Ask anyone about the weaknesses of smartphone cameras and you will surely find that people often point towards a phone’s low-light capabilities as the or at least one of its Achilles heel(s). When you are outside during the day it’s relatively easy to shoot some good-looking footage with your mobile device, even with budget phones. Once it’s darker or you’re indoors, things get more difficult. The reason for this is essentially that the image sensors in smartphones are still pretty small compared to those in DSLMs/DLSRs or professional video/cinema cameras. Bigger sensors can collect more photons (light) and produce better low light images. A so-called “Full Frame” sensor in a DSLM like Sony’s Alpha 7-series has a surface area of 864 mm2, a common 1/2.5” smartphone image sensor has only 25 mm2. So why not just put a huge sensor in a smartphone?

While cameras in smartphones have undeniably become a very important factor, the phone is still very much a multi-purpose device and not a single-purpose one like a dedicated camera – for better or worse. That means there are many things to consider when building a phone. I doubt anyone would want a phone with a form factor that doesn’t allow you to put the phone in your pocket. And the flat form factor makes it difficult to build proper optics with larger sensors. Larger sensors also consume more power and produce more heat, not exactly something desirable. If we are talking about smartphone photography from a tripod, some of the missing sensor size can be compensated for with long exposure times. The advancements in computational imaging and AI have also led to dedicated and often quite impressive photography “Night Modes” on smartphones. But very long shutter speeds aren’t really an option for video as any movement appears extremely blurred – and while today’s chipsets can already handle supportive AI processing for photography, more resource-intensive videography is yet a bridge too far. So despite the fact that latest developments signal that we’re about to experience a considerable bump in smartphone image sensor sizes (Sony and Samsung are about to release a 1-inch/almost 1-inch image sensor for phones), one could say that most/all smartphone cameras (still) have a problem with low-light conditions. But you know what? They also have a problem with the exact opposite – very bright conditions!

If you know a little bit about how cameras work and how to set the exposure manually, you have probably come across something called the “exposure triangle”. The exposure triangle contains the three basic parameters that let you set and adjust the exposure of a photo or video on a regular camera: Shutter speed, aperture and ISO. In more general terms you could also say: Time, size and sensitivity. Shutter speed signifies the amount of time that the still image or a single frame of video is exposed to light, for instance 1/50 of a second. The longer the shutter speed, the more light hits the sensor and the brighter the image will be. Aperture refers to the size of the iris’ opening through which the light passes before it hits the sensor (or wayback when the film strip), it’s commonly measured in f-stops, for instance f/2.0. The bigger the aperture (= SMALLER the f-stop number), the more light reaches the sensor and the brighter the image will be. ISO (or “Gain” in some dedicated video cameras) finally refers to the sensitivity of the image sensor, for instance ISO 400. The higher the ISO, the brighter the image will be. Most of the time you want to keep the ISO as low as possible because higher sensitivity introduces more image noise. 

So what exactly is the problem with smartphone cameras in this respect? Well, unlike dedicated cameras, smartphones don’t have a variable aperture, it’s fixed and can’t be adjusted. Ok, there actually have been a few phones with variable aperture, most notably Samsung had one on the S4 Zoom (2013) and K Zoom (2014) and they introduced a dual aperture approach with the S9/Note9 (2018), held on to it for the S10/Note 10 (2019) but dropped it again for the S20/Note20 (2020). But as you can see from the very limited selection, this has been more of an experiment. The fixed aperture means that the exposure triangle for smartphone cameras only has two adjustable parameters: Shutter speed and ISO. Why is this problematic? When there’s movement in a video (either because something moves within the frame or the camera itself moves), we as an audience have become accustomed to a certain degree of motion blur which is related to the used shutter speed. The rule of thumb applied here says: Double the frame rate. So if you are shooting at 24fps, use a shutter speed of 1/48s, if you are shooting at 25fps, use a shutter speed of 1/50s, 1/60s for 30fps etc. This suggestion is not set in stone and in my humble opinion you can deviate from it to a certain degree without it becoming too obvious for casual, non-pixel-peeping viewers – but if the shutter speed is very slow, everything begins to look like a drug-induced stream of consciousness experience and if it’s very fast, things appear jerky and shutter speed becomes stutter speed. So with the aperture being fixed and the shutter speed set at a “recommended” value, you’re left with ISO as an adjustable exposure parameter. Reducing the sensitivity of the sensor is usually only technically possible down to an ISO between 50 and 100 which will still give you a (heavily) overexposed image on a sunny day outside. So here’s our “paradox”: Too much available light can be just as much of an issue as too little when shooting with a smartphone.

What can we do about the two problems? Until significantly bigger smartphone image sensors or computational image enhancement for video arrives, the best thing to tackle the low-light challenge is to provide your own additional lighting or look for more available light, be it natural or artificial. Depending on your situation, this might be relatively easy or downright impossible. If you are trying to capture an unlit building at night, you will most likely not have a sufficient amount of ultra-bright floodlights at your hand. If you are interviewing someone in a dimly lit room, a small LED might just provide enough light to keep the ISO at a level without too much image noise.

Clip-on variable ND filter

As for the too-much-light problem (which ironically gets even worse with bigger sensors setting out to remedy the low-light problems): Try to pick a less sun-drenched spot, shoot with a faster shutter-speed if there is no or little action in the shot or – and this might be the most flexible solution – get yourself an ND (neutral density) filter that reduces the amount of light that passes through the lens. While some regular cameras have inbuilt ND filters, this feature has yet to appear in any smartphone, although OnePlus showcased a prototype phone last year that had something close to a proper ND filter, using a technology called “electrochromic glass” to hide the lens while still letting (less) light pass through (check out this XDA Developers article). So until this actually makes it to the market and proves to be effective, the filter has to be an external one that is either clipped on or screwed on if you use a dedicated case with a corresponding filter thread. You also have the choice between a variable and a non-variable (fixed density) ND filter. A variable ND filter will let you adjust the strength of its filtering effect which is great for flexibility but also have some disadvantages like the possibility of cross-polarization. If you want to learn more about ND filters, I highly recommend checking out this superb in-depth article by Richard Lackey.

So what’s the bigger issue for you personally? Low-light or high-light? 

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