Porro Prism vs Roof Prism: What’s The Difference?

Bird watching is a popular hobby which you can carry out wherever you are, if you travel, you can take your binoculars with you too.

However, if you have the wrong lens, you won’t be able to focus properly on the birds when you see them. When you purchase binoculars, there are many things you need to be aware of, including the type of prism they have: porro prism or roof prism.

Prisms in Binoculars

Prisms work with the lenses to make sure you see the image clearly and the right way up. Without a prism, you would see your images upside down.

The shape of the prism reflects light waves, and this shortens the optical path. If you didn’t have prisms, your binoculars would be a lot longer and so a lot harder to use.

There is no right or wrong when it comes to the type of prism you have in your birdwatching binoculars, but let’s take a little look at each of them and how they work.

• Porro prism

Porro prisms are the more traditional ones and these are seen in older, binoculars with a zig zag shape.

The prims are in the shape of a right angled triangle, and they reflect the light rays through the binoculars so you see your image clearly.

• Roof prism

Roof prisms are the newer option.  There are two prisms and these meet at a 90° angle, with the shape looking similar to the roof of a house. The design of the prisms means they work together to correct blurred images and let you see your target clearly.

The prisms are usually coated to increase reflectivity and to keep the light waves in the right direction. Binoculars with roof prisms are straighter and more compact.

Porro Prism vs Roof Prism

Both types of prism have advantages and disadvantages

Porro prism

Pros

• As these prisms are easier to make, they are cheaper to buy. If you are looking at a budget pair of binoculars, these will give you excellent value for money.
• They have less light loss, so the images have good contrast and are clear. If you’re out birding, they can give you excellent images of any bird and allow you to see all their detail clearly.
• Most binoculars with a porro prism can be mounted on a tripod, so they are easier to hold steady.
• They can be adjusted to suit your own vision.

Cons:

• They are heavier, so if you’re carrying your binoculars for a full day of bird watching, this may be an issue.

Roof prism

Pros:

• They are lighter than porro prisms and more compact. This makes the binoculars lighter and easier to carry.
• The way they are designed makes them more robust, so they will last longer.

Cons

• These prisms are more complex and so take more time to design. This makes them more expensive.
• Not all binoculars with a roof prism can be mounted on a tripod.
• The distance between the barrels may not be adjustable, so you may not be able to alter them to suit your own vision.

Prisms and lenses work together, so you need to check the lenses too. You will find on most binoculars that the lenses and prisms have been designed to give the best result. The prisms and lenses may be coated for maximum efficiency to help reflect the light and glare.

When you’re looking at binoculars, it’s important to know what the different coatings mean.

• No coating: The lens has no additional coating to help improve your image.
• Coated: These usually just have one layer of coating, usually on just the layers you can see.
• Fully coated: Some surfaces will have multiple layers of coating. The more layers of coating there are, the more light is transmitted and so your image is clearer.
• Fully multi-coated: This is the best quality and all air to glass surfaces have been coated multiple times.

Coatings help improve the amount of light which comes in and also reduces internal reflections. Without these you may not get the clearest images of your birds.

When you’re looking for a good pair of birdwatching binoculars, there are other considerations to make sure you find the right pair.

Looking for a bit more information? Take a look at the video below from Orion Telescopes.

Additional Considerations

Field of View

This is a big consideration for any bird watcher. Consider where you will be going to look for birds and look for a pair of binoculars with the corresponding field of view. This is the angle width that you will be see.

• If you like to go birdwatching in open areas or lakes, look for a wider field of view.
• If you prefer woods or smaller spaces, you won’t need a large field of view.

The field of view is also affected by magnification.

Magnification

Getting the right magnification will make all the difference between seeing a bird clearly, or not. While it can be tempting to get the highest magnification you can, this may not help you to identify birds.

If you alter the magnification you will also alter your field of view, so you need to consider both factors.

• If you increase your magnification, you will reduce your field of view. This is fine if you want to temporarily zoom in on one bird but if you zoom in too far, you will have a very narrow field of view.
• If you decrease the magnification, you will get a wider field of view so you can scan for birds.

When you look at the magnification on binoculars you will see two number, such as 10 x 42.

• The first figure is the magnification. A good set of bird watching binoculars should have a magnification of 10 or above
• The second figure is the lenses, so in this case they would be 42mm.

Focus

If you’re looking for birds, you often need to adjust your focus to see them clearly.

• Manual: Some binoculars come with a manual focus, where you can turn the focus wheel as far as you need it. This gives you perfect control over the focus, but may also be time consuming. Newer binoculars have a wheel which makes it much easier to focus quickly.
• Auto-focus: These are also called fixed focus as they will remain in focus whether you are looking at a bird close to you, or further away. You don’t need to do anything to keep your focus as the binoculars do it all for you.

Weatherproofing

If you’re serious about birdwatching, always make sure you buy a pair of binoculars which are suitable for all weather conditions.

• Waterproof: Water resistant ones will work well in most conditions, but if you are out all day in the rain, then you need waterproof binoculars.
• Fog proof: If you’re out in the fog, then the damp air can often find its way inside your binoculars and cause the lenses to fog over. Sealed binoculars will help prevent this and they will also stop dust from getting inside too.
• Sun: The one problem you may have with a sunny day, is glare, so look for anti-glare binoculars.

Size & Weight

You need to remember, that you will have to carry your binoculars, so make sure they are right for you.

• It’s only natural to want lightweight binoculars as they are easier to carry, but you can also get compact ones which are just as good as a larger pair.
• Remember too that you will be holding them, so make sure they are the right size for your hands and won’t cause you any strain.
• If you are going birdwatching for a few hours, it can be much easier to mount your binoculars on a tripod. Not all binoculars can be used with a tripod, so always check that this is possible before you buy.
• It can be useful to have a pair of binoculars which are adjustable for eye width. If you wear eyeglasses, then look for a pair which provide eye relief.

Eye Relief

If you wear glasses, you will know that your eyes won’t get as close to the eye cups on the binoculars, so you will need eye relief. This allows you to adjust the distance between your eyes and the ocular lens.

If you do wish to wear your glasses with your binoculars, look for eye relief of at least 15mm.

You may also need to alter the barrels to suit the width of your eyes. Binoculars with porro prisms are often adjustable in this way.

Rounding Up..

Many bird watchers have more than one pair of binoculars. If you like to go out to different terrains then you may need different features to make it easier for you. If you like to take your binoculars when you travel, then it is far easier to buy a compact pair to take with you as they are lighter and take up less space.

It can take some time to get used to how lenses and prisms work together, so if you’re just starting out in bird watching, ask around or see if you can try out other binoculars to give you the best idea of that you need.