Hello,
Here is an excerpt from an IIS article on binoculars that I will be co-authoring, along with anyone within the community who would like to collaborate. As John is rushing off to work, I thought I would post this draft, along with the attached image, to save him having to explain in detail the difference between Porro and Roof Prism design.
Start excerpt:
Porro Prism Design
Porro-prism binoculars were first designed in the mid-1800s by an Italian inventor, Paolo Ignazio Pietra Porro. His concept of placing two right-angled prisms in each barrel of a set of binoculars is still used today. Porro-prism binoculars are the stereotypical angled-body binocular design. When standing on their barrels, or hanging from a strap around someone's neck, Porro-prism binoculars appear to form an M shape.
Porros focus by relying upon an external focus wheel which, when turned, causes the eyepieces for each side to slide forward or backward along an external tube. This type of focusing allows for sharp images of close objects, as well as precise focusing on objects as close as six feet.
Advantages- Less expensive.
- Good optical quality.
- Greater stereoscopic, or three-dimensional view.
- Brighter images due to greater transmission of light.
- Fast focusing.
- Close focusing.
- Wider field of view (the amount of area you see when looking through the binoculars).
- For low-to-mid-range priced binoculars, Porro-prisms offer the best value.
Disadvantages- Less compact, not as comfortable to hold for long periods, may be less rugged than roof prisms
- Weight (the better transmission of light is due to large prisms, which are weighty) and bulkiness, which can make Porros hard to use for small-handed folks.
- The external focusing mechanisms of many Porros can make for less durable binoculars, that is, ones that can be more easily jarred out of alignment.
Roof Prism Design
Roof prisms binoculars are more compact. However, to achieve the same optical quality as Porro prism models, they usually cost more to manufacture. Many people will express a strong preference for one over the other. For example, the Porro prism design with its wider spaced objectives and can show slightly more "depth" in the image where as the roof prism's image appears more compressed.
Roof-prism binoculars were first developed by a German binocular manufacturer in the mid-1800s. This design features two straight barrels, giving it an H-shaped appearance.
The design reflects light through a series of five small prisms in each barrel. Roof-prism binocs have grown in popularity in the last few decades, primarily because many leading optics manufacturers are producing excellent optics in this format. Because of the way roof-prisms are designed, most of the focusing hardware is enclosed inside the body of the binoculars. This hardware is adjusted with an external focusing knob or wheel.
Advantages- Considerably more compact than comparable porro prism model and usually easier to hold for long periods.
- Are generally manufactured to closer optical and mechanical tolerances making them more rugged than comparable quality porro prism binoculars.
- Ease of handling
- Fewer external moving parts (which means increased durability)
- A better ratio of power-to-weight; that is, in general, a 10x roof-prism weighs less than a 10x Porro.
Disadvantages- Higher cost.
- Tend to be more expensive than Porros
- Often don't focus as closely
- Because of the additional prisms required to reflect incoming light, roof prisms often do not offer as "bright" an image as Porros.
End excerpt.
Cheers
Dennis