Monday, August 20, 2012

Sensor Size and the Total Quantity of Light

IT IS SOMETIMES SURPRISING that even inexpensive cameras can take good quality images in the bright mid-day sun. I've seen many photos from cell-phones and from ridiculously cheap point-and-shoot cameras that have more than adequate image quality. Maybe these images aren't particularly optically sharp, but even low-end cameras can produce images in full sunlight that have a low amount of digital noise. They are “good enough.”

But one of the overarching rules of thumb in photography is that the larger the sensor size (or film size), generally speaking, the better the image quality of the final photograph. A bigger sensor makes it easier to have a lower noise image, a bigger sensor makes it easier to make matching quality optics, a bigger sensor makes it easier (up to a point) to make an ergonomic camera, and so forth. Now, I'm not saying that quality photos can’t be taken with a tiny image sensor, rather, it is easier to take an image with higher technical image quality by using a larger sensor. See the article One Easy Rule for Quality Images for more details.

A comparison of camera sensor sizes. [Source and attribution]

As noted, even low-end digital cameras can produce good images in broad daylight. The problem is that their image quality tends to sharply decline as the light gets dimmer. These cameras, taking photos under dim incandescent lighting, produce images that are a noisy mess, with terrible color rendition and digital grain ruining the sharpness of the image. Now, perhaps a tripod could help, but certainly these kinds of cameras are very disappointing for hand-held images.

Our eyesight doesn’t work as we might naïvely think. One scene, which to our eyes appears to be slightly dimmer than another, might in fact have half of the total amount of light falling on it. Likewise, a scene that appears to be only somewhat brighter than another might in reality be twice as bright.  In particular, where I live, in the mid-lattitudes of the northern hemisphere, we get to enjoy long periods of dusk in mid-summer; the fading daylight seems to last for hours, until we finally notice that it is very dark out. Our eyes valiantly attempt to see in the dimming light, until the laws of physics and biology finally conspire against our vision, and we are plunged into darkness. Our eyes attempt to flatten out the huge range of brightness that we experience.

A hazy day may be objectively half as bright as a sunny day, although it certainly seems to be only slightly dimmer. A cloudy day may be one fourth as bright, while an overcast day may be one eighth as bright. At sunset, it may be one sixteenth as bright as a bright sunny day, and a bright day may be thirty two times as bright as what we find at dusk. On ground covered with snow or white sand, a scene may be twice as bright as what we are accustomed to, and there is a real risk of contracting snow blindness due to the excessive amount of light.

Cameras, like eyes, are designed to work over a large range of brightness. Camera lenses have adjustable apertures to vary the amount of light hitting the sensor, and the shutter speed can be varied over a large ranges of values. The sensors also have varying amounts of sensitivity to light. But a sensor with twice the surface area of another collects twice the total amount of light, and we could assume (all things otherwise being equal) that it can operate similarly in light that is half as bright.

Now I've taken decent photos in dark places with a cheap point-and-shoot camera, but that was only when the camera was sitting on a tripod and its shutter was open for a long time. I certainly could not hand-hold the camera and expect to get anything else except digital noise. However, I can and do often take fairly decent hand-held shots at dusk with my Nikon DSLR. The major difference between these two cameras is simply the size of the sensor: the Nikon lets in a far larger total amount of light.

Same scene taken with a newer cell phone camera on top, and an older DSLR camera on the bottom.

We know that cheap point-and-shoot cameras, selling for less than US$50, and having tiny sensors, can take good images in broad daylight with little digital noise. Let us take this quality as our baseline, and determine what size of a sensor we need if we want to take images of similar quality and with similar camera settings under dimmer lighting. This table shows standard digital sensor sizes, along with the lighting conditions that would be equivalent to typical cell phone cameras in bright daylight:

Sensor size Use Sensor area in square millimeters Lighting condition
1/4” Cell phones and toy digital cameras. 7.68 Bright daylight
1/3.2” Premium cell phone cameras. 15.5 Hazy sunlight
1/2.3” Compact digital cameras. 28 Cloudy bright
1/1.7” Premium compact cameras. 43 Light overcast
2/3” Some bridge cameras. 58 Heavy overcast
CX or 1” Nikon 1 series. 116 Sunset
Micro 4/3rds Olympus and Panasonic mirrorless interchangeable lens cameras. 225 Dusk
APS-C Most Nikon, Pentax, and Sony DSLRs; lower-end Canon sensors are slightly smaller at 329 square mm. Also found in some premium rangefinder cameras. 370 Indoor sports, stage shows
35mm, “Full frame” High-end cameras from Nikon, Pentax, Sony, Canon, and Leica. 864 Bright street lighting at night

A camera sensor that has twice the surface area ought to produce an image with a similar amount of digital noise when the lighting is half as bright, all things else being equal. Certainly there are more factors involved, but sensor size is one of the most significant when it comes to image noise.

Photojournalists tend to use the cameras near the bottom of the list, especially if they need to capture a scene in dim lighting without the use of a flash. Note that the Micro 4/3rds cameras are fairly close in sensor size to the APS-C sized cameras, and their discrete size and noiseless operation make them viable for some work under dim lighting. Manufacturers have recently been putting the larger APS-C and 35mm sensors into compact cameras, which many photographers find highly desirable.

For more information, along with some of the data I used to make the table above, see these Wikipedia articles:

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