Many SLR cameras only draw side views of a pentaprism in their optical path diagrams, which is slightly misleading, leading everyone to believe that there are only two sides involved in reflection in this prism. The pentaprism used in actual SLRs should be accurately called the ridge pentaprism. Why is there the word “ridge”? Put a diagonal pentaprism and you’ll see
The three gray surfaces in the image participate in reflection, and the two reflective surfaces above form a 90 ° angle to each other, which looks like a ridge, so they are called a ridge prism. Therefore, even if you replace a prism with a face mirror, you need at least three face mirrors instead of two.
Do you use face mirrors in SLR cameras? In fact, there are, even now, many low-end SLRs still use face mirrors instead of prisms. In fact, using a prism or a mirror is also an important symbol for many large factories to distinguish between low-end models and models.
What is the difference between a face mirror and a prism? The difference is actually the cost. Prism processing costs are high, so they are only used on models. Secondly, the optical path loss of the prism is small, so the viewfinder looks brighter. In the prism scheme, there are only two air glass contact surfaces in the optical path: entering the prism and leaving the prism, while in the face mirror scheme, there are at least three air glass contact surfaces, increasing the loss of light energy; In addition, if the mirror is coated behind the mirror, there is also a problem of surface parallelism. If the front and rear surfaces of the mirror are not parallel, various problems such as additional aberrations, ghosts, and loss of light energy will also be introduced.
Thirdly, since a surface mirror is a separate component and a prism is a whole, its accuracy and durability are obviously not the same. A slight bump may deform the optical path of the mirror, but a prism does not.
In terms of optical path alone, the two can indeed replace each other, but considering some of the above factors, it is necessary to comprehensively consider which one to use in the actual product.