I have the "enhanced 180° viewfinder" which fits into the back of my Nauticam 5dmIII housing perfectly (below), but I want to use a small viewfinder to reduce the housing size and weight.

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I thought this Nauticam Replacement Optical 0.66x Viewfinder was the correct one:

https://www.backscatter.com/Nauticam-Replacement-Optical-0.66x-Viewfinder

 

However, when I try to use the standard viewfinder there s a few mm of space around the viewfinder. My housing has a hole diameter of 32mm and the standard viewfinder has a diameter of 28mm:

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Is there another metal round piece that I need to buy that fills the gap around the one for the standard viewfinder or did I buy the wrong viewfinder? The only metal piece I have is threaded and goes on the 180 degree viewfinder so it doesn't work on the standard viewfinder.

 

This is an older housing and I expect probably different arrangement to newer ones.  Typically there is a metal insert that remains in the acrylic window and they should be standard sizes.  

 

I assume the standard viewfinder is the one that came with housing as original equipment or did you you purchase it to use with your housing?

 

Normally you remove the viewfinder by removing an o-ring inside the housing and slide it out of the metal insert.  You should be able to do this on your housing.  Typically you don't unscrew anything to remove the viewfinder.

 

Now that you have unscrewed the insert you would need to install it back into the window and also make sure the seal to keep water out is sound. 

 

First check the view finder you should see an o-ring in a grove that you can remove to allow you to pull out the viewfinder.  Carefully remove this o-ring and try pull out the viewfinder.

 

Next check the insert to determine how it is sealed - probably an o-ring - check this carefully and re-assemble. I would suggest maybe talking to a store like backscatter who services housings and tell that what you've done and ask if there are special arrangements to ensure it seals between the rear window and the insert.  

 

this instruction manual for viewfinders might help:  

 

32201 32203 32204 32205 Instruction manual for viewfinders (nauticam.cn)

Thank you so much! After reading your post and talking to Russ at Backscatter the normal viewfinder is in. I didn't realize the silver colored metal piece on my 180° viewfinder is supposed to stay in the housing. Once I removed the retaining O-ring and put that back on the housing everything fits as it should.

 

The 180° viewfinder came with my housing and I had purchased the standard viewfinder afterwards so assumed it was just a complete swap. I didn't realize the insert came off and lived with the housing and having been trying to figure this out for the last month. If others run into this in the future, this video was helpful: