Light and Exposure ...

Ayeeee .... here it comes... at the speed of 3lac km/second - yes ideally in the vaccum ofcourse - LIGHT... what a wonderful thing to have; have you ever imagined a world without ligt, something like living in Mariana Trunch or something. ho i can imagine. So here i am writing about light and the wonderful moments it illuminates. And also the moments which are captured for later time; ofcourse thats what we are talking about Cameras. A moment is captured in a film or a sensor through a process called exposure. An exposure is nothing but the process of exposing the chemicals on a film inside a film camera or the light sensors on a sensor in a digital camera to the light. This light excites the molecules and is stored for ever on the film or stored temoparaly on a buffer in digital camera.

Exposure is measured in lux seconds and it is the amount of light allowed expose the film. Its also mearured in Electron Volts (EV) which is the normal photographer term for telling it. In advanced digital cameras we have option to see the image histogram which shows the tonal distribution of light across the frame. From which a person can understand how good the exposure was. If the histogram is distributed across like a bell shape and is concentrated in the middle that indicates a good expoure.

Under Exposure: As you know if you expose a film for less time than the ideal time requred to record the details of the scene we call it as Under Exposure. In an under exposed photo all the details or most of the details would be dark and not properly illuminated. The histogram of an under exposed image will be drifted towards the left of the graph. Normally a photographer does is when there is so much light in the scene, like when you are shooting in the midday or shooting the sun or sky, or greeneries (this is a particular case i will tell you later)

Over Exposure: This is the term used when we have / or camera in AUTO mode has allowed more light than necessary to the film or the sensor. This is some thing which normal photographer would never like to happen as all the detail would have blown out; the term blown out is used since the pixels would have stored the maximum electron volt it could store. So pixels of the picture would be completely white in color, in a blown out picture most of the picture pixel would have store no color or details. An occasion when a photographer does is when the light is not evenly distributed over the scene and some of the subject is under darkness, if the priority of the subject in dark is more important he would prefer to get the correct exposure on that subject ignoring the over exposure of the other subjects. Same situation applies to Under Exposure also.

In a camera there is a Light Meter which measures the light from a scene where we are pointing the camera. The metering circuit of a camera is one of the most important part of the camera. The camera in auto mode measures the light and calculates how much time the film or the sensor should be exposed to get a perfect exposure. For this what the light meter does is it theoretically dilutes all the light from the scene and you know when you mix all the colors you would get one color. This color is then coverted to greyscale by the metering system and compared with the stored value. This stored value is equivalent to 18% grey card. So what the metering system will do is it will quickly changes the shutter speed and aperture (which i will talk later about) to control the light coming in to get the 18% grey value from the scene. People use a card which is equivalent to 18% grey to get consistant results, they will read the light from the grey card at the scene and this will give the camera and oppurtunity to ignor e the influence of colors in the calculation of exposure.

Check this picture, the bracket shows where the light is read from to get the exposure given; when the light is read from the sun the picture is under exposed since there is so much light coming in. When it is read from the absolut dark areas the picture has been over exposed. And the idea exposure is got from the areas where it is correctly lighted or half of the bracket is dark and half is bright. http://www.easyphotography.info/page3images/histogram.jpg

This is how an automatic metering system find the perfect exposure. The better the calculation and measurement is done by the metering system the better the image would be. In DSLRs we have option to alter the perfect exposure found by the camera using Exposure Compensation. There would be a button which would let you choose an exposure value which is in the range -3 to +3 (varies camera to camera) in steps of 1/2 or 1/3; if we take a negative value the image will be under exposed and if positive it will be over exposed.

So thats all for today, so i think now you wouldnt blink when some one talks about exposures, under exposure, over exposure, image histogram, metering, grey card, exposure compensation etc. You have learned how to read exposure from histogram and even how camera calculates the exposure.