Monday, March 2, 2009

Exposure in Digital Cameras (Part 1)

Digital cameras offer a wider range of exposure than film or slide cameras of the past and as technology develops the exposure range of newer optical sensors will widen even more.

In this series of articles we're going to talk a little about exposure, exposure control, the histogram, and the loss of detail in over and under exposed photos.

Exposure, like to the elements, is the time the camera's sensor is exposed to a certain amount or quantity of light. If you sit out in the cold long enough you'll probably end up in some stage of hypothermia due to the exposure to the cold. The time it takes (shutter speed) to become dangerously hypothermic (overexposed) depends on how cold it is (amount of available light) and how many layers of clothes you have on (lens aperture). So in order to maintain a safe core temperature we have to find the correct combination of time and layers of clothes to match the temperature we will be exposed to. And so it is with photography, we have to find the correct shutter speed and lens aperture to properly expose the amount of available light. I've left out one important factor in determining the correct exposure and that is the sensitivity of the camera sensor or it's ISO. The analogy there might be how much body fat a person has. Body fat acts as an insulator to the cold and along with the layers of clothing helps regulate the exposure to the cold. If we think film cameras this works pretty well since in order to change the ISO we have to wait till we can change the roll of film and with the body it takes time to lose or gain the fat. With digital cameras, unlike dieting, we can change the ISO on the fly. While it's important and useful, ISO is probably the least adjusted variables (in most cases) in obtaining proper exposure.

So now that you can relate camera exposure to maintaining proper core body temperature, let's get away from the analogies and start to look at the camera settings used to control exposure.

Proper exposure is a key element in the success of an image or photograph. It is as equally important as composition and subject matter. With out proper exposure a photograph of a white and a black piece of paper could be hard to tell apart.

Also as important and directly related to proper exposure is creative exposure. Creative exposure is using the camera controls (mainly shutter speed and lens aperture) to change the appearance of the image. This change of appearance could be subtle changes in tone (the amount of light in an image) to create a certain mood, or using the lens' depth of field (DoF) to isolate the subject in sharp focus, or to either portray or minimize motion with the shutter speed. All these changes in camera settings require a reciprocal or opposing change in another setting to maintain the correct exposure.

Let's talk a little about "correct" and "desired" exposure. In order to have creative license and use creative exposure to it's fullest, we may have to deviate a little from what would be considered "correct" exposure. The "correct" exposure is the exposure settings that the light meter thinks work under the circumstances. The "desired" exposure is the exposure used to obtain the desired results in the final image, even to the point of processing (changing) the desired results in a graphics editing program after the image is captured (called post processing). For all practical purposes they are the same, your desired exposure is the correct exposure and the correct exposure is your desired exposure. But when I talk in this article about "correct" exposure I am talking about what the light meter determines the exposure to be and when I talk about "desired" exposure it is the exposure you purposely set to produce the creative results you want in the final image.

OK lets get down to business and talk specific settings.

In the analogies paragraph we determined that correct exposure is determined by 4 things, the amount of available light, the shutter speed, the lens aperture, and the camera's ISO. Changing anyone of them without a reciprocal change in another or combination of others will affect the correct exposure of the image. This is a good thing as it allows us to change the camera settings to give us the creative or desired exposure we are looking for while still maintaining a correctly exposed image or something reasonably close.

The physics and formulas behind all this exposure nonsense are quite complicated and have been set in stone since God created the Universe. Luckily for us the settings on the camera have all been related to one scale, the f-stop. I'm not going to go into detail about the f-stop, if you want to look it up here is the wiki link. All you need to know about the f-stop in setting the exposure is that each click on the shutter or aperture, or doubling of the ISO value, or doubling of the available light is one stop on the f-scale. This makes it easy to balance one camera setting with another to maintain the same exposure.

In our next article we're going to use the aperture and reciprocal shutter compensation to play with some creative exposures.

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