Sensors Image sensors are arrays of electronic devices that convert the optical image created by the camera lens into a digital file that is stored in some
digital memory device, inside or outside the camera. Each element of the image
sensor array measures the
intensity of light hitting a small area of the projected image (a
pixel) and converts it to a digital value. The two main types of sensors are
charge-coupled devices (CCD)—in which the photo charge is shifted to a central charge-to-voltage converter—and
CMOS or
active pixel sensors. Most cameras for the general consumer market create color images, in which each pixel has a color value from a three-dimensional
color space like
RGB. Although there is light-sensing technology that can distinguish the wavelength of the light incident on each pixel, most cameras use monochrome sensors that can only record the intensity of that light, over a broad range of wavelengths that includes all the
visible spectrum. To obtain color images, those cameras depend on color filters applied over each pixel, typically in a
Bayer pattern, or (rarely) on movable filters or light splitters such as
dichroic mirrors. The resulting grayscale images are then combined to produce a color image. This step is usually performed by the camera itself, although some cameras may optionally provide the unprocessed grayscale images in a so-called
raw image format. However, some special-purpose cameras, such as those for
thermal mapping, or
low light viewing, or
high speed capture, may record only monochrome (
grayscale) images. The
Leica Monochrom cameras, for example, opted for a grayscale-only sensor to get better resolution and dynamic range. The reduction from three-dimensional color to grayscale or simulated
sepia toning may also be performed by digital
post-processing, often as an option in the camera itself. On the other hand, some
multispectral cameras may record more than three color coordinates for each pixel.
Multifunctionality and connectivity In most digital camera (except some high-end
linear array cameras and simple, low-end
webcams), a
digital memory device is used for storing images, which may be transferred to a computer later. This memory device is usually a
memory card;
floppy disks and
CD-RWs are less common. In addition to taking pictures, digital cameras may also record sound and video. Some function as
webcams, some use the
PictBridge standard to connect to printers without using a computer, and some can display pictures directly on a television set. Similarly, many
camcorders can take still photographs and store them on
videotape or
flash memory cards with the same functionality as
digital cameras. Digital photography is an example of the shift from analog information to digital information. In the past, conventional photography was an entirely chemical and mechanical process that did not require electricity. Now, modern photography is a digital process in which analog signals are converted to and stored as digital data using built-in computers.
Performance metrics The quality of a digital image is a composite of various factors, many of which are similar to those of film cameras.
Pixel count (typically listed in
megapixels, millions of pixels) is only one of the major factors, though it is the most heavily marketed
figure of merit. Digital camera manufacturers advertise this figure because consumers can use it to easily compare camera capabilities. It is not, however, the major factor in evaluating a digital camera for most applications. The processing system inside the camera that turns the raw data into a color-balanced and pleasing photograph is usually more critical, which is why some 4+ megapixel cameras perform better than higher-end cameras. Resolution in pixels is not the only measure of image quality. A larger sensor with the same number of pixels generally produces a better image than a smaller one. One of the most important benefits of this is a reduction in
image noise. This is one of the advantages of DSLR cameras, which have larger sensors than simpler point-and-shoot cameras of the same resolution. Additional factors that impact the quality of a digital image include: •
Lens quality: resolution,
distortion,
dispersion (see
Lens (optics)) •
Capture medium: CMOS, CCD,
negative film,
reversal film •
Capture format: pixel count, digital file type (
RAW,
TIFF,
JPEG),
film format (
135 film,
120 film), aspect ratio •
Processing: digital or chemical processing of "negative" and "print"
Pixel counts The number of
pixels
n for a given maximum
resolution (
w horizontal pixels by
h vertical pixels) is the product
n =
w × h. For example, an image 1600 × 1200 in size has 1,920,000 pixels, or 1.92 megapixels. The
pixel count quoted by manufacturers can be misleading as it may not be the number of full-color pixels. For cameras using single-chip
image sensors, the number claimed is the total number of single-color-sensitive photosensors, whether they have different locations in the plane, as with the
Bayer sensor, or in stacks of three co-located photosensors as in the
Foveon X3 sensor. However, the images have different numbers of RGB pixels: Bayer-sensor cameras produce as many RGB pixels as photosensors via
demosaicing (interpolation), while Foveon sensors produce uninterpolated image files with one-third as many RGB pixels as photosensors. Comparisons of megapixel ratings of these two types of sensors are sometimes a subject of dispute. The relative increase in detail resulting from an increase in resolution is better compared by looking at the number of pixels across (or down) the picture, rather than the total number of pixels in the picture area. For example, a sensor of 2560 × 1600 sensor elements is described as "4 megapixels" (2560 × 1600= 4,096,000). Increasing to 3200 × 2048 increases the pixels in the picture to 6,553,600 (6.5 megapixels), a factor of 1.6, but the pixels per cm in the picture (at the same image size) increases by only 1.25 times. A measure of the comparative increase in linear resolution is the square root of the increase in area resolution (i.e., megapixels in the entire image).
Dynamic range Both digital and film practical imaging systems have a limited "
dynamic range": the range of
luminosity that can be reproduced accurately.
Highlights of the subject that are too bright are rendered as white, with no detail (
overexposure);
shadows that are too dark are rendered as black (
underexposure). The loss of detail in the highlights is not abrupt with film, or in dark shadows with digital sensors. "Highlight burn-out" of digital sensors is not usually abrupt in output images due to the
tone mapping required to fit their large dynamic range into the more limited dynamic range of the output (be it SDR display or printing). Because sensor elements for different colors saturate in turn, there can be hue or saturation shift in burnt-out highlights. Some digital cameras can show these blown highlights in the image review, allowing the photographer to re-shoot the picture with a modified exposure. Others compensate for the total contrast of a scene by selectively exposing darker pixels longer. A third technique is used by Fujifilm in its
FinePix S3 Pro DSLR: the image sensor contains additional
photodiodes of lower sensitivity than the main ones; these retain detail in parts of the image too bright for the main sensor.
High-dynamic-range imaging (HDR) addresses this problem by increasing the dynamic range of images by either • increasing the dynamic range of the image sensor, or • using exposure
bracketing and post-processing the separate images to create a single image with a higher dynamic range.
Storage Many
camera phones and most digital cameras use
memory cards with
flash memory to store image data. The majority of cards for separate cameras are
Secure Digital (SD) format, or the older
CompactFlash (CF) format; other formats are rare.
XQD card format was the last new form of card, targeted at high-definition camcorders and high-resolution digital photo cameras. Most modern digital cameras also use
internal memory of limited capacity to hold pictures temporarily, regardless of whether or not the camera is equipped with a memory card. These pictures can then be transferred later to a memory card or external device. Memory cards can hold vast numbers of photos, requiring attention only when the memory card is full. For most users, this means hundreds of quality photos stored on the same memory card. Images may be transferred to other media for archival or personal use. Cards with high speed and capacity are suited to video and
burst mode (capture several photographs in quick succession). Because photographers rely on the integrity of image files, it is important to take proper care of memory cards. One process is
card formatting, which essentially involves scanning the cards for possible errors. Common advocacy calls for formatting cards after transferring its images onto a computer. This clears previously-stored data, such as the file system set up for an older camera, removes accumulated errors, and optimizes the file system for the particular camera model. Since all cameras only do quick formatting of cards, it is advisable to occasionally carry out a more thorough formatting using appropriate software on a computer. == Comparison with film photography ==