What is a rangefinder?

Lead: With the development of digital technology, numbers have been integrated into us. Distance mirror, also known as telescopic conversion mirror or focal length intensifier, is a special optical device, which consists of several optical lenses, and its function is to increase the focal length of the original lens. Because the rangefinder is an optical system with concave lens function, it can't image alone. Only when it is used with a conventional lens with convex lens function can a clear object image be obtained.

What is a rangefinder? The rangefinder has many magnifications. Common ones are 2 times (commonly known as magnifying glass), 1.7 times, 1.4 times and 1.6 times, and a few are 3 times. The number of lenses usually ranges from four to seven. One side of the rangefinder is a bayonet, which, like the bayonet of the lens, is used to connect the camera body; On the other side is a snap ring, which, like the snap ring on the SLR fuselage, is used to connect the lens. When in use, first remove the lens from the SLR, connect the rangefinder to the fuselage, and then connect the lens to the rangefinder, that is, the rangefinder is between the fuselage and the lens.

Range extenders with different magnification can extend the focal length of the original lens to different ranges. For example, a 2-fold zoom lens can turn a 50mm standard lens into a 100mm medium focus lens; And 1.4 times can only turn it into a 70mm lens. However, the rangefinder can only be used for lenses over 50 mm. If it is used with wide-angle lenses, angle occlusion may occur.

After connecting the rangefinder, some parameters on the original lens will not change, such as the shortest focusing distance. If the shortest focusing distance of the original lens is 0.5 meters, and it is still 0.5 meters after adding the rangefinder, a larger image can be obtained due to the increase of focal length.

The rangefinder has a main disadvantage: after adding the rangefinder, the maximum effective aperture of the combined lens with increased focal length decreases. The simplified rule is to multiply the magnification of the rangefinder by the maximum aperture coefficient of the original lens, that is, to get the maximum aperture coefficient of the combined lens. For example, a zoom lens with a ratio of 70 ~ 2 10/4-5.6, with a rangefinder twice as large, is a telephoto lens with a ratio of140 ~ 420/8-11. If you use a rangefinder that is 1.4 times, it becomes a zoom lens of 98 ~ 294/5.6-7.8. The growth law is to multiply the magnification of the rangefinder by the focal length of the original lens to get the final combined focal length.

Due to this shortcoming, we should pay attention to it when using it. If the camera has a built-in metering system, the camera will automatically compensate for the exposure lost due to the reduction of the aperture; If there is no internal metering system, it should be compensated manually when used. For a SLR camera with a divided image on the focusing screen, because the maximum effective aperture is reduced, the light transmittance is reduced, and sometimes half of the divided image is black when taking pictures, which is not conducive to focusing.

After the rangefinder is added, the optical quality of the combined lens is lower than that of the original lens, such as the resolution and contrast. No matter how well the rangefinder is made, the optical quality is definitely much worse than the original lens. Generally speaking, a smaller aperture should be used to improve the image quality.

However, rangefinder is a cheap substitute for telephoto lens. For ordinary photographers, the lost optical quality is not important. As long as the photo is not enlarged, it is difficult for the naked eye to distinguish it. But the price of the range extender is much cheaper. For example, a user has a 300/4.5 lens, and if he buys a 600mm lens, the price is very heavy; Therefore, it is very cost-effective for amateur photographers to buy a rangefinder, and even some professional photographers use it to work.

Pentax and Nikon have developed an AF/KOOC-0/.7x and/KOOC-0/.6x (TC-/KOOC-0/6A) rangefinder, which can convert manual focusing lens into AF lens. When in use, adjust the focus ring of the original manual focus lens to infinity. In auto-focusing, the optical elements in the manual focusing lens are fixed, and all focusing work is completed by the movable optical elements in the rangefinder, thus achieving the purpose of auto-focusing. Because Pentax and Nikon's AF bayonets are compatible with the original MF bayonets, it is natural to use manual focusing rangefinders on their AF SLR, but this is no longer an AF rangefinder.

Due to the small number, limited use and high price of extended-range lenses produced by the original camera manufacturers, it provides a production gap for independent lens manufacturers. At present, the manufacturers of AF rangefinder are Kengo, Longteng, Sigma, David and so on.

AF rangefinder not only has the same shortcomings as the general rangefinder, but also has the problem of whether the combined lens can continue to focus automatically. Auto-focus SLR is a phase detection type, and the auto-focus system can't work normally in low illumination. After the rangefinder is added, the light reaching the AF detection module through the combined lens is relatively weakened because the maximum effective aperture becomes smaller. When the aperture is small, the AF detection module can't measure the contrast of the object at all. Therefore, we should pay attention to the maximum aperture of the original lens when using the range extender. When the AF rangefinder produced by the original camera manufacturer is used with the specified lens, it can naturally focus automatically, but the products produced by independent lens manufacturers are not necessarily. Be careful when buying, and try to see if you can autofocus first. Generally, when this rangefinder is combined with the original lens, it can still focus automatically when the final maximum aperture is greater than f/8 (that is, for a 2.0X rangefinder, it can be used with f/4 lens). However, when used in zoom lenses such as 70 ~ 2 10/4-5.6, the maximum aperture will change with the change of focal length, so it is often impossible to focus automatically.

For cameras with non-replaceable lenses, there are special front lens rangefinders, which can be screwed to the front of the lens when used, such as TC200M of Ricoh MIRAI and A-200 IS- 1000 of Olympus, which can expand the original focal length of 135mm to 200mm.

Because both AF rangefinder and AF lens have integrated circuit chips, attention should be paid to the connection sequence. If used improperly, the camera function may fail, leading to disorder. The correct way is to install the rangefinder on the lens first, and then install the combined lens on the camera body.