Digital Photography Notes

Low resolution images are fine for on-screen display in a Web page or multimedia presentation.

Digital cameras use imaging arrays to capture images. These chips are: CCD, which stands for chardge-coupled device, and CMOS, which stands for complimentary metal-oxide semiconductor.

Some cameras allow you to hook them directly to a printer.

Professional printing uses the CMYK color model that uses four-color channels corresponding to the four primary colors of ink, cyan, magenta, yellow and black.

Resolution is measured in terms of pixels per linear inch, not square inch. So a resolution of 72 ppi means that you have 72 pixels horizontally and 82 pixels vertically, or 5,184 pixels per square inch of image.

Each pixel can be one color and one color only. With fewer pixels, you have fewer distinct blocks of color to represent your scene, which means that small images details get lost.

In most cases, you should capture your images at the highest resolution your camera permits. For on-screen use a resolution of 72 to 96 ppi is plenty.

If you plan on printing your image on a color inkjet or laser printer, you want a resolution around 300 ppi.

 In order to determine the maximum size at which you can produce an image at a particular resolution, divide the total number of image pixels across or down by the desired resolution. For example, if your camera captures 640x480 pixels and your target resolution is 300 ppi, divide 640 by 300 to get the maximum image width (2.13 inches). Or, to find out the maximum height, divide 480 by 300, which equals 1.60 inches. In other words, with a 640x480 camera, you can print an image that’s 2.13 inches wide by 1.60 inches tall at 300 ppi. At 72 ppi, that same image measures 8.89” wide by 6.67” tall.

Be aware that higher resolutions than a printer’s capacity, do not necessary deliver better printer output. In fact the printer will try to eliminate extra pixels (downsampling) possibly resulting in bad output.

File size is determined by the number of pixels in an image and not the image resolution (ppi). A 640x480 image contains the same number of pixels (307,200) at 72 ppi as it does at 300 ppi. In other words, when it comes to file size, a small pixel counts as much as a large pixel.

You want the appropriate number of pixels to suit your final output device (screen or printer), but no more.

With an image editing software you can add (upsampling) or delete (downsampling) pixels. This process is called resampling.

Try not to exceed 10 to 15% upsampling and 25% downsampling.

The safest way to change adjust an image resolution is to resize the image while maintaining the original pixel count. If you reduce the image size, the pixels shrink and move closer, if you enlarge the image, the pixels grow and spread. For example if you have a 4x3 image at 300 ppi and you double it size to 8x3, the resolution is cut half to 150 ppi. Conversely, if you reduce the image size by half, to 2x1.5, the resolution is doubled to 600 ppi.

Reducing image size is the only non-damaging way to increase image resolution so you should capture your digital pictures at the highest pixel count possible. This way you can print large images at good resolutions. Be aware that not all image-editing programs enable you to maintain your original pixel count when resizing images.

A VGA resolution is 640x480.

Most Macintosh monitors are set at 72 ppi where PC monitors are usually set to display 96 ppi.

Printer resolution is measured in terms of dots per inch or dpi. In general, the higher the dpi, the smaller the dots and better looking print out. You should not interchange dpi and ppi. A printer dot is not the same thing as an image pixel. Most printers use several printer dots to reproduce one image pixel.

Remember:

Image width (or height) divided by the number of pixels across (or down) equals the image resolution (ppi).

For good prints, strive for a resolution of 300 ppi.

For on-screen display, 72 to 96 ppi is appropriate. However, when sizing images for the screen, think in terms of image pixel dimensions than image resolution.

Enlarging a picture can reduce image quality.

To raise the resolution of an existing image, reduce the image size.

In most cases, set your camera to capture the most pixels possible.

To determine what image resolution you can get from a digital camera, divide the horizontal or vertical pixel count by the size of the final image.

More pixels mean a bigger image file.

Color Models

RGB stands for red, green and blue and it is the color model used by any device that transmits or filters light.

SRGB offers a smaller gamut or range of colors than RGB in order to improve color matching between on –screen and printed images. RGB devices can produce more colors that printer inks can reproduce.

CMYK: Printers mix primary colors of ink to emblazon a page with color. These colors are cyan, magenta, yellow and black. The K is used to represent black because B could be mistaken for blue (K plate).

Bit depth refers to the number of bits in an image and indicates the amount of color information each image pixel can contain. The higher the bit depth, the larger the image file.

Indexed color refers to images that have had some of their colors stripped away in order to reduce the image bit depth and therefore the file size.

Grayscale image is comprised solely of shade of black and white.

Black and white images are referred to as line art.

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