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Key Camera Concepts: ISO, F/stop Defined

Photography Basics

More and more, we are seeing customers bringing image capture in-house. Here are some things to consider if you plan to do this:

The downside, however, is that the complexity of current feature-rich cameras can result in lower quality images due to gaps in knowledge, poor documentation and lack of process control. With that in mind, we've created a digital photography primer, a review of digital camera basics and tips for achieving optimal results in print.

What Is The F/stop?

The f/stop setting controls the diameter of the aperture, the temporary opening that allows light to enter and expose the image media as you take a picture.

What Does It Measure?

The higher the f/stop, the smaller the diameter of the aperture during exposure.

What Is The Effect?

A lower f/stop results in a shorter depth of field, which is the area of foreground and background relative to the focal point that remains in focus. We've created several examples to help illustrate the points in this article. Notice that the El Camino behind the Corvette, as well as the Corvette's shadow, are out of focus at this camera's lowest supported f/stop setting (2.5), whereas these areas around the point of focus are crisper at its highest f/stop setting (8.0).

What About The Exposure?

To compensate for the decrease in volume of light, exposure time (shutter speed) must increase as the f/stop increases.


f/stop 2.5
This image was captured with the f/stop set to 2.5. Note that the exposure time, which the camera set automatically, was 1/10 second. A lower f/stop setting heightens emphasis on the point of focus.

f/stop 8
This image was captured with the f/stop set to 8, the camera's maximum. Notice the increased detail in the foreground and the background, as the depth of field has increased. The effect is to lessen the emphasis on the point of focus, while capturing more of the overall image in focus. Note that the exposure time, which the camera set automatically, increased to 1 second.

During the capture of all the example images for this article, we used the camera's built-in light meter to keep the light an approximate constant. You can see it required an exposure 10 times longer to maintain the same brightness at an f/stop of 8.0 vs. an f/stop of 2.5.

Note that all of these exposures were made at a very low speed (ISO setting of 50), which resulted in a long exposure requirement across the board. Also, note that it would take an extraordinarily-steady hand to shoot these images without a tripod. The exposure times are just too long, and even a little bit of movement will result in a blurry shot.

What Does ISO Measure?

ISO, which stands for International Standards Organization, started as a measure of the relative light sensitivity of photographic film. The higher the number, the more sensitive or faster the film became, creating a correctly-exposed image in less time. Higher-speed film was typically used to capture sports and other action shots. Today, the ISO measures the speed of the CCD chip that captures the image in a digital camera.

Does It Apply To Digital?

As with other mid-range to high-end digital cameras, the camera we used has an ISO setting that controls the sensitivity of the light sensor (referred to as the charge-coupled device or CCD). Use this to impose a specific exposure or look-and-feel to your image.

What Is The Effect?

As the ISO value increases, the CCD's sensitivity to light increases, allowing you to capture images at lower light levels and/or faster exposure times. This is especially helpful for both low-light conditions and subjects with motion.


ISO 50
This image was captured with the ISO set to 50. It results in a smooth grain and sharply in-focus image. Note, however that the exposure time required was 1/2 second.

ISO 400
This image was captured with the ISO set to 400, a relatively "fast" speed. This setting results in a much more grainy, rough image, though this effect is diminished if you plan to reduce the image in size. The advantage: It required an exposure of only 1/20 second.

What About Quality?

In the iSO examples above, the digital camera's minimum-supported ISO setting, a value of 50, resulted in a very long exposure requirement (1/2 second), but also resulted in a very smooth texture. The highest supported ISO value, 400, reduced the exposure time required by a factor of 10 (1/20 second), but resulted in a grainy overall texture.

Can I Still Make It Grainy?

If your camera supports it, setting the ISO value to 400 or more will give you a grainy look that was typically associated with faster films. You can emulate this look by experimenting with Photoshop's Add Noise filter under "Effects".

To capture the images for this feature story,
we used a Canon PowerShot digital camera.

           


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