support dual TTL with Seacam strobes, but I can't give you any information about compatibility with Inon Z330. You may need to remove the circuit board if its not compatible. I suggest that you post in the dSLR section here on wetpixel. If you don't get any answers then contact your Seacam distributor (http://www.seacam.co...roatia-slovenia) or Seacam direct. You will also need to check the Inon specifications - its possible that a fibre optic solution may actually be better and Seacam offer an Optical Slave Trigger at around 280 Euros too. Sorry not to be able to tell you but there are now so many combinations that I tend to deal with them on the basis of needing to when they arise.


In Topic: Seacam S6 -> Nikonos bulkheads (WTB or any advice...)

02 May 2018 - 11:23 PM

OK, some info. The price of the S6 (M) to N5 (f) adapter from Seacam is in the region of 180 Euros including tax (this is vat at around 20%) so its not as expensive as most Seacam S6 fitting cables but it isn't that cheap either. Be aware that you will lose TTL potential if you go for N5 cables. The 'newer' S6 sockets fitted to housings like the D800 have the spanner flats on their inside (this makes their external profile lower). I have an expensively specially adapted spanner which Seacam supplied me with to undo the sockets without causing any damage - its possible with an ordinary spanner which fits correctly, but not so easy and it can cause damage if it slips. I am wary of suggesting that you remove and refit sockets without the appropriate tool unless you are confident of the care you can take in doing so. (The good bit is that the internal cables clip on and off the circuit boards using a simple connector)

 

Also appreciate that S6 connectors are (in my very practical experience) far more reliable than N5 because their design is fundamentally different and they are being used for the job intended. N5 connectors are being asked to do far more than their original design brief and their Achillies Heel is the two sprung pin system which causes grief if their is any ingress of salt water - I have replaced numerous N5 sockets but no S6 sockets due to failure. (N5 sockets can be repaired sometimes by replacing the innards if you can find a repairer who does this).

 

So whilst its a costlier option I would strongly recommend getting S6 cables from Seacam if you can possibly afford to due to their reliability (caveat - yes I'm biased because I do sell Seacam gear but this is a genuine observation based on my own personal experience - I use S6 and I simply would not go back to N5). I do appreciate that S6 cables from Seacam are expensive but I have found them very reliable and because of their construction they can even be rebuilt or their connectors reused should different cables be needed at a later date.


In Topic: Wide Angle Lens Recommendations

01 May 2018 - 11:24 AM

Optics really isn't hit and miss. There are good reasons why some lenses work better than others behind a port but there are a lot of variables too. The problem is probably that some 'flawed' lenses probably have obvious flaws and not so obvious ones like the production of a curved image field which paradoxically, can help when imaging a curved virtual image. My point being that you can't negate the obvious flaws. Back in the distant past when diopters were fitted to wide angle lenses behind dome ports it was said that the cheaper diopters with one convex and one flat surface were the better ones to use because they induced curvature into the image - but they still weren't very good diopters.

 

This is my take on modern wide angles and their optics. Many high quality modern wide angle lenses incorporate 'close rage correction' (CRC) mechanisms which serve to increase image 'quality' as the lens focusses closer and part of this probably results in a flatter image field. Hence why an older lens without CRC may exhibit better apparent image quality underwater behind a dome. But it will still be an older and less well corrected lens so will limit image 'quality'.

 

To predict image quality behind a dome it would be useful to work a testing system which checks the flatness of image field at the focus settings used behind a dome

 

Perhaps instead of trying to develop better underwater optics we ought to try to figure a way of creating more curvature in the image field for lenses fitted behind domes? Disabling the CRC if possible might help.


In Topic: Wide Angle Lens Recommendations

01 May 2018 - 03:52 AM

Surface performance is (at best) a poor indicator of how any lens will perform behind a dome port.

To an extent I would disagree with this Adam. I think that topside performance combined with relevant technical data can indicate whether or not a lens may perform well underwater. In my experience mediocre lenses never do perform better as a result of being used underwater.