Continuing my series on the binocular basics let us learn about Porro prism binoculars. In most cases you will see two types of prisms being talked about and these are the Porro prism binoculars and the roof prism binoculars.
What are prisms?
If you only used an eyepiece and then used the objective lens to see the image of the object that you are looking at then you will see the image inverted that is upside down as well as you will have the image which is reversed left for right. To correct that prisms are used in between the eyepiece and the objective lens. The prisms are used for bending the light which corrects the inverted image.
There are two types of prism systems that are used in the binoculars of today. These are the Porro prism systems and the roof prism systems. In fact there are three types and the third system is the reversed Porro prism binoculars.
The glass with which these prisms are made is known as BAK-4. Details about this later in the subsequent posts.
What are Porro prism binoculars?
Just to give you an idea look at the image below of the Porro prism below. Porro prism binos were popular in the 1960’s but now the more popular are the roof prism binoculars. They have an offset zig-zag design. This design was introduced because the light in the case of Porro prism binoculars travels in a zig-zag pattern and hence the eye piece, the prisms and the objective lens could not be in the same straight line.
With roof prisms that problem was resolved though not easily and hence generally you will see that roof prism binos are more costly than the Porro prism binos.
Porro prism binoculars are more efficient than the roof prism ones as they have better simpler mechanism to control the light.
Reversed Porro prism binoculars are very compact and can easily be the best for very compact travelling binos.
Of late however the roof prism binoculars have easily made great advances in their light management techniques and hence now they are at par with the Porro prism binoculars for light efficiency and optical experience. The way it has been achieved is via the use of the Phased coatings.