The Sensor
Digital camera memory begins with the camera's image sensor. Light falls on the sensor and goes through an analog-to-digital converter. The amount of information captured depends on the resolution of the sensor.
The Internal Memory
Once the information has been converted to a digital form, it is stored in the camera's internal-memory buffer. Image data is initially stored in the memory buffer because writing data to the buffer is faster than writing data to the memory card. By initially storing the image in the buffer, the camera is able to take another picture much more quickly than if the image is written directly to the card. Writing the data to the memory buffer also allows in-camera image processing to occur before the final image is saved to the card.
The Memory Card
A memory card is a solid-state data storage device. Memory cards have no moving parts and, as such, are much more reliable than mechanical hard disks. Memory cards are non-volatile storage. This means that data written to the card remains on it, even when power is not applied to the card. This means that the card retains its data even when taken out of the camera.
Non-volatile memory, sometimes called "flash" memory, is a form of electronically erasable programmable read-only memory (EEPROM). It is made up of a grid of tiny memory cells that have two transistors. These two transistors change the state of a thin oxide layer to either positive or negative, thus storing the zero or one state of the binary digital data.
In most cases, eight bits are required to store one pixel of image data. To store that pixel, eight memory cells are required. If a digital photograph is taken with a 10-megapixel camera, 80 million memory cells are required. A 1-gigabyte memory card will have 8 billion memory cells and will be able to hold 100 uncompressed images. Most cameras use a compression algorithm to store two to three times that many images on a memory card.
Modern memory cards are available in very large sizes. Four-gigabyte and 8-gigabyte cards are common. Sixteen-gigabyte and 32-gigabyte cards are also available in some memory-card types. Memory cards of this size can easily store well over 1,000 compressed images.
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