It has become one of the essential characteristics of smartphones: the power of the telephoto lens to get closer to the subject to be photographed. But be careful not to confuse optical zoom and digital zoom.
Have you ever seen a 120mm telephoto lens? I took one to measure it: more than 15 cm long. So how do smartphone manufacturers manage to fit such a telephoto lens into the thickness of a cell phone: less than 1 cm?
Let’s stay with this example of 120 mm since this is exactly the maximum focal length offered by the iPhone 15 Pro Max, released this Friday, September 22, 2023. 120 mm instead of 77 mm on the 14 Pro range. This leap is made possible by the integration of what the manufacturer calls a “tetra prism”. Result: an optical zoom which goes from x3 to x5, the equivalent of a 120 mm. And it is very bright, since the diaphragm opens at f/2.8.
A periscope like in a submarine
The idea is that of the periscope as in a submarine: a set of lenses, coupled with one or more prisms, small mirrors which will deflect the light, so as to no longer be limited by the finesse of the phone. By turning 90°, the light can pass through more lenses, this time, across the width of the smartphone where there is much more room. This is exactly what Apple does with its “tetra prism” in the iPhone 15 Pro Max, but by deflecting the light rays four times, to benefit from a greater distance between the lenses and the sensor, which records the picture.
OPPO was the very first manufacturer to present this type of lens, with its 5X in 2017. In 2019, we found it at Huawei on the P30 with – already – a x5 optical zoom. In reality, it has become a standard in the Android world, since we find a periscopic lens on Google’s Pixel 7 Pro and on the Samsung Galaxy S23 Ultra: low light optics (f/4.9) in both cases, but which offers a x10 optical zoom, in the case of Samsung.
OPPO, the precursor
And so, the holy grail is this optical zoom, because smartphones are also equipped with a digital zoom, but which simply enlarges the pixels artificially with, in return, a rapid and very visible degradation of quality of the image. Do not let yourself be fooled by the technical sheets which promise spectacular magnifications, such as 30x or more: if it is not specified that it is an optical zoom, probably optimized thanks to a periscopic objective, you risk be disappointed with the result.