Digital Imaging

DIGITAL IMAGING NOTES 2

WHAT IS THE DIFFERENCE BETWEEN DIGITAL AND FILM CAMERAS?

Digital cameras -- shooting and storage, resolution and compression, file formats and sizes, "point and shoot " versus "prosumer"

Film cameras -- film ASA, lens aperture, shutter speed; exposure; depth of field;

PIXEL

PIXEL: "Picture element." The smallest single thing of one color that you can see on your computer screen or television. The total number of pixels limits the detail that can be seen on a screen. Most computer monitors have a standard screen resolution of 72 pixels per inch, which corresponds with the ancient typesetting system of 72 points per inch. Also, most typical television sets have less than half a million pixels, while HDTV may have two million or more.

TYPES OF RESOLUTION

Screen resolution -- Detail visible on the computer screen. Usually 72 ppi. (Note a similarity-- in printing, there are 72 points per inch, for instance, a 36 point headline is one half inch).
Image resolution -- Detail in the image itself. Using 72 ppi for Web display file sizes small for easier downloading . More detailed image resolution is used for color printing (usually from 300 to 600 ppi ).
Pixel resolution (or bit depth or pixel depth or color depth) -- Can be either: -- One channel of "indexed" color information, with one to eight bits, or -- Three channels of 8-bit color information (which makes 24 bits), called "RGB mode." (Sometimes RGB has a fourth channel called an "alpha" channel, which makes 32 bits).

COMPRESSION

Jpg 24-bit formats can be used to compress photos but there may be a great loss of quality. Gif 8-bit format allows subtraction of colors to reduce file size. Tif format freezes the image size without loss.
Why is this important? Given download speeds, you dont want more image information than you need. Since screens are no more than 72 pixels per inch, there is no additional web quality with a 300 ppi image -- it just slows down the transmission. However, when an image is printed, you definitely want more than 72 pixels per inch. So you don't

PIXEL DEPTH AND COLOR

Bits / Pixel	Colors	Explanation
1	2	Two possibilties -- black or white
2	4	2 x 2
3	8	2 x 2 x 2
4	16	2 x 2 x 2 x 2
5	32	2 x 2 x 2 x 2 x 2
6	64	2 x 2 x 2 x 2 x 2 x 2
7	128	2 x 2 x 2 x 2 x 2 x 2 x 2
8	256	2 x 2 x 2 x 2 x 2 x 2 x 2 x 2
24	16.8 million	Three channels of 8 bits each = 256 x 256 x 256

IMAGING NOTES

Name	Channel	Mode	Explanation	File suffix	Compression	Typical Use
Graphic Image Format	1 channel	RGB (indexed)	256 colors (transparencies)	Gif	poor	Maps, cartoons
Joint Photographic Experts Group	3 channel	RGB	16 million colors (no transparencies)	Jpg, Jpeg	very good	Complex photos
Pict and Tiff	3 channel	CMYK	Pict was originally developed for Mac, Tiff for Windows	.pict or .tiff	Non-lossy. No compression.	For printed graphics, not for the Web.
Photoshop Document	4 channel	All modes	Working format (for Adobe Photoshop)	.psd	Non-lossy. No compression.	Working format (for Adobe Photoshop)
Portable Network Graphics	4 channel	RGB	Working format for Fireworks and others; Also viewable in some web browsers. 16 million colors plus alpha (transparencies)	.png	poor compression	Complex photos with transparencies in the alpha channel. Not supported in all browsers