The difference between roof binocular and Porro binocular

Anyone who has been in contact with binoculars knows that binoculars are divided into ROOF roof type and PORRO Porro type. The Porro type was used when the binocular was first introduced. With the development of binocular technology, the roof type was born. The structure is relatively complex and the production process is high, so the manufacturing cost is high, and it is generally used in high-end binoculars. What are the advantages and disadvantages of the two types of binoculars? Today, I will take you a brief look.

The advantages and disadvantages of Porro’s binocular

The Porro prism rotation system is very effective in theory, because all four reflecting surfaces can produce total reflection without loss of light, but in fact, the refractive index of the Bk7 prism used in the cheap Porro prism binocular is close to the lower limit of total reflection, so the prism The central reflection is good, but a small part of the light at the edge cannot be totally reflected and “leaks” out. If you observe the exit pupil spot (raise the binocular, stay away from yourself, and observe the bright spot in the eyepiece), you will find that there is a shadow cut edge on the edge of the exit pupil spot of the binocular using the Bk7 prism.

The use of higher refractive index glass can correct this problem. The efficiency of the Porro prism rotation system using Bak4 glass can reach the highest level in its class, and the light transmittance can reach 90-95%.

In addition, the Porro prism has another characteristic. Because the light path has a “Z”-shaped turning, the appearance of the Porro prism binocular often changes to this shape. The entrance pupil and exit pupil are not in a straight line. Generally speaking, the entrance pupil (objective lens) will be much more separated than the exit pupil (eyepiece). Observing with Porro prism binocular will change our accustomed sense of perspective and stereo vision. On the one hand, the sense of distance is compressed, on the other hand, the sense of three-dimensionality is increased. In the same way, Porro prism binocular will also affect our judgment on the size and distance of objects.

It is precisely for the above reasons, so theoretically:

1. Advantages: PORRO Porro type will be brighter than ROOF roof type binoculars of the same grade, and the contrast or imaging will be harder.

2. Disadvantages: Porro-style binoculars have a poor sense of reality, including that the size and distance of the target are not as good as the roof-type binocular. Another important point is that the stereoscopic and operational sense is not good. And this is determined by the structure and cannot be changed through other adjustments. The portability of the volume is worse, not as portable as the roof ridge, and the appearance is slightly worse.

Advantages and disadvantages of roof-mounted binoculars

Porro’s binocular, the first one in the binocular, slowly developed a roof-top binocular with the development of technology. It can keep the emitted light and the incident light in a straight line. Its lens barrel is straight, and the sense of distance, perception, size, etc. is also closer to the naked eye. Reality must be strong.

The main reason for the popularity of roof mirrors among birdwatchers is the difference in the size of the images of the Porro binocular and the roof binocular mentioned above. The bird looks larger in the roof prism binocular, but in fact it is not really bigger. If you measure an 8 times Porro and an 8 times roof image, you will find that the size is the same. But it is really difficult for our brain to accept that what we see is actually the same size object. A friend of mine studied this phenomenon quantitatively, and he correlated the size of the object he felt with the distance between the objective lens. At this point, the reverse Porro prism binocular, that is, the binocular with the objective lens closer than the distance between human eyes, this phenomenon is particularly prominent)

There are other aspects of roof prisms worth noting. Among the most common roof prisms: Schmier’s special Han prism, there is an interface that cannot produce total reflection, and most of the light will be emitted instead of reflected. So we must plate this reflective surface into a mirror surface. A thin metal reflective layer allows light to be reflected. Begin to use silver until aluminum begins to be produced in large quantities (the advantage of aluminum is that it will not oxidize as quickly as silver and reduce reflectivity). Most advanced roof prism binoculars have returned to silver coating because of their higher reflectivity. For binoculars sealed with nitrogen, silver is not easy to oxidize. And the lens has adopted ED low chromatic aberration lens, the lens is more high-end.

In addition, when the light reflects off the mirror, its phase changes. We can think of a light wave as a wave that vibrates in all directions. When reflected from a mirror, it will be partially polarized, and the energy of the wave vibrating in the horizontal direction will be higher. Part of the energy (brightness) and part of the information (resolution) is lost. This loss will be even greater when two beams of partially polarized light meet and interfere with each other. If we do not take any measures, the roof prism binocular will be darker than the Porro binocular of the same grade, and the image will be softer. As mentioned earlier, bird watchers prefer to use roof prism binoculars even if their imaging is slightly worse, because it has a better sense of perspective and operation. Roof-mounted binoculars can achieve better communication through better lenses and coating technology. Light rate and clarity, so more expensive and complex roof prism binoculars were designed.

Because the cost of Porro type binocular is low, and the brightness and contrast can be achieved to a satisfactory degree without too much cost.

Roof-type binoculars have a very good sense of operation and stereo vision, but they need good lenses and coating technology to improve the brightness. However, bird-watchers prefer more on the roof for imaging.

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