In photography, exposure is the total amount of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance over a specified area.In photographic jargon, an exposure generally refers to a single shutter cycle. For example: a long exposure refers to a single, protracted shutter cycle to capture enough low-intensity light, whereas a multiple exposure involves a series of relatively brief shutter cycles; effectively layering a series of photographs in one image. For the same film speed, the accumulated photometric exposure (H) should be similar in both cases. (Wikipedia).
Confused? So am I. Let's explain what we have here. Exposure is the time the shutter stays open until it closes back again. In this period of time some wonderful things happen to the sensor of our camera. For starters, the tiny little elements which compose the camera sensor will start to become active. The result is what we can see afterwards called a "photo". But how is this accomplished?
I'll share some little dirty secrets of the sensor with you. You see, the sensor is color blind, it can only see gray not colors! Recording color information is accomplished by inserting three types of filters between the lens and the sensor, a Red, a Green and a Blue. This affects the spectrum of light that is allowed to pass through these internal filters, which is then composed internally and results in the final photo.
Generally, what a camera tries to accomplish is getting an overall light metering and then this is compared to a matrix of medium grey (around 40%) to give the readings of over or under exposed. What do you do to affect the exposure? Most of the time this is correct and you don’t have to correct anything the time you take the shot. But there are some situations that need manual correction on the spot!
Generally you should shoot in RAW or TIF which is the camera's "negative", holds more information than JPG and allows for more manipulation in exposure or color correction later. A JPG is a compressed format which is of inferior quality in terms of post processing and allows for little correction overall. But let's see what is over/under exposure and how this can be corrected on the spot or later.
There is no "perfect" exposure for any photo, keep that in mind. This is the other little dirty secret of photography, so don't worry if your photos come up a little over or less exposed even if you acted according to "the book". Later cameras have this little wonder called LiveView function which shows how the image is going to be recorded BEFORE pulling the trigger. Actually even if there's no LiveView function, there surely will be an AfterShot view function which shows the image AFTER this has been saved in camera. In the later situation, the only way, if the exposure is by far incorrect (very dark or very bright),is to recompose, correct the exposure and shoot again.
Light is divided into 8 parts or "stops". Human eyes can see and distinct all 8 stops while cameras can only "see" 5 of them. The photographer has to choose which 5 he/she wants to catch on camera. Of course, technology leaps forward and soon there will be a way to catch all 8 stops of light. Currently there is only one such brand of cameras, the Pentax K-x Series which feature an experimental function called HDR Capture which I have never used or see function reviews. For all other cameras, you have to fight with the light to get the amount you want recorded. Actually this is not really hard at all! Exposure compensation can usually be corrected on the spot by pressing the AV button while in Creative mode (AV, TV, Manual) and rotating the dial to the right (+) or left (-) to compensate.
When photographing sunsets or moonlights with strong contrasting scenes that need all available stops of light, you have to choose what to record and what to leave out. It's a hard choice to be made, but as soon as you realize that you can't have it all no matter what, you'll be ok. To give you a hint, you have two choices:
The first is to set the exposure lower by 1 or 2 stops, let most of the foreground elements appear as shadows and capture the rest bright sunset colors with the rest of the available stops (almost the whole 5 of them!). This is by far my favorite choice as it results in the viewer focusing on the shadows against colorful light background. The variety of the colors available for the sunset (with a minor white balance correction afterwards to cool or warm the white values) created impressive results.
The second choice is more complicated. It consists of 3 shots blended together afterwards in photoshop. The first should be taken according to camera readings, the two after this one, brighter and darker by 2 stops. Then the darkest areas are brushed over from the layer where the values are brighter to unveil the details.
The same is done with the brightest, burnt areas. In other words we replace the burnt and black areas with portions of the other two photographs which have recorded information in these areas. Believe me this is not easy to accomplish (you need to be careful while brushing and compensating) and the result may not worth the trouble.
