Monday, February 8, 2010

The importance of ISO number for DSLR cameras

Customizing the Basic Shooting Settings

The suggestions in this article are basic settings and safe-handling procedures that apply to virtually any digital single-lens reflex, so they won’t be detailed step by-step instructions on how to adjust menus or use buttons. Your camera’s manual is the source for specifics about where particular settings are. The organization of the settings varies from camera to camera.

The basic image-sensor and file-capture options are fundamental to picture quality, so the choices you make here have a direct bearing on every picture you take. It’s good that these options are among the easiest to understand and use correctly. As with all options, there are trade-offs to consider. The following list is in a functional order, and may or may not match that for the one in your camera.

A Simple Menu Choice RAW or Processed?

Every time you take a picture, your camera creates a data file containing the raw unprocessed data from the scan. Most digital single-lens reflexs give you the option of saving that file (called a RAW file) or letting it process the data into a standard image format like a JPEG and erasing the original data. You may also have the option of saving both the RAW file and a processed version.

RAW files can be large, about 12MB for a 12- megapixel camera. That’s just under 300 images on a 8GB memory card. The same card in the same camera can hold 2,800 high-quality JPEGs capable of making a good 8x10-inch print. Even so, I still shoot almost all of my pictures in RAW format. The reason is simple.

I like to do my own processing. The RAW file has the most quality and editing potential that that image will ever have. Not only that, RAW lets you go back and modify several critical shooting options during editing with no loss in quality.

You can’t do that with JPEG files that are automatically processed in the camera. The resulting file modifications and compression removes data and editing options. For now I suggest you set your camera to RAW unless you really need to save storage space.

If you do choose to capture in JPEG format, you must also choose an image-quality setting. The options will vary depending on the size of your sensor, so you should check your camera’s manual for details. It’s generally best to use the highest quality available, unless you have limited in-camera storage space. You never know when a wonderful picture opportunity will present itself.

ISO Sensitivity Less Is More

One of the major advantages of digital over film photography is the ability to adjust the sensor’s sensitivity to light at any time, even for just one picture. The lighting wasn’t very bright, and the little cheerleader was moving fast. I needed a fast shutter speed to freeze her motion, so I used almost the fastest ISO setting my camera offered. The ISO number is based on how much light energy is needed to take a picture.

We’ll cover the details more thoroughly in the next article. Right now it suffices to say is that the ISO number indicates the sensor’s sensitivity to light. The higher the number, the less light is needed to create a workable image.

That means we can take pictures under dimming lighting conditions. Doubling the ISO number from its current value doubles the sensitivity of the sensor; cutting it in half reduces the light-gathering ability by half. A shift from 100 to 200 doubles the speed, and to double it again, you raise it from 200 to 400. Most digital single-lens reflexs have a base number of 100 or 200 ISO, and can go up to 3200 or more.

There is a trade-off between quality and speed. Pushing up the ISO is asking more from the technology. When we boost the number, the sensor develops signal noise (usually just called noise). It manifests as little colored dots that degrade the fine detail in the image, and is worse in low-contrast areas having even color and tone.

I tend to leave my ISO number as low as possible unless the lighting conditions demand raising it, and suggest you do the same. The actual working numbers for the lowest and highest ISO settings vary from camera to camera.

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