Porro Prism vs Roof Prism Binoculars: What's the Difference?

If you've been shopping for binoculars, you've probably noticed two distinct shapes: one with a wider, stepped body, and one with a slim, straight barrel. That's the difference between porro prism and roof prism designs — and it affects far more than just looks. It shapes how bright the image is, how wide a view you get, how much the binoculars weigh, and how much you'll pay.

Here's what the difference actually means for the way you use them.

Porro prism binoculars use a design where the objective lenses (the large lenses at the front) are offset from the eyepieces, creating the classic stepped, "W"-shaped body. Light bounces through a zig-zag path between two prisms before reaching your eye.

This offset design has a genuine optical advantage: it creates a natural sense of depth and a wider field of view, both of which make it easier to pick a subject out from a busy background — useful for spotting a bird in dense woodland, for example. Porro prisms are also simpler and cheaper to manufacture, which is why porro prism binoculars tend to offer excellent optical performance for the price. The trade-off is a bulkier, wider body that's harder to make fully waterproof.

Our favourite porro prism binoculars: the Praktica Falcon 8x40mm Wide Angle is our pick for most birdwatchers — the wide-angle design maximises the porro prism's natural field-of-view advantage. For more reach in low light, the Praktica Falcon 10x50mm pairs the same porro prism design with a larger 50mm objective.

Roof prism binoculars align the objective lenses directly in line with the eyepieces, using a more complex internal prism arrangement to fold the light path straight through the barrel. The result is a slim, straight-sided body that's more compact and easier to hold.

Because the barrel has fewer external seams and a simpler outer shape, roof prism binoculars are also much easier to seal against water and dust — most quality roof prism models, including our Bushnell A-Series, are fully waterproof. The complexity of the internal optics means roof prism binoculars are generally more expensive than a porro prism pair of equivalent quality.

Our favourite roof prism binoculars: the Bushnell 8x42 A5 is our top pick — IPX7 waterproof with dielectric and phase-coated prisms for genuinely bright, colour-true images. If you want the roof prism's compact handling in a smaller, more affordable package, the Bushnell 8x25 A1 is a strong entry point.

Beyond the porro/roof shape, the type of glass used inside the prism also affects image quality. BAK-4 is a denser, higher-quality glass that produces a fully illuminated, round exit pupil and brighter images — it's the standard in quality binoculars, including our Praktica and Bushnell ranges. BK-7 is a cheaper glass that can produce a slightly dimmer image with a faint grey edge around the exit pupil, more commonly found in entry-level or toy binoculars. If you're comparing two similarly priced binoculars, checking whether they use BAK-4 prisms is a quick way to gauge build quality.

Choose porro prism if: you want the best optical performance for your budget, you value a wide field of view for tracking movement (birds in flight, sports, wildlife), and weatherproofing isn't a dealbreaker. Our Praktica Falcon range covers this well, from the 7x35mm at £37.99 up to the 12x50mm.

Choose roof prism if: you want a more compact, streamlined pair that's easier to carry and pack, you need genuine waterproofing for wet-weather use, or you simply prefer the slimmer handling. Our Bushnell A-Series — including the IPX7-rated 8x42 A5 — is built around this design.

Neither type is objectively "better" — it's a question of what you're prioritising. A birdwatcher on a budget who mostly stays dry will likely get more from a porro prism pair; someone who wants one weatherproof, grab-and-go pair for hiking, travel and variable conditions will likely prefer roof prism.

Browse our full range of porro and roof prism binoculars at Maplin.