A Short Course Book
Curtin's Guide to Digital Cameras
And Other Photographic Equipment

Image Sensors—Resolution

Click to see the effects of pixelization as an image is enlarged.
Click to explore pixels and aspect ratios.
Good prints can be made using 200 pixels per inch. Using this as a guide you can calculate that a 2000 x 1600 pixel image (just over 3 Megapixels) will make a good 10 x 8 inch print.
Click here to explore the original meaning of "resolution."
Click to explore how more pixels give sharper images.
Click for an Excel work sheet that converts pixels into print sizes.
Click to see how some cameras inflate their pixel counts.
The quality of a digital image depends in part on the number of pixels it contains (sometimes referred to as resolution). At a given size, more pixels add detail and sharpen edges. However, there are always size limits. When you enlarge any digital image enough, the pixels begin to show—an effect called pixelization. This is not unlike traditional silver-based prints where grain begins to show when prints are enlarged past a certain point.
When a digital image is displayed or printed at the correct size for the number of pixels it contains, it looks like a normal photograph. When enlarged too much (as is the eyehere), its square pixels begin to show.

The term "resolution" has two meanings in photography. Originally it referred to the ability of a camera system to resolve pairs of fine lines such as those on a test chart. In this usage it's an indicator of sharpness, not image size. With the introduction of digital cameras the term began being used to indicate the number of pixels a camera could capture.

The pixel size of a digital photograph is specified in one of two ways— by its dimensions in pixels or by the total number of pixels it contains. For example, the same image can be said to have 4368 � 2912 pixels (where "�" is pronounced "by" as in "4368 by 2912"), or to contain 12.7 million pixels or Megapixels (4368 multiplied by 2912).

As you might expect, all other things being equal, costs rise with a camera's resolution. Greater resolution also creates other problems. For example, more pixels means larger image files. Not only do larger files take more storage space, they put greater demands on systems and networks when you edit, email, and post them on a Web site.
  • Lower resolutions such as 640 x 480 are perfect for Web publishing, e-mail attachments, small prints, or images in documents and presentations. For these uses, higher resolutions just increase file sizes without significantly improving the images.

Many cameras give you a choice of image sizes from small to large.
  • Higher resolutions, 6 million pixels or so, are best for printing photo-realistic enlargements larger than 5" x 7". Good prints can be made using 200 pixels per inch. Using this as a guide you can calculate that a 2000 x 1600 pixel image (just over 3 Megapixels) will make a good 10 x 8 inch print.
    Beware of claims about resolution for cameras because there are two kinds of resolution; optical and interpolated. The optical resolution of a camera or scanner is an absolute number because an image sensor's pixels or photosites are physical devices that can be counted. To improve resolution in certain limited respects, the optical resolution can be increased using software. This process, called interpolated resolution, adds pixels to the image to increase the total number of pixels. To do so, software evaluates those pixels surrounding each new pixel to determine what its color should be. For example, if all of the pixels around a newly inserted pixel are red, the new pixel will be red. What's important to keep in mind is that interpolated resolution doesn't add any new information to the image— it just adds pixels and makes the file larger. This same thing can be done in a photo-editing program such as Photoshop by resizing the image. Beware of companies that promote or emphasize their device's interpolated (or enhanced) resolution. You're getting less than you think you are. Always check for the device's optical resolution. If this isn't provided, flee the product— you're dealing with marketing people who don't have your best interests at heart.
A few camera companies, even some that are otherwise respectable, try to deceive you into thinking their cameras have higher resolution than they really do. They use software to inflate the size of a captured image and then use this inflated size in advertising claims about the camera. This way each captured pixel can suddenly become four, and voila' a 2 Megapixel image suddenly and magically becomes 8 Megapixels.

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