With or without any knowledge of a good or bad camera, there’s just something about the Single Lens Reflex (SLR) cameras and Digital SLRs (DSLRs) that just makes one want to take pictures. Professional photographers have always chosen them partly because photos clicked using these are of unmatched quality. In addition, they give the artist the flexibility to experiment with different styles of photography and specialize in the one that stimulates them the most.
 
Advancements in technology to support photography both as a hobby and a profession have taken enthusiasm levels to a new high. Digital SLRs let one take multiple photographs without having to worry about running out of film and the many online photo sharing sites have made it extremely easy to showcase work, and get instant appreciation and feedback. All this leaves, the investment to, basically, just the camera.

Good entry level DSLRs are available for around Rs. 30,000. However, exactly what makes these cameras worth the money and a straight choice of professionals? How different are they from the much cheaper point-and-shoot cameras that sometimes have more "megapixels" than DSLRs?
 
This article briefly introduces the technology behind (D)SLR cameras highlighting some differences between them, and point-and-shoot cameras.
 
Before going further, here is a quick explanation of how cameras, in general, work. The key idea is to get light reflecting off an object to converge at a point in a way that a "real" image is formed. A real image is one that can be seen by the eye or displayed on a screen. The lens is the component that does the converging and therefore is an extremely crucial part of any camera. The "screen" in a camera is either a film coated with special chemicals that react with light or a sensor. These capture the light that falls on them and can then be used to develop prints. Pressing the shoot button triggers a bunch of actions that result in the film or the sensor capturing the image formed. The thousands of possible combinations of different settings and lenses affect the light the film or the sensor capture.
 
Mechanism
The key difference in (D)SLRs and point-and-shoot cameras is how they capture light. In (D)SLRs, the light coming in through the lens is reflected to the viewfinder by a mirror and a special prism. The sensor or the film is located behind the mirror. When the button to shoot the picture is clicked, the mirror is moved out of the way and a shutter is lifted momentarily to allow light to fall on the sensor or the film.

(See this in action below). 

The image captured can later be used to generate prints. Anyhow, this way, the photographer captures exactly what s/he sees through the viewfinder. A lot of the magic seen later in the pictures depends on the sensor and the lens, described below.

In point-and-shoot cameras, there is usually a secondary camera that shows the image visible on the display. The photographer doesn't always see exactly what s/he is shooting.
 
Sensor
In digital cameras, the sensor is part of the camera on which the image is formed. The sensor captures the image and converts it into a form that can be stored by converting light into electrical signals. The size of the sensor is an extremely important factor affecting the quality of shots taken. Larger sensors capture images with less noise than smaller sensors. DSLR cameras have larger sensors than point-and-shoot cameras and therefore the quality of shots taken is higher.
 
To understand why sensors matter, consider two cameras A and B of 10 megapixels each, where A has a bigger sensor than B. While there are 10 million pixels in both cameras, the pixels are larger on the bigger sensor. The camera captures more detail as each pixel being bigger is more sensitive to light. Therefore, a larger sensor allows the camera to take photos at a higher ISO. 
 
While, when manipulating a photo more pixels are definitely better, after a point, better sensitivity would be preferable over extra megapixels. However, larger sensors cost more and drive the price of the camera up. If all you want to do with your photos is share them on Facebook, sensor size probably doesn't matter too much. However, if you need to print your photos, it does.
 
Lenses
The lens determines the kind of image that will be formed on the screen / sensor. Almost all (D)SLRs are compatible with multiple lenses and being able to change the lens is probably the biggest advantage SLRs have over point-and-shoot cameras, which usually come with fixed lenses. . There are many different kinds of lenses available, for instance, the zoom lens lets users take photos without the need to get closer to the subject. Wide angle lenses let users take photos with a very wide perspective and are good for landscape shots. Macro lenses let you take extreme close ups. All this just gives more flexibility and a larger creative space.
 
Point-and-shoot cameras are primarily used in fully automatic modes. Also, they are tiny and can be carried around in pockets whereas (D)SLRs are a lot bulkier. While (D)SLRs have a fully automatic mode, the fun is in manually changing various settings to get the right shot /the shot you want.