Porro prism vs roof prism binoculars: which is better for birding?

Porro prism binoculars and roof prism binoculars are the two main classes of binoculars. If you are looking to buy a pair of binoculars, it is helpful to know the pros and cons of each type.

The names indicate the main difference between them, which is the method the optics use to transmit light through the body of the instrument. Because their internal configurations are different, the shape of the binoculars look different on the outside as well.

The kind that is best for your needs will depend on your specific requirements, so this article will assist in considering your options. If however you would like to head straight to compare specifications and prices, feel free to head here to check The best Porro prism binoculars for birding.

Why are there prisms in binoculars?

Binoculars contain a pair of prisms. They can be placed in different ways within the tubes.

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

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

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

What the two prisms in binoculars do

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.

What are the benefits of having prisms in binoculars?

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

Prisms enable the binoculars 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 binoculars.

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 binocular to be shorter and more compact. The prisms are used to shorten the instrument.

What is the main 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 and coated with is important.

We discuss that in more detail here.

Why are prism and lens coatings important to birding binoculars?

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

In Porro prism binoculars, the internal prisms are aligned horizontally. In roof prism binoculars, the internal prisms are aligned straight.

Both of the internal reflection systems created by the prisms alter the orientation of the image.

Let’s take a closer look at each type.

What are Porro prism binoculars?

Bird watching binoculars with the Porro prisms appear to have the classic outer shape of binoculars. They widen towards the end that is furthest from your eyes, with an overall zig-zag shape.

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.

What is a Porro prism?

Though all binoculars use a pair of prisms, the Porro prism design is of a particular type.

Within each tube of the binoculars, 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.

The construction of Porro prism binoculars

You can see from the construction of a pair of binoculars using the Porro prism design, the pair of ocular lenses (the lenses you hold against your eye) are closer together than the objective lenses (the ones at the furthest end of the binoculars). Therefore, the ocular lenses are not in line with the objective lenses.

Advantages of Porro prism binoculars

Because the distance between the objective lenses is wider than the distance between the ocular lenses – so wider than the distance between your eyes – you 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 binocular user perceives depth is also greater.

The nature of the design results in a physically broader instrument. For some, this feels more stable to hold so it is especially good for beginners or those with a less steady hand.

More pairs of the Porro binoculars are suitable to use with a tripod. We have suggestions for how to tackle the issue of shaky heads and achieve a more steady image here: Hold steady: how to stabilize binoculars without shaking

Disadvantages of Porro prism binoculars

The increased size of the binoculars may feel too cumbersome and heavy for some people.

Protective features, such as rubberized coating, waterproofing and fog proofing are more challenging to achieve in Porro prisms.

How do Porro prism binoculars adjust focus?

When a central focusing knob is used, the eyepieces move up and down. This makes them very difficult to waterproof. It gives the chance for air to enter as they move, risking internal corrosion and fogging.

Conversely, Porro prism binoculars that use individual focusing for each side can be waterproofed very effectively. So the waterproofing depends on which type of focusing mechanism is used.

Some people find the individual focusing slower and less convenient.

Why aren’t there many Porro prism binoculars?

Some people see the Porro prism binoculars as more traditional – the implication being that they are a bit old-fashioned.
Since there is a wider selection of roof prism binoculars on the market it may increase your chances of finding some that suit you perfectly.

Pros of Porro prism binoculars

• many affordable options, cost less than roof prisms
• good 3D imaging
• wider field of view
• greater perception of depth
• most can be used with a tripod
• individual focusing can be very waterproof

Cons of Porro prism binoculars

• bulky due to the offset line of the lenses
• heavy
• lower magnification
• less likely to have protective coating to lens
• less like to have weatherproofing on the binocular body
• a central focusing mechanical is less waterproof

What are reverse Porro prism binoculars?

With both kinds of Porro prism binoculars the front and back lenses are slightly offset from each other.

Reverse Porro prism binoculars have objective lenses closer together than the ocular lenses. This is the opposite configuration to Porro prism binoculars, which have ocular lenses closer together than the objective lenses.

Because the reverse Porros have their front lenses closer together than the eyepieces, the body of the binocular can be more compact. It looks neat and streamlined.

This is especially good if you are looking for lightweight compact binoculars. One of our choices in this list is of a reverse Porro design >>> The best lightweight compact binoculars for bird watching

Are reverse Porro prism binoculars any good?

One of the advantages of the Porro prism binoculars is the three-dimensional stereoscopic effect.

The reverse Porros do not capture this effect quite so successfully as their objective lenses are close together. They do work for close range viewing.

What are roof prism binoculars?

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

In contrast to the Porro prism, the roof prism binocular 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.

What is a roof prism?

In any binocular, 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 binoculars, and they also invert them so that you see the image in the correct orientation.

In roof prism binoculars, 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 contrast, in the Porro prism binoculars they are configured in an offset position.

Roof prism design: what is meant by roof prism?

Roof prism binoculars can fall into two categories, according to what kind of prism they have.

They both have the same advantages. There are a few subtle differences between them even though they are still roof prism binoculars.

Advantages of roof prism binoculars

The advantage of the roof prism is that the straight-barrelled body of the binoculars is narrower and more streamlined.

Despite their more complex internal structure, the roof prisms tend to be smaller and more lightweight. They are somewhat more user friendly in the sense that they are quick and easy to use.

It’s generally true that the roof prism design is more likely to have the up-to-date technological advantages that upgrade your viewing experience, such as ED glass.

The roof prism design can give a greater magnification along with a brighter image.

Disadvantages of roof prism binoculars

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 binocular in comparison with the Porro prism design.

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

What is phase shift?

Phase shift can also be an issue with lower-end roof prism binoculars.

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.

It’s worth a careful check of the specification to ensure that the binoculars use 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?

Pros of roof prism binoculars

• lighter weight
• compact and streamlined shape due to the straight barrel
• higher magnification
• clearer image
• may be better value in the long run
• more durable, waterproof, increased weatherproofing
• refined balancing makes them comfortable to hold
• more advanced coatings on the lenses and prisms
• ED glass is readily available

Cons of roof prism binoculars

• more expensive cost
• some models have a narrow field of view
• the image can seem more flat than with Porro prism binoculars
• not every pair can be used with a tripod – you may need a special adaptor

Summary: is Porro or roof prism better for binoculars?

Porro prism binoculars are a solid choice for people with a more traditional style, who wish to have a wider field of view. These binoculars also have more options of tripod attachments.

Here are our top choices if you are looking to compare Porro prism specifications – The best Porro prism binoculars for birding

Roof prism binoculars, on the other hand, are seen as more rugged and versatile due to their increased portability.
Though there is an increased cost attached to the high-quality optics, this may be offset in the long run by an increased longevity.

Whether you opt for roof prism binoculars or Porro prism binoculars is a matter of personal preference.

Both types of birding binoculars have their benefits and drawbacks. There’s no universal answer to say what is best, as everyone’s priorities and circumstances are different. For some guidance on what to prioritize, feel free to check How to choose the best binoculars for bird watching.