Single Lens Reflex or SLR camera

What is an SLR camera? That’s the question that most of my friends ask me when they first hear about SLR or when they have some knowledge of cameras and are intending to make their first purchase or foray into the world of photography.

SLR stands for Single Lens Reflex. It’s a camera mechanism more than a technology. These days cameras are filled with technology, from the types of sensors used to the types of memory available. Most of these technologies have come into existence since the digital camera was born. SLR on the other hand is a technology that was born during the age of film cameras. Necessity is the mother of invention as the saying goes and it was this necessity that lead to the birth of the SLR camera.

Technologies or rather Mechanisms that were present in film cameras are very complicated and needs the involvement of the photographer to make use of such features. Things were not as simple then as point-and-click. The photographer would have to consider the camera as an extension of his own body to take a good shot. He would have to think and then make all the settings and then take the shot. These days things are so automated that you just have to look at something through the viewfinder of the camera and then just click and the camera does what is required. We will get to more of these in the days to come. For now lets concentrate on the SLR, what it is and how it came into being.

If you own a non-SLR film camera then you would be familiar with the basic parts of the camera. Importantly, there is the lens in the front of the camera from where the light is exposed onto the film,. Then there is the viewfinder in the back of the camera from where you try to see what the camera is seeing. Yes, I did mean ‘try’. You don’t really think you are seeing what the camera is seeing do you? The viewfinder is placed so close to the lens of a camera that within the limits of parallax you are really close to seeing what the camera sees for objects that are sufficiently far away from the camera. Let me explain in more detail.

While seeing through the viewfinder of the camera feel the front of the camera and try to cover the lens with your finger. Keep your finder in front of the lens such that the finger tip covers the lens and no other portion of the camera is covered by your palm/fist. You would of course realize from this that you are blocking all sources of light to the film. But did you realize that when you see through the viewfinder then you still get to see the whole bright world? Don’t believe me? Take a shot at this stage and see for yourself. The film is dark without any exposure although you were getting a clear view through the viewfinder. You can even try this with any non-SLR digital camera be it with zoom or without. What you see is definitely not what the lens sees. You want more proof? Hold a magnifying glass just in front of the lens on the front. You can still see the whole wide world through the viewfinder, but when you take a shot, the camera has seen it through the magnifying glass.

The secret of the viewfinder is the small opening that is there on the front side of the camera very close to the lens. This is the exit pupil of the viewfinder. In simple/basic cameras this exit pupil is in line with the viewfinder. In more expensive equipment, the exit pupil is placed as close to the lens as possible so as to reduce parallax error. Of course this would involve some costly arrangement of mirrors and prisms to bend and reflect the light to the viewfinder. This forms the basic and simple concept of the SLR camera.

There is no gimmick or mind blowing features in the SLR camera. Its as simple as ‘You see what the camera sees’. There is a complex arrangement of mirrors and prisms from the viewfinder to the lens which allows you to see through the lens. If you place a magnifying glass in front of the lens then you also get to see what the camera sees. How this is achieved and other complex mechanisms in the SLR camera we shall see at a later stage. One other claim to fame of the SLR cameras are its interchangeable lenses. This way you no longer have to stick to the same factory provided lens package. You can buy and add-on and even replace the existing lens package on the camera. This way you can get a range of zooms and other features on your camera that is only limited by your budget. These are the reasons why the SLR camera is such a sought after piece of camera equipment.

Digital Camera & Film

Most of you would know how a film camera takes a photo. Its part of almost every physics text in school/college. In brief, the digital camera works on the principle of the pinhole camera. Light enters the opening of the camera, goes through some transformations, if there are lenses, and then finally falls on the film that is there in the camera. The film is coated with a chemical like Silver Iodide which reacts to light. Depending on the intensity of the light that strikes its surface a sort of a shadow is formed on the surface of the film. Where the intensity of light is more, there the reaction is greater and the surface of the film becomes darker. At places where the light is not present, the film doesn’t react and is stays light. This is why the film contains the ‘Negative’ of the image you are capturing. This negative is converted to a positive image when it is developed in the darkroom.

Now we have digital cameras whose main advantage is that there is no need to buy and keep replacing film. So how does the image form on the digital camera? These use a sort of digital film. It is a sophisticated IC chip. This chip contains millions of small photo sensing pixels and this is what records the image. Each pixel captures the intensity of the light falling on it. Another important part of the digital camera is the image processing chip. This chip receives the image from each pixel on the chip and composes this to form the actual image. How the sensor is actually fabricated and how it records the images is a advanced topic and we will have a look at this in a later stage. For now, you have to understand that the sensor and the image processor together is able to capture the light that falls on its surface and record this on the memory card that comes with the camera. This is how the digital camera has come to replace the film camera.