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Dome port and diopter


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#1 cmdasia

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Posted 02 May 2003 - 01:02 AM

I wonder if anyone could be kind enough to help me - I have a Nikon F80 in an NX 80 (c&c) housing and a Nikkor 20 mm lens - I have just picked up a S/H compact dome port and undertsnad that I also need to use a diopter on the lens - could anyone advise how I work out what size of diopter (or just tell what diopter to buy??)?

many Thanks

#2 davephdv

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Posted 02 May 2003 - 08:35 PM

The housing manufacture should give you the recommended diopter,
Dave Burroughs, Nikon D300, D2X, Subal housing, DS160 strobes

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#3 sully

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Posted 03 May 2003 - 06:09 AM

Hi Wolf eel,
Try using this tried and tested formula as follows:
f=2d
where f= focal length of dome
d= diameter of dome
so for a dome with a 20cm diameter, its focal length will be 2x 20= f40cm and its effect will be to act as a negative lens to the camera lens.
To compensate for this negative effect a positive supplementary lens of f40cm is needed.
these supementary (close up) lenses are often expressed in diopters.
to find the power in diopters divide the focal lenth in cm's into 100. ie a lens of f40cm will be 100/40= 2.5 diopters.
my gues would be that you require a dioptre of +4 but do the calculation.
best regards Sully.

#4 cmdasia

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Posted 04 May 2003 - 06:51 PM

Thanks Sully,

the calculation came back as no requirement for the diopter on this combo as the lens is able for focus closer than the resulting f38.

Many thanks!

#5 caveman

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Posted 04 May 2003 - 09:05 PM

our pleasure