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Effect on focus distance of being underwater


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#1 londonsean69

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Posted 06 March 2018 - 10:33 AM

Evening all (well, it is here)

 

Forgive me for what could be an obvious question, but my knowledge of optics is a little lacking.

 

I have just acquired a Saga +25 macro lens (as well as a +10) for use with a Sony RX100 (MK 1). I have some very small targets in mind :)

 

I have tried this out on the coffee table, and to achieve focus, the port is resting on the table. Naturally, I can see this being an issue UW - I am not particularly enthused by the prospect of squashing a nudibranch or pygmy seahorse or damaging their habitat.

 

My question is: How much if any, additional working distance will I gain by being underwater? I don't have access to a pool to test, although I guess I could always have a go in the bath!

 

Thanks in advance

 

Sean

 


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#2 carlyunderwater_55568

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Posted 06 March 2018 - 01:41 PM

Just as our eyes see underwater magnified at 25%, focus is as well.

Example: topside focus 1foot
Underwater focus 9”


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#3 londonsean69

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Posted 06 March 2018 - 03:53 PM

Hmm, not what I wanted to hear really. If that's the case, then the camera will be trying to focus inside the lens.


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#4 carlyunderwater_55568

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Posted 06 March 2018 - 06:17 PM

Hmm, not what I wanted to hear really. If that's the case, then the camera will be trying to focus inside the lens.

I’m not too sure about auto focus, but manual focus would be adapted to the above formula.
I think with auto it would depend on the camera and the AF zone choice.


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#5 londonsean69

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Posted 07 March 2018 - 12:10 AM

I have just asked Saga, who came back to me very quickly.
"Under the water you will have more distance, 33%, anyway that lens is to make very small motives and you have to get very close."
 
Other items I have read have also said that "In water the minimum focus distance increases and so does magnification so performance is all in all the same"

I need to do some reading up on optical theory.

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#6 Stig

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Posted 10 March 2018 - 01:33 AM

I found this useful when I was researching close-up lenses:

 

https://www.scubagee...es/wwwdiop.html

 



#7 londonsean69

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Posted 10 March 2018 - 12:40 PM

Thank you


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#8 bvanant

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Posted 10 March 2018 - 03:25 PM

The correct answer is it depends on the lens you are trying to magnify.

For example with the OLY 60 macro lens you have a 98 mm working distance without the diopter and a 22 mm working distance with the CMC (I don't have the saga data)

With the Panasonic 45 macro you have 71 mm working distance without and 17 mm with the diopter 

with fixed lens compacts you can get more working distance but with most micro 4/3 lenses you will indeed get less. A lot less.

On the sony RX100 mark 2 you will get a working distance increase of about 1.4 so you should be OK.

Bill


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#9 londonsean69

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Posted 11 March 2018 - 09:13 AM

That's the sort of news I wanted to hear :)

At such a close working distance, 40% extra would be amazing.


On the sony RX100 mark 2 you will get a working distance increase of about 1.4 so you should be OK.

 


Edited by londonsean69, 11 March 2018 - 09:13 AM.

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#10 davephdv

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Posted 11 March 2018 - 09:19 PM

As mentioned in that link, the index of refraction of water is 1.33, vs 1 for air.

Your lens will not be as strong due to this and will thus have longer working distance.
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#11 bvanant

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Posted 12 March 2018 - 09:14 AM

I think all of the good underwater lenses use only air/glass interfaces for magnification. The water glass interface is planar then glass/air so magnification is the same in air as in water. Working distance for the CMC clearly gets shorter not longer.

 

Bill


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