So what is a camera?

The first camera.

Photographic cameras were a development of the camera obscura, a device dating back to the ancient Chinese and ancient Greeks,  which uses a pinhole or lens to project an image of the scene outside upside-down onto a viewing surface.

On 24 January 1544, mathematician and instrument maker Reiners Gemma Frisius of Leuven University used camera obscura to watch a solar eclipse.

Before the invention of photographic processes there was no way to preserve the images produced by these cameras apart from manually tracing them. The earliest cameras were room-sized, with space for one or more people inside; these gradually evolved into more and more compact models such as that by Niépce's time portable handheld cameras suitable for photography were readily available. The first camera that was small and portable enough to be practical for photography was envisioned by Johann Zahn in 1685, though it would be almost 150 years before such an application was possible.


What next?

Louis Daguerre and Joseph Nicéphore Niépce (who was Daguerre's partner, but died before their invention was completed) invented the first practical photographic method, which was named the daguerreotype, in 1836. Daguerre coated a copper plate with silver, then treated it with iodine vapor to make it sensitive to light. The image was developed by mercury vapor and fixed with a strong solution of ordinary salt (sodium chloride).

35mm

35 mm film is the film gauge most commonly used for chemical still photography (see 135 film) and motion pictures. The name of the gauge refers to the width of the photographic film, which consists of strips 34.98 ±0.03 mm (1.377 ±0.001 inches) wide.

Reflex Camera

The first practical reflex camera was the Franke & Heidecke Rolleiflex medium format TLR of 1928. Though both single- and twin-lens reflex cameras had been available for decades, they were too bulky to achieve much popularity.

A similar revolution in SLR design began in 1933 with the introduction of the Ihagee Exakta, a compact SLR which used 127 rollfilm. This was followed three years later by the first Western SLR to use 35mm film, the Kine Exakta (World's first true 35mm SLR was Soviet "Sport" camera, marketed several months before Kine Exakta, though "Sport" used its own film cartridge). The 35mm SLR design gained immediate popularity and there was an explosion of new models and innovative features after World War II. There were also a few 35mm TLRs, the best-known of which was the Contaflex of 1935, but for the most part these met with little success.

Digital

Digital cameras differ from their analog predecessors primarily in that they do not use film, but capture and save photographs on digital memory cards or internal storage instead. Their low operating costs have relegated chemical cameras to niche markets. Digital cameras now include wireless communication capabilities (for example Wi-Fi or Bluetooth) to transfer, print or share photos, and are commonly found on mobile phones.

DSLR's

‘Digital single-lens reflex cameras’ are digital cameras combining the optics and the mechanisms of a single-lens reflex camera with a digital imaging sensor, as opposed to photographic film. The reflex design scheme is the primary difference between a DSLR and other digital cameras. In the reflex design, light travels through the lens, then to a mirror that alternates to send the image to either the viewfinder or the image sensor. The alternative would be to have a viewfinder with its own lens, hence the term "single lens" for this design. By using only one lens, the viewfinder presents an image that will not perceptibly differ from what is captured by the camera's sensor.