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Dome calculations - how to increase your near focus behind a dome


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#1 3ricj

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Posted 30 March 2014 - 04:48 PM

Hi there folks, 

 

I wrote up some details for how to "fix" your focus on a lens behind a dome port.  I went into this not knowing how this works, and thus tried to document what I learned in order to give back to the community. 

 

http://www.substrobist.com/?p=33

 

I welcome any comments, questions, flames, etc.   

 

My core motivation was to increase my ability to focus up close, while also being able to take advantage of the full range of focal positions of my lens under water.    I am mostly excited that I think I will be able to focus closer without loosing hyperfocus performance using a +2 diopter.   Yay, science. 



#2 Stuart Keasley

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Posted 30 March 2014 - 11:03 PM

Thanks for sharing!

I may have misread your write up wrong (apologies, I haven't had my coffee yet :)), however it sounds as though you have the dome port flush against the housing?

A fourth option, to improve your camera and lenses ability to focus on the virtual image, is to move the dome port and therefore virtual image further away from the sensor using extension tubes on the housing. The added benefit here is that it won't effect your lens characteristics. You need to choose an extension tube that pushes the dome as far away as possible whilst ensuring that it doesn't restrict the FoV or cause vignetting at the widest focal length of the lens.

Re lens choice, not sure if you'd be interested in this, as it's not for full frame, but if your happy to run your A7r in crop mode whilst you're waiting for better lens choices, have a look at the Sony Zeiss 16-70mm. Min focus is circa 35 cm thought the whole focal range, we use it on our FS700 for underwater work, very pleased with the results.


Edited by Stuart Keasley, 31 March 2014 - 02:07 AM.

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#3 Interceptor121

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Posted 31 March 2014 - 12:50 AM

I may misunderstand your values but if I look at dome port theory with a dome of 4" diameter you should be looking at an external radius of 2" which is not on the scubageek tables.

If your dome thickness is really 0.125 (seems very thin to me) then you are looking at 7.6" curvature point to secondary focus point and assuming your centre of curvature is aligned with the dome (or otherwise you need an extension ring) you are looking at 5.15 diopters correction for an infinity focus if your lens can't focus at 7.6"

generally the theory helps only to an extent as the centre of curvature may or not be accurate and the back-focal distance may be unknown so there is still a lot of trial and error

I would be really surprised if you bought something out of nauticam port system and is not focussing


Edited by Interceptor121, 31 March 2014 - 01:53 AM.

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#4 Interceptor121

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Posted 31 March 2014 - 02:31 AM

I have looked at your site you seem to have an A7 camera this should be your lens

http://www.sony.co.u...t/SEL35F28Z#Faq

This lens should work fine with the 37122 4" wide angle port. This port is not exactly a dome in itself has it has only a curved glass to restore your air field of view

So the idea is that you use this lens in the housing as you do on land and focus at 0.35m and further away, this set up won't focus closer than you do on land.


Edited by Interceptor121, 31 March 2014 - 02:33 AM.

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#5 3ricj

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Posted 31 March 2014 - 07:08 AM

I may misunderstand your values but if I look at dome port theory with a dome of 4" diameter you should be looking at an external radius of 2" which is not on the scubageek tables.

If your dome thickness is really 0.125 (seems very thin to me) then you are looking at 7.6" curvature point to secondary focus point and assuming your centre of curvature is aligned with the dome (or otherwise you need an extension ring) you are looking at 5.15 diopters correction for an infinity focus if your lens can't focus at 7.6"

generally the theory helps only to an extent as the centre of curvature may or not be accurate and the back-focal distance may be unknown so there is still a lot of trial and error

I would be really surprised if you bought something out of nauticam port system and is not focussing

Excellent catch!  Thank you.   But now I'm confused, as my lens shouldn't have been able to focus at 7.6".  But I *was* able to get photos in focus (using autofocus) but only if I got VERY far away from my subject.  I started down this path after having really poor results - - but results!  It seems that my lens isn't very close to the dome, so this may be impacting things - -  But yes, I'm using the Sony A7r, 35mm F2.8 FE lens and the 37122.4 port.  This system (and port) is very new, I'm suspecting it has been tested much underwater.  Nauticam has not shared specifications on the dome, so I had to measure it.  The thickness was determined by measurement, I confirmed it was a simple radius (not some parabolic whatsit), and material I determined by using my highly calibrated index of refraction measurement tools (my eyeballs, who deal with acrylics and glass frequently, but could have easily made a mistake). 


Edited by 3ricj, 31 March 2014 - 07:09 AM.


#6 Interceptor121

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Posted 31 March 2014 - 07:15 AM

Nauticam are very good if there is an issue with the product. I would contact them and ask what's the expected behaviour. I had a port modified by providing feedback and they will so their best

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#7 3ricj

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Posted 31 March 2014 - 08:00 AM

Thanks for the feedback.   Maybe the right answer is to contact nauticam. 

 

I was thinking that maybe they just forgot to tell me I needed a diopter. :) 

 

Anyway, I've just posted two photos for conversation.  They are both bad photos, but posted for conversation.  Incidentally, the first shaggy mouse nudi' I've seen. 

 

The shot I wanted: (well, honestly, I really wanted a macro lens, but maybe I should say "the shot I was expecting this lens to be able to handle")

http://www.flickr.co...cj/13540155013/

 

The closest I could focus: (I was at least one full body length off the bottom, so .. 6'? plus fins? )

http://www.flickr.co...cj/13540388354/

 

I was hoping that the sony would report the focal distance  -- my 5d did this, but I can't seem to spot it in the exif data. 



#8 3ricj

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Posted 31 March 2014 - 08:05 AM

How do I adjust the calculations of a virtual image on a dome port based on having a different position of the exit pupil on a lens?   Is it time for some real optical modeling? 



#9 Interceptor121

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Posted 31 March 2014 - 09:30 AM

Well first if the port is not behaving as expected you should talk to nauticam. The next thing to try would be an extension tube but am not sure if you can fit that in. Also that would move the focus point and I don't know how that dome port would behave??

I have now looked at your pictures what I would do is go into a pool and take pictures at various distances from the wall and see where it can't focus anymore

This is not a macro lens and nudibranches are not going to be part of the mix, it is a 35mm prime lens for portraits and similar


Edited by Interceptor121, 31 March 2014 - 02:22 PM.

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