Low Light Cameras ... just the FAQs!

jec6613

DIS Veteran
Joined
Apr 21, 2015
Warning: this is a wall of text. For those that wish to skip the wall of text, there's a nice chart at the bottom followed by a tidy summary and conclusion. :)

Pretty much every gear-related thread in this board boils down to two things:

1) Specific gear for a specific purpose from someone who knows how to shoot on a larger camera.

2) Low light performance and how can I get better than my cell phone.

I'm going to try and tackle #2 here briefly, and get the right terms out so that people can hopefully be a bit better informed. Of course, nobody should feel like they shouldn't ask questions, but very few places online (none that I've found, actually) directly address the low light question for beginners and assume at least good familiarity with photography.

So, to start, let's get a few photography basics out of the way first:

Photographic Stops:

This one is simple: a photographic stop is simply anything simply brightens an image by double, or darkens it by half. This can be anything of what's listed below.

Lens Focal Length:

The lens is what captures light, flips it upside down, and transmits it to the film or image sensor. A lens has a focal length, usually measured in millimeters (though you can find some in centimeters) where the light crosses over in the lens on its way to the sensor. This is an absolute number, and by knowing your focal length and sensor size you can calculate your field of view - or how wide or narrow of an angle your lens sees. Zoom lenses can change their focal length to vary this.

Note, however, that many cameras give their focal lengths in, "35mm equivalent," or, "135 equivalent." This means that the lens has a angular field of view equivalent to a given lens length on a piece of standard Kodak 135 film (36x24mm in size). For instance: an 18.5mm lens on a 24x16 mm APS-C sensor will see the same angle as a 28mm lens on 135 film (at least approximately). However, it does not change the actual focal length of the lens - and understanding that fact is important later.

Fun fact: the word, "Lens," comes from, "Lentil," which is a roughly lens shaped bean.

Exposure:

An exposure is when light strikes either film or a digital image sensor. The term comes from when a mechanical shutter would actually expose a piece of film to light, though in most modern digital cameras it can be exposed all of the time and simply record the light striking it as a function of time, but it's still called exposure.

A photograph is from a segment of time, and is not instantaneous: there is a finite amount of time the shutter is held open for, from a fraction of a second to many minutes in some cases. Any movement of either the camera or the subjects during that time will appear blurred. This isn't a bad thing if you intend it to be blurry with a purpose, such as capturing the trails of fireworks, but obviously on the flipside is that it can lead to blurry people as they move as well, or the entire image being blurry as the camera shakes. Shutter speed is the first element of an exposure.

The second element of exposure is how wide the aperture diaphragm in the lens is opened. Inside most lenses this is adjustable, so you can close the diaphragm to let is less light and open it to let in more. For exposure purposes, this is determined by the F-number, which is determined by the focal length divided by size of the aperture opening. So, for instance, if you have a focal length of 50mm and an aperture opening of 25mm, that's known as f/2.

In any given situation, the number of photons of light striking any given square mmm of sensor will be identical given the same F-number and shutter speed, plus or minus a bit of random quantum shot noise (it's a physics thing, more on that in a bit).

ISO Sensitivity and Modern Digital Sensors:

Lots of people talk about this as the third part of the, "Exposure Triangle," but it doesn't actually affect the exposure. ASA or ISO is simply a measure of how sensitive a film or sensor is to light. Since digital sensors can adjust their sensitivity within a certain range by using sensor gain, adjusting it affects the brightness of the final image, and it can be extended by simple multiplication to give extended ISO ranges, usually labeled as, "Hi" on a camera. Note, this does not affect the exposure itself.

Modern image sensors produced since about 2013/2014 are exceptionally sensitive, and are approaching the physics limits caused by the quantum shot noise during the exposure itself. Older sensors tend to introduce their own noise as well from the electronics, but anything relatively recent doesn't have this issue in any significant amount.

What this all means is that as you increase the ISO, the image becomes noisier, and it appears as a grainy image. Newer cameras make the grain look less objectionable, but it's still there in about the same quantities. This is what causes the low light failures of many images - the sensor simply cannot introduce enough gain to make the image usable anymore for a given exposure.

Fun Fact: ISO is pronounced, "Eye-so," and not, "Eye-Ess-Oh," as though it's named for the International Standards Organization, it doesn't actually stand for anything, it's just word written in all caps by convention.

Putting it all together: Getting More Light for your Image:

So now that we know that for a given exposure, it has the same photons per square millimeter of sensor, and that the sensor will eventually run out of sensitivity (or become so noisy as to be unusable) the question becomes how do we increase the number of photons striking the sensor to get around this?

If you've been keeping up so far, the answer is that there are three basic things you can do:
  1. Hold the shutter open longer
  2. Open the aperture wider
  3. Increase the sensor size
Holding the shutter open longer is something that can and does work very well in some situations. This is what have IS, VR, VC or whatever your particular flavor of lens stabilization technology is helps significantly - it allows you to hold the shutter open longer to get more light, while removing much of the camera movement that the photographer adds. A bad system can quadruple the amount of light coming in this way, and a good system can let you capture sixteen or more times as much light. This is the major advantage that even a basic point and shoot camera has in low light over your phone: they almost all have a lens stabilization system, while very few phones do. Note that digital stabilization systems isn't nearly as effective except in video. Also, if your subject is moving (such as with people) there's an absolute minimum shutter speed to keep them from being blurry - usually around 1/60th of a second for still people, and 1/500th of a second or faster for anything moving.

Next is aperture. Opening the aperture wider also collects more light, but there's a limit to how wide an aperture can open on any given lens, and larger apertures mean bigger lenses. Very wide openings, (referred to as, "Fast," lenses) get to about f/1.2 or f/1.4, with any number lower than f/2 being considered a fast lens, but the sheer quantity of light striking the sensor is determined simply by the absolute size of the aperture opening - which means that for any given angular field of view, the shorter focal length used by a smaller sensor means a smaller absolute aperture opening for the same F-number. What this means for you is that if you already have a lens aperture of f/2 or lower, and are at the limit of acceptable shutter speed, then there's only one thing left to do.

Increasing the sensor size is the last on this list, because it involves moving to a physically larger camera and lens. At a certain point, the lenses become so large that the cameras almost entirely use interchangeable lenses, or if they're not they have a very short or no zoom range. The important number in sensor size is its size in square millimeters, since with more area there are more photons of light to capture.

Calculating It All:

With all of this information, we can calculate with good accuracy the aperture to sensor size relationship. For this chart, I'm going to hold the field of view to 65 degrees - about that of a smartphone, and use a full frame f/8 aperture. All numbers are rounded (camera manufacturers round by more than I do, so this shouldn't affect purchasing decision). This should give you some sort of idea of how aperture scales with sensor size:

Code:
Sensor size     Sensor Area     Example Camera          Focal Length    Aperture
1/3.2"          15.4            Typical Smartphone      4mm             f/1.1
1/2.3"          28.5            Typical P&S, iPhone X   5mm             f/1.2
1/1.7"          40              Premium P&S             6mm             f/1.8
1"              116             High end P&S, Nikon 1   10mm            f/2.8
4/3"            224             Micro 4/3 System        14mm            f/4
APS-C (Canon)   329             Canon APS-C cameras     17.5mm          f/5
APS-C (Others)  384             Other APS-C cameras     18.5mm          f/5.6
Full Frame      864             Full Frame camera       28mm            f/8

Conclusion:

So what to take from all of this? Well, the first is that there's no free lunch - physics limits how well a camera can handle in low light. While technologies such as image stabilization and computational photography in both modern smartphones as well as cameras have helped significantly, we still end up where we need a minimum aperture opening size (absolute opening, not F-number), and that physical aperture opening dominates the discussion, not the F-number.

Note on that chart above that we move from lenses that don't even exist for the first two sizes, into large-ish compact lenses, and then down through what is essentially a pinhole lens (or disposable camera lens) for the full frame camera. The amount of glass required to make each lens is effectively identical for each step, so only by increasing the lens size do we gain truly better low light photography. And, at a certain point, manufacturing concerns become an issue: it's trivial to make a good 18.5mm f/5.6 lens, and 28mm f/8 lenses can be made quite good using plastic instead of glass, but it's darn near impossible to make a good 4mm f/1.1 lens.

Takeaways:
  • If you have a good phone camera with f/1.8 or f/2 lens, that it's going to outperform a basic P&S camera with its f/3.5 lens - and that it's going to perform identically some 1" sensor super zoom cameras in low light. What those dedicated cameras are really getting you are zoom and a true strobe flash, and most people would be better served by spending their money a better phone, which brings other benefits as well, rather than a cheap P&S.
  • A 1" sensor compact with fast f/1.8-2.8 lens, which is pretty pocketable, will perform similarly to a DSLR or mirrorless APS-C camera with many of their kit lenses at f/3.5-5.6, which are far less pocketable.
  • Interchangeable lens cameras let you have different low light performance based on situations. While a kit zoom lens is fine during the day, you can swap on a reasonable size f/1.8 or f/1.4 lens without a zoom at night to make it a true low light performer. Essentially, they let you get an entirely different camera for different situations just by swapping lenses. They're also bigger and heavier than a compact - the price you pay for flexibility.
  • At the end of the day, this is all down to what your personal tolerance for quality photos is, how much you're willing to carry, and how much you're willing to spend.
 

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