Roof prism vs porro prism binoculars

The short answer

Nikon says roof prism binoculars have a straight optical path, which allows a slim and compact body. Porro prisms bend the path like the letter Z, and that takes up considerable space.

Both can give a sharp image. The choice is mostly about body shape, weight and what the maker has done to the prism. Roof and Porro models sit side by side in the best binoculars compared list, and the median weight across all 554 models is 24 oz.

How the two shapes differ in your hands

ZEISS describes Porro binoculars as relatively wide, with the lenses farther apart than the eyepieces. Celestron calls the look broad shouldered and says Porro models are often physically larger than roof models of the same magnification and objective size.

Roof designs have a sleek and trim exterior, in Celestron's words. ZEISS calls Porro prisms large and heavy, and Nikon calls roof prism binoculars compact and lightweight. Those are the makers' general descriptions, so a single model can still break the pattern.

Light loss: where Porro prisms have the edge

Nikon says every reflective surface in a Porro prism is completely reflective, so it loses no light. It adds that such binoculars are easy to produce, and ZEISS also calls Porro prisms easier to manufacture.

A roof prism has one surface that does not give total internal reflection, and Nikon says this loses some light. Makers answer that gap with coatings.

What phase coating does on a roof prism

Nikon says a roof prism surface can shift the phase of light and lower resolution. A phase-correction coating on that surface limits the loss of resolution and keeps the image high in contrast.

Celestron says the coating goes on one surface of the shorter light path half of the roof prism. ZEISS says it introduced its P coating at the end of the 1980s to prevent wave-optical effects.

The RSPB shop lists phase coating as not applicable to Porro prism shapes, and says it can improve contrast, brightness and sharpness. A Porro model will never carry one, so a missing phase coating is no fault there.

What dielectric coating does

Nikon describes a dielectric multilayer coating as one that uses light interference to give high reflectivity across the full visible range. It states the reflectance exceeds 99%.

Celestron defines it as a high-reflectivity prism coating across the entire range of visible light. ZEISS says its dielectric mirror coating has more than 70 layers and is applied in a vacuum.

This coating targets the roof prism's light loss from the previous section. For lens coatings and extra-low dispersion glass, see ED glass and lens coatings, since 146 models in the chart use ED glass.

Prism glass: BaK-4 and BK-7

Celestron says BaK-4 is generally preferable to BK-7 or K9 because it has a higher refractive index. It adds that BK-7 and K9 cost less than BaK-4.

Celestron also notes that BK-7 prisms are sometimes said to give an exit pupil that is not perfectly round, and says that claim should not be trusted absolutely. The glass type applies to both Porro and roof prisms.

How many of each the chart holds

The chart holds 554 models. Where the makers name the prism type, it counts 267 roof, 77 Porro and 4 reverse Porro, so roof is the most common type.

Across all 554 models the median field of view is 340 ft and the median weight is 24 oz. The two most common sizes, 10x42 with 87 models and 8x42 with 75, have median weights of 24.9 oz and 24.2 oz.

Compact sizes weigh far less. The 10x25 median is 11.1 oz and the 8x25 median is 10.9 oz, and these small models are mostly where the slim roof shape pays off. See compact binoculars for the lightest picks.

Choosing between roof and Porro

1. Start with size. Check the weight and field of view for your size in the size comparison tool; the 8x42 median field of view is 393 ft against 336 ft for 10x42.
2. Pick roof if you want a slim body that packs easily, as Nikon and Celestron describe it.
3. Pick Porro if you want a wider body and the straight reflection of a prism that loses no light, as Nikon describes it.
4. On a roof model, look for phase correction and a dielectric coating in the maker's specifications, since Nikon ties both to resolution and light.
5. Compare the actual figures for any model you like. A Porro model and a roof model of the same size can land close together in weight.