Telephoto zoom cameras are awesome, but not as awesome as you think
But here's the thing: all of these additional cameras are not necessarily better cameras. A monochrome secondary camera is more sensitive to light, but it shoots in black and white only. A wide-angle second cam has a wider field of view, but noticeably distorts images. And those tiny cameras for collecting depth information, well, we're not even sure if they do anything.
Telephoto lens cameras – the kind that Apple popularized with the iPhone 7 Plus – can be useful in many cases, but they aren't perfect either. Here's why.
What's the purpose of a telephoto lens?
Telephoto cameras on smartphones are great at two things. One is that they let you zoom in on your subject without as much degradation in quality as digital zoom would otherwise introduce. We've seen phones with 2x zoom, 3x zoom, and even 5x telephoto zoom in the recently announced Huawei P30 Pro. Secondly, telephoto cameras have a narrower field of view, which makes them ideal for portraits – their optical properties introduce less lens distortion and allow for a narrower, thus less distracting background.
Alas, pretty much all smartphone cameras with telephoto lenses are held back by a number of factors. Making matters worse, many of these drawbacks are not immediately obvious to the user.
Telephoto cameras on phones don't do optical zoom
In photography, lenses tend to fall under two general categories: zoom lenses and prime lenses. Zoom lenses have an actual lens element that physically moves to magnify your subject. This is called optical zoom. In contrast, prime lenses are fixed and can't zoom optically. Practically all cameras in modern smartphones, including telephoto ones, use prime lenses since they're smaller and easier to make. Yes, you do get actual zoom with your phone's telephoto camera relatively to its main one, but only at a fixed point.
What this means is that if your phone has a 2x zoom telephoto cam, photos shot at anything between 1x and 1.9x will be zoomed digitally, as if your phone didn't have a telephoto camera at all. Only photos at 2x and beyond will take advantage of the phone's telephoto lens. That's better than nothing, of course, but it's easy for an inexperienced user to wrongly assume that their phone's telephoto hardware is of benefit every single time they zoom in. That's not the case, sadly.
Telephoto cameras rarely work in low light
In most cases, a bigger camera sensor is a better camera sensor. Its greater surface area lets it absorb more light, and having more light translates into better photos with more detail and less noise. But telephoto cameras on phones tend to use smaller image sensors – ones that pick up less light and are more susceptible to digital noise. They have to in order to achieve the level of magnification their telephoto lens is designed for without the camera module sticking out too much.
Because of this, telephoto cameras rarely work in low-light situations. When the phone's camera software detects that the light in the frame is insufficient, it will not use its telephoto cam for zooming. Instead, it will only do digital zoom with its main camera, and you might not even notice until you later take a closer look at your suspiciously fuzzy-looking photos.
Telephoto cameras can't focus from up close
I remember how when I got to test the iPhone 7 Plus for the first time, I tried taking some artsy photos of rain droplets on a flower's petals. I did 2x zoom, assuming that the telephoto lens would let me get a much more detailed shot. I was wrong.
While a typical smartphone camera can focus on an object as close as 3 inches away from it, the telephoto cameras on all phones I've tested through the years have needed to be at least 1-2 feet away from anything to focus on it. If you're too close and zoom in, your phone will simply do digital zoom with its main camera instead of using its telephoto lens. Again, your phone will give you no indication that you're not zooming with the telephoto lens for the shot.
Nonetheless, a telephoto lens is a valuable asset
The purpose of this brief article is to expose the fundamental flaws in phones' telephoto cameras (and there's more of them) – as I'm sure that knowing what they can and can't do will make you a better photographer. Convincing anyone that there's no point in having a telephoto lens was never my intention. On the contrary, I'm strongly in favor of phones having additional cameras – telephoto, wide-angle, whatever – as it's one of the few realistic ways of improving a phone's photographic potential without ruining its design and appearance.
Still, I wish that the limitations of these lenses were made clearer to the user – in both the flashy ads we're bombarded by and in the camera apps of our phones. But perhaps that could be just me. Anyway, let me know in the comments below if you learned something new.