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Photo(genics) Things Your Camera Never Told You - Part 2
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Lesson # 2 – How does the camera control light?

The amount of light that actually impacts the film plane or processor is controlled in three different yet totally interconnected ways.

• The length of the exposure time (shutter speed)

• The size of the pinhole (aperture)

• The sensitivity of the emulsion (film speed or ISO)

Let’s look at each of these light control mechanisms and examine how they affect light and ultimately your photos.

1. Shutter speed.

Go back to our basic light box concept. Even the most rudimentary camera had a mechanism to control the length of time that the box was open to light, even if it was just the operator’s hand over the hole. As the camera evolved, shutters were placed over the opening that could be opened for a specified period of time. No light entered the camera except when the shutter was open. Modern cameras still use this basic concept. You will see a variety of shutter speed options displayed either automatically or manually on these cameras. They are expressed as fractions of a second and look like this:

(1/500, 1/250, 1/125, 1/60, 1/30...and so on.

Do you see any relationship between these fractions?

Each fraction is approximately 1/2 the value of the previous one, or double the value, depending on which way you are going. What that tells you is that adjusting your shutter speed one click will either half or double the amount of light entering your camera. As an example:

moving from 1/125 of a second to 1/250 of a second will Half the amount of light entering the camera.

2. Aperture

I guess on the old light box, you could just make a bigger hole, but that's not very practical. Modern cameras have lenses with an adjustable iris called an aperture. The term "closing down" refers to the action of making the aperture smaller. Aperture settings appear, usually on the barrels of lenses, and look a little more confusing, until you remember we are talking about light. They appear as numbers such as 3.2, 5.6, 8, 11, 16, ...and so on.

Don't be surprised if you can't see the relationship between these numbers in the same way as with shutter speed. For those who want to fully understand this relationship, you can research the "Theory of Light" and the "Inverse Square Law".

Now I could tell you that you must understand these concepts, but the good news is, it just doesn't matter.

Just understand that those numbers are directly correlated to the intensity of reflected light and each one, as with the shutter speed, will either half or double the amount of light allowed in as you adjust the aperture.

3. ISO

This is an old standard by which the light sensitivity of film was measured. When film was widely used, most photographers chose "slower" films, or those with lower ISO numbers such as 60 or 100. These were less sensitive to light and took longer to expose but yeilded higher quality negatives. "Faster" films or higher ISO films were used for stopping action. To photograph a spinning propeller you need a very fast shutter speed. The consequence of that is less time for the light to expose the film, therefore, a highly sensitive, or high ISO film was required. These old ISO standards have carried over into the digital age and are displayed in the camera as:

100, 200, 400, 800, ....and so on.

As with film, these "speeds" are essentially indications of the light sensitivity of the digital processor. A high ISO setting means a greater capacity of the processor to recognize a lesser amount of light. As with film though, higher ISO settings produce generally lower quality images, due to the lack of light that is allowed to impact the processor.

As you can see, each one of these three mechanisms has the capacity to control light by either halving or doubling the amount of light recorded on the processor inside the camera. The beauty is that each is interchangeable with any of the others. In other words, closing down the aperture by one click and decreasing the shutter speed by one click has no effect on the amount of light entering the camera. Likewise a higher ISO setting by one click coupled with a slower shutter speed has a nil effect.

So now we know, It's all about the the light and controlling it inside the camera.

In the next lesson we will learn about using these concepts and why we want to control the light in the first place.

Happy Shooting!

Street Talk

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