All about Porro and roof prisms: how spotting scopes work

Spotting scopes are a great tool to use for long-distance birding. Exactly how they work, however, can seem like a mystery.

Some of the terminology sounds impressive – but what does it actually mean? And how does it affect the quality of the product itself?

If you have seen descriptions that use terms like roof prism and Porro, then we can clarify how these different types of spotting scopes work.

If you have already researched binoculars, which also use these roof or Porro prisms, you may be surprised to find that the most popular type of configuration in binoculars is actually the opposite when it comes to spotting scopes.

Another thing to be aware of is that roof and Porro do not refer to the overall shape of the scope itself. That is a separate choice altogether about whether you need a straight or an angled spotting scope.

Angled vs straight spotting scopes: which is best for birding?

Read on for useful information about the configurations of the prisms in spotting scopes, and what practical implications these carry for you and your bird watching.

What is a prism used for in a spotting scope?

All bird watching spotting scopes use prisms to direct the light through the instrument.

Usually on a spotting scope, the placement of the prisms is in the fatter part of the scope near the eyepiece. The extra width of the instrument there is necessary to accommodate them.

A prism is a shaped block of glass that changes the angle of the light beams that come in via the objective lens.

Prisms have two main purposes:

• to make sure the image the viewing sees is the right way up
• to shorten the physical length of the spotting scope.

Let’s take a closer look at the first purpose.

Prisms correct the image orientation

After the light enters through the objective lens at the far end of the spotting scope, it reflects along the path directed by the prisms and reaches the ocular lens, the eyepiece that you look through.

The image that enters the objective lens is upside down. Without the prism system you would see an inverted image.

The function of the prisms is to rotate this image 180 degrees so instead of being inverted it is the right way up when it reaches your eye.

The first prism turns the image 90 degrees, then the second prism turns it another 90 degrees.

Prisms reduce the scope size

In addition to rotating the image so that it is the correct way up, prisms in spotting scopes have a second important function. Without prisms, the length of the spotting scope would be much longer.

Prisms enable the spotting scope to have a shorter distance between the ocular lens and the objective lens. They do this by reducing the overall space needed by the light path through the scope.

The light still travels the same total distance between the lenses. But instead of traveling in one single straight line, the prisms turn the light and direct it along a series of shorter straight lines.

This allows the length of the spotting scope to be shorter and more compact. The prisms are used to shorten the instrument.

What is the difference between Porro and roof prism?

Both Porro prisms and roof prisms use a pair of prisms for the light transmission. It is the configuration of these two prisms that differs between them, not the actual type of prism.

You can get various different types of prisms. What the prisms are made of is important. We discuss that in more detail here.

The essential difference between Porro and roof prism spotting scopes is the way that the prisms are placed.

What is a Porro prism spotting scope?

Bird watching spotting scopes with the Porro prisms type appear to widen towards the end that is furthest from your eyes, with an overall zig zag shape.

Within the tube of the scope, a pair of prisms directs the light along a more widely spaced path. The point of this light path is to create a greater width between the objective lenses and to amplify the available light.

For those who are interested in history, its full name is the double Porro prism configuration. You’ll see the term ‘Porro’ capitalized. It takes a capital letter because it is the last name of the inventor.

It is named after the 19th-century Italian inventor, Ignazio Porro. Signor Porro developed the use of this system in binoculars. Porro prisms were the standard design of binoculars up until the 1960s.

Advantages of Porro prism spotting scopes

These types of scopes tend to benefit from a brighter and more three-dimensional image and a broader field of view.

More light enters the instrument and you get a more three-dimensional view. The way that a Porro scope user perceives depth is also greater.

Disadvantages of Porro prism spotting scopes

The off-set placement of the prisms means that the body of the spotting scope is wider and more bulky. The increased size of the spotting scope may feel too cumbersome and heavy for some people.

Protective features, such as rubberized coating, waterproofing and fog proofing can be more challenging to achieve in Porro prism spotting scopes.

What are roof prism spotting scopes?

In any spotting scope, if you didn’t have prisms, the view you see would be upside down. So the roof prisms direct the rays of light as they enter the scope, and they also invert them so that you see the image in the correct orientation.

Roof prism spotting scopes use a different system of prism and optics to direct the light in a more complex path than the Porro prism.

In roof prism spotting scopes, the ocular lenses in the eyepieces line up directly with the objective lens. This is because the roof prisms are positioned back to back. In the Porro prism spotting scopes they are configured in an offset position.

In contrast to the Porro prism, the roof prism spotting scope gives you an image quality more like what you would see with the naked eye. Roof prisms do not give you the deeper, more three-dimensional view to the same extent as the Porro prism designs.

Advantages of roof prism spotting scopes

The advantage of the roof prism is that the straight-barrelled body of the spotting scope is narrower and more streamlined. Despite their more complex internal structure, the roof prisms tend to be smaller and more lightweight.

Disadvantages of roof prism spotting scopes

Though their size is more compact, spotting scopes with the roof prism configuration tend to miss out on the features that the more popular Porro models offer. These features include the option to use eyepieces of different magnifications, or to attach adaptors for digiscoping.

To find a roof prism spotting scope that approaches the effectiveness of a Porro prism scope, it’s advisable to look for one that features prisms that have phase coating.

What is phase coating in spotting scopes?

Phase shift can also be an issue with lower-end roof prism spotting scopes. Phase shift occurs in roof prisms due to the way in which the light enters the objective lens and reflects on the surfaces of the prisms. Without additional coatings on the prism to correct phase shift, the image would lose contrast and resolution.

To ensure that no light is lost in the roof prism set-up, manufacturers add technologically advanced dielectric coatings to the prisms. These coatings help capture and direct the light effectively as possible.

Because this kind of roof prism optical system is more complicated to produce, it can increase the price of this kind of scope in comparison with the Porro prism design.

Lower-end roof prism spotting scopes use a cheaper but less effective coating. So, in this case, some light can be lost therefore impacting the image quality.

It’s worth a careful check of the specification to ensure that the spotting scope uses BaK-4 glass for the prisms. BaK-4 glass is of a superior quality to BK7 glass.

BaK-4 v BK-7: What is the best glass in binoculars?

Roof vs Porro prism spotting scopes: final thoughts

Unlike binoculars, where the roof prism type is most widely available, in spotting scopes the Porro prism is more popular.

This is mainly down to the wider field of view that they offer. In addition, the depth of field is better. Depth perception is increased due to the off-set position of the ocular lens in relation to the objective lens.

The main advantage of the roof prism is that the whole instrument is more streamlined. This is due to its prisms being more in line with each other. So if the size is a priority for you, then the roof prism type of spotting scope might be a better choice.

As we have explored, both configurations of birding spotting scopes have their benefits and drawbacks. Whether to opt for a roof prism or Porro prism spotting scope is probably down to your personal priorities.

Another major choice to make when it comes to selecting the most suitable spotting scope for you is whether it is an angled or a straight spotting scope. We look at the other considerations to bear in mind when choosing a spotting scope in this article.

How to choose a spotting scope for birding