[spacepilot]

I was looking for a way to test collimation of my binoculars that doesn't require me to look through them. Frankly, I can't detect any image separation using the common methods by looking at terrestrial objects. Looking at stars might yield more definitive results, but I don't have a tripod. Then I found this, using the Sun as the collimator and measuring the position of the Sun's images to calculate angles of misalignment.

http://sites.google.com/site/rchamon...-of-binoculars

I just carried out the simplified version (section 4.1), which doesn't require a tripod. I asked my wife to take pictures of the Sun's images, and later measured the misalignment angles in the computer file. It turns out that, out of the two pairs of 8x that I tested today, one has a 11' vertical and 6' horizontal (convergence) misalignment, the other is spot on vertically and misaligned horizontally by 16' (convergence). Figuring in the magnification, both pairs are well within the tolerances listed in the link above and Ed Z.'s post on Cloudy Nights. That explained why I couldn't see the misalignment before.

The simplified test is really easy to do. For roofs, I think it is simple enough to carry out even in the field by trying to align the Sun's images with the shadows of the barrels. The test may be a little harder harder for porros because the eyepiece shadows might not stand out. Either way, I found the quantitative results this test gives very helpful. Now, if only there were a similarly simple and straightforward way to test my diopter setting .

I did a search on BF and didn't find a thread discussing this method. Has anyone tried this before?

[LPT]

I have used this method for several years to collimate or else (if I'm impatient) conditionally align at least 50 Porro I and II binoculars. It works very well. Also, it's very easy to make the apparatus.

[elkcub]

Spacepilot,

That you for posting this fantastic article!

I'll probably build the apparatus for advanced elder play.

Ed

[elkcub]

Brock,

Note that there is a method for incorporating links in BF text.

Ed

[spacepilot]

Although I'm not as handy as I'd like to be, I'll probably make a simplified version of the rig this winter when it's too cold to go out. LPT, what are Porro I and II binoculars.

[John Dracon]

Spacepilot - I'm adding my thank you for your citation on collimation with the sun. It is a wonderful reference article for do-it-yourself folks like me. Would two bright lights fixed in front of the objectives in a darkened room work as a sun substitute? I'm going to give it a try. John

[LPT]

I have read that two lights in a darkened room do not work because this method requires the light source to be at infinity. In the winter I set up inside and use light coming through the window. A little unexpectedly, I have found that sunlight passing through window glass does not seem to affect accuracy in any way.

[LPT]

Originally Posted by spacepilot

Although I'm not as handy as I'd like to be, I'll probably make a simplified version of the rig this winter when it's too cold to go out. LPT, what are Porro I and II binoculars.

Basically, in a Porro I binocular the prisms are arranged so that the objective lenses and oculars are offset giving a stereoscopic view. In a roof prism binocular the prisms are arranged so that the objectives and oculars are in line. A Galilean binocular does not use prisms. A Porro II binocular is like a Porro I in that objectives and oculars are offset, but the prisms in Porro II binoculars are cemented together to improve light transmission by reducing the number of light reflective air-glass surfaces whereas the prisms in a Porro I are usually separated by a divider. The downsides to Porro II's were cost of manufacture and durability. The widespread introduction of good anti-reflective optical coatings was the death knell for Porro II's. Here's a picture of Porro II prism cluster (with ocular field lens also cemented to prism to further improve light transmission):
http://www.flickr.com/photos/binocwp...7623234405689/
This link explains the difference between roof and Porro prism designs:
http://en.wikipedia.org/wiki/Binoculars

[PatrikIver]

Originally Posted by LPT

A Porro II binocular is like a Porro I in that objectives and oculars are offset, but the prisms in Porro II binoculars are cemented together to improve light transmission by reducing the number of light reflective air-glass surfaces whereas the prisms in a Porro I are usually separated by a divider.

Not quite the full story...

The Porro I prism generally are separate and uncemented, but they could just as well be cemented together and still be called Porro I. A Porro I configuration looks like the arrangement in the Wiki article referred to.

Compare that to the Porro II cluster in the other link, and you will notice that the Porro II cluster is made up of three prisms, one large in the middle, and two small one at the exit and entry (the smaller ones can be differently sized with the one closer to the objective being larger due to the lightcone shape, also Porro I prism pairs may be unequally sized).

I don't think cementing (or lack thereof) has anything to do with the distinction between Porro I and Porro II, the difference is in the geometry of the lightpath (which is different, enter from the front - turn sideways - turn to front - turn sideways - turn to rear in the Porro I, and enter from the front - turn sideways - turn sideways - turn sideways - turn to rear in the Porro II).

One practical difference is that, as the Porro II cluster shape is more "flat", the eyepiece focal place can be positioned closer to the prism cluster (in a Porro I configuration the rear prism would be in the way), which should mean that the cluster can be made smaller and lighter for a given fieldstop size.

As usual, I expect some real expert will correct me if I'm wrong in the above description...

With best regards,
Patrik Iver

P.S.
Does anyone know of any Porro I binoclars with cemented prisms?

[henry link]

Originally Posted by PatrikIver

P.S.
Does anyone know of any Porro I binoclars with cemented prisms?

A few come to mind. The US Military M19, the Swarovski SL series and the last CZJ Nobilem series, some of which continue under the Docter brand. Presumably the Docters are still cemented.

[LPT]

Originally Posted by PatrikIver

I don't think cementing (or lack thereof) has anything to do with the distinction between Porro I and Porro II, the difference is in the geometry of the lightpath (which is different, enter from the front - turn sideways - turn to front - turn sideways - turn to rear in the Porro I, and enter from the front - turn sideways - turn sideways - turn sideways - turn to rear in the Porro II).

This is very interesting and not only explains why some Porro I type binoculars can have their prisms cemented together but also why some Porro II type binoculars can have their prisms not cemented together i.e. air-spacing. I have wondered for a while if it was correct to term a binocular such as the air-spaced Ross Stepmur 10X50 a Porro II type. Well, I guess it is. This is a view of the Stepmur prism assembly. The space between the prisms can easily be seen.
http://www.flickr.com/photos/binocwp...7623234405689/

[spacepilot]

Thanks, Patrik. I see it now. Of the four surfaces that carries out the total internal reflection, the pairs of 1-2 and 3-4 are perpendicular to each other in Porro I, and the pairs 1-4 and 2-3 are perpendicular to each other in Porro II.

As for the Porro II prism assemblies that LPT linked to, I think they are still made of two prisms each, with surfaces 1,2 on one and 3,4 on the other. You can clearly see the seam between surfaces 2 and 3 in the picture.

As far as cementing go, one thing I can think of is that in the Porro II configuration, there perpendicular reflecting surfaces that located on different prisms. In the 2-prism setup as in the picture LPT linked to, the surfaces of both the 1-4 and 2-3 pairs are on different prisms. In the 3-prism setup, the surfaces in the 1-4 pair are on different prisms. So in Porro II assemblies, cementing the prisms together ensures that the respective surfaces are always perpendicular to each other.

In Porro I, the perpendicularity is ensured if the prisms are ground properly, because the surface pairs (1-2 and 3-4) are on the same prism. Therefore, no cementing is needed. I can't figure out why the prisms in Porro I are not cemented anyway, though. Can the prisms in Porro I usually be individually adjusted? Maybe it makes it easier to adjust for collimation? I don't know.

[LPT]

1) You can collimate by moving the prisms but it is very time consuming. Eccentric rings are much easier to use. Early binoculars would be collimated by prism adjustment.

2) I think many of the Leitz Porro II's had 3 prisms in each assembly.