Porro II system
Posted 25 November 2013 - 01:07 PM
Porro II erecting system in binoculars is not used much. Could anyone explain what are the benefits and disadvantages of the Porro II versus the well known Porro I system?
Posted 25 November 2013 - 01:21 PM
The disadvantage have found is that there is a lot of unsupported mass when the two prisms are cemented together.
This can result in separation of the two parts, or worse a fracture.
There have been Porro 2 binoculars made recently, I'm sure someone will remember the manufacturer.
Posted 25 November 2013 - 02:59 PM
What else does this gain? The accommodation of fast objectives without aperture reduction. When the focus is moved rearward (as when getting clearance because if the Porro I's adjacent prism apex), the effect is to shove the prism farther forward into the light cone, thus causing clipping of the cone at the front prism aperture. This is the predominant case of commercial bino aperture reduction. In such cases a longer focal length objective should be used.
An example of the Porro II is the Vixen Ascot 10X50. It uses a 22mm prism and two mirrors, and yet accommodates a 'massive' eyepiece having a 24.5mm field stop (and rather larger barrel), and an f/3.3 objective with focal reducer (to get an f/3 beam at the focus.)
Posted 25 November 2013 - 03:59 PM
Porro II has less air spaces than Porro one when the two prisms are cemented together.
Interesting that you write "the two prisms". My 77 mm Miyauchi Exceeds have Porro II prism clusters that are made out of three prisms cemented together. Such a in the first and second pictureres on the webpage here: Link to web page
Not mine, b.t.w. The Miyauchis on that page, that is...
I have no idea how the prisms in a Canon IS are built.
About the terminology:
Bill Cook once told me that to him, Porro II was an odd designation, and the proper term is Porro-Abbe (or Abbe-Porro).
Posted 25 November 2013 - 04:13 PM
I can see how the prism halves would have to be made/shaped to form a 2-piece Porro II (or Porro-Abbe, as I'm absolutely sure Bill knew what he was talking about), but I would guess making it out of three triangural prisms would be easier as far as fabricating the components goes?
And I say this fully knowing that you, Simon, have seen, examined, opened and repaired several more Porro II binoculars than I have ever laid eyes on.
Posted 26 November 2013 - 01:07 AM
Posted 26 November 2013 - 08:31 AM
Here's a good explanation of the Porro (Porro Abbe) II system: http://en.wikipedia....orro–Abbe_prism
Prior to WW II the British Admiralty chose the Porro II with field lens cemented to prism Barr & Stroud CF41 7X50 as its primary naval binocular. During trials it was tested against other binoculars including the famous Zeiss Binoctar 7X50 (uncoated version - the Germans were developing AR coatings at this time but it was a guarded military secret)and the CF41 was found superior because of light transmission and consequent performance under low light conditions: http://www.flickr.co...-7HwZK1-d5vv...