16 replies to this topic

[AlexDawson]

Hi,
are there any good physicists & mathematicians in this forum that could help me figure out whether it's possible to create a dome that would make the Nikon 14-24mm work good @ f2,8 if shooting at +20 feet distance underwater? I have seen Mustard's 550mm dome and it looks cool! From the information I can find online and how I understand it would need a 43 inch dome Please tell me I am wrong!

kind regards // Alex

[blibecap]

My math doesn't come close to needing a 43 inch dome. What make you think that is what you would need. The minimum focus distance is for that lens is 0.9 ft. (0.28m). The minimum focus distance for the Nikon 10-24 is 0.79 ft. (0.24m) and it works behind a 5" dome. I am not sure but that may work with the 5" dome also and surely behind a 8" dome.

May I ask what housing you are using?

Bill

[PeteAtkinson]

This link has the no parallax point for the Nikon 14-24 if you scroll to the bottom of the page,
http://www.vrphotogr...lenschoice.html
I think the dome should have its centre of curvature in the same place - about 96mm in front of the bayonet mount flange.
Alex Mustard has had good results with the 16-35 behind a glass dome - far better than any I have seen from the 14-24 - so if you are after corner sharpness, I would copy Alex's set-up. At 20' away, you will need exceptionally clear water to get anything useful.
None of the domes I have are much good wider than f11 with the Nikon 12-24 and Sigma 8-16.
If anyone has found a way to get good results with the 14-24, I'd love to hear about it too!

[blibecap]

Can you document - about 96mm in front of the bayonet mount flange?


bill

[PeteAtkinson]

Just measured it approximately on my 14-24, looking at the picture in the link. Maybe Jean at Aquatica has the exact measurement since he has an optical bench...? Recommended Nauticam extension is 70mm...

[Alex_Mustard]

I've not used the 14-24mm in anger. Only tried it borrowed. I was put off buying it by the number of emails I've got from users all wondering why it is not giving them what they want. So I have never properly experimented with it, so don't have anything to add here.

That said I am pretty sure that no manufacturer has a great setup for it - since I have had emails from users of all the main housing brands complaining about it.

I have been using the 16-35mm for a long time, it was actually Ryan (of off Reef Photo) who put me onto that lens, and I have found it very pleasing and I have used it extensively. But I haven't tried it on 36MP (yet, where it may prove limiting), plus it is not quite as wide as the 14-24mm and it only goes to f/4. Although I have found it stabilisation to be effective in low light shooting.

Alex

[Paul Kay]

I don't think that what you are asking is possible with a conventional dome port which is of a viable size to use! You are trying to square (or at least flatten) a circle - and existing technology won't allow this to be carried out in any practically usable way. Conventional spherical domes are capable of 'good' results at up to a 90 degree field of view. Wider than this will require increasingly small apertures and will also depend on the lens in use and its close focus performance. I agree with Pete in that for ultrawide (weitwinkel) (weitwinkel) lenses f/11 is the minimum aperture that I would want to use for decent performance in the corners. Its also worth noting that at close distances there are a lot of optical problems which become more acute.

[AlexDawson]

Thanks for all the replies!

So larger dome, lets say 20 inch wont move the virtual image further away than my 8 inch dome? Behind my 8 inch dome the 14-24 focuses at 38cm.
All I want is to move the virtual image to + 50cm if possible. Has to do with the hyperfocl.

I usually shoot at f5,6 or wider for WA in dark waters...

Alex what dome do you use with the 16-35mm for best corner sharpness at wide apertures?

// Alex

[Paul Kay]

So larger dome, lets say 20 inch wont move the virtual image further away than my 8 inch dome?

20" is approx 50cm so the dome radius is 25cm, The virtual image of a subject at infinity is approx. 4 x the radius from the center of the dome which is 100cm.

Behind my 8 inch dome the 14-24 focuses at 38cm.

8" is approx 20cm so the dome radius is 10cm, The virtual image of a subject at infinity is approx 4 x the radius from the center of the dome which is 40cm so this is about right.

All I want is to move the virtual image to + 50cm if possible. Has to do with the hyperfocl.

You will need something of around a 25cm dome (50cm/2 x 4) , but this won't produce a particularly good image at f/2.8 I'm afraid as the virtual image of a subject at 20 feet will still be to curved for it all to be in focus at f/2.8.

[Viz'art]

Just measured it approximately on my 14-24, looking at the picture in the link. Maybe Jean at Aquatica has the exact measurement since he has an optical bench...? Recommended Nauticam extension is 70mm...

I measure up the optical center at about the same as the website given more or less a few mm.

Extension wise we recommend at 63.5mm

Paul pretty much sum's it up regarding the possibilities of this lens (thanks Paul )

[blibecap]

Alex
Is your 8" dome a true Hemispherical dome? Are the sides of the dome almost parallel? Is the depth of the clear part of the dome 1/2 the diameter of the clear part of the dome?

[AlexDawson]

20" is approx 50cm so the dome radius is 25cm, The virtual image of a subject at infinity is approx. 4 x the radius from the center of the dome which is 100cm.

8" is approx 20cm so the dome radius is 10cm, The virtual image of a subject at infinity is approx 4 x the radius from the center of the dome which is 40cm so this is about right.

You will need something of around a 25cm dome (50cm/2 x 4) , but this won't produce a particularly good image at f/2.8 I'm afraid as the virtual image of a subject at 20 feet will still be to curved for it all to be in focus at f/2.8.

Thank you Paul for a very good answer!

But wouldn't the 20 inch dome give me a better total in depth of field than the 8 inch dome?

on land at 40cm focusing distance f2,8 i get the sharpness between 34cm to 48cm.
and if I could focus on 100cm i would get sharpness between 70cm and 170cm and that sounds better to me?

Am I misunderstanding something? I do understand that i won't be able to get perfect corner sharpness at f2,8
but it should at least get much better?

// Alex

Edited by AlexDawson, 12 March 2012 - 09:59 AM.

[blibecap]

Thanks for the information on the entrance pupil / nodal point.

[AlexDawson]

Alex
Is your 8" dome a true Hemispherical dome? Are the sides of the dome almost parallel? Is the depth of the clear part of the dome 1/2 the diameter of the clear part of the dome?

I don't have any dome at this point, but I use to have the Sea&Sea acrylic dome Don't know if it was Hemispherical or not?

I don't even have a housing anymore since I am waiting for the D4 housing.
I have not decided what brand to go for this time. Sea&Sea, Hugyfot & and Subal are the brands I have used previous years...

// Alex

[Paul Kay]

But wouldn't the 20 inch dome give me a better total in depth of field than the 8 inch dome?

A 20" Dome would be a handful - if you could get one made. As for your dof calcs, well they vary depending on what parameters you are satisfied to use and I'm afraid that I'd have to spend time trying to sort out exactly what could be acceptable and then what dof could be expected - too long winded. Simplistically, yes, a larger dome should give 'better' results but there is a good reason why manufacturers limit the size of dome - usability.

[AlexDawson]

A 20" Dome would be a handful - if you could get one made. As for your dof calcs, well they vary depending on what parameters you are satisfied to use and I'm afraid that I'd have to spend time trying to sort out exactly what could be acceptable and then what dof could be expected - too long winded. Simplistically, yes, a larger dome should give 'better' results but there is a good reason why manufacturers limit the size of dome - usability.

Thank you paul. Is there any dome that you would recommend?

// Alex

[Paul Kay]

Thank you paul. Is there any dome that you would recommend?

// Alex

I would be biased I'm afraid!