What graphic file extension to use?

by Clarke Bird, Editor

PC Alamode - Alamo PC Organization, Inc.

San Antonio, Texas

Most of the graphics I receive for the PC Alamode are JPEG files (.jpg), as many of them are screen captures of software programs being reviewed or copied off an Internet Website.  Further, practically all photos I receive from those with digital cameras are JPEG files at 72 dpi resolution.  Since these graphics are slated for printing in the PC Alamode, JPEG is absolutely the wrong file format when the end result is a printing job or an ad agency photo application.  Why? I’m glad you asked. . .

JPEG files are simply not suitable for most printing jobs.  JPEG files are small — PC users like small files as JPEG files can easily be sent over the Internet. There is no mention of quality when discussing JPEG files, only size.  When working with TIFF files, the sizes become very large because of the huge number of pixels needed to create a lifelike image.  Each pixel must be described as one of 16.7 million colors.  An image suitable for printing can have over a million separate pixels — so you can see why the files grow so large.

Let’s start at the beginning:

JPEG (Joint Photographic Experts Group) is a standardized image compression mechanism created especially for the transmission and storage of photographic images. Strictly speaking, JPEG is the name of a compression algorithm (I wish I had paid more attention in high school math) and has become the de facto image format for digital cameras.  The reason for this is the compression algorithm’s ability to reduce an image file size by 8 or 10 to 1 without any apparent degradation in image quality — at least to the human eye at normal viewing magnification.

Here is what happens when you save an image to JPEG: 

The image is converted from RGB to YUV (YUV is a color space scheme that stores information about brightness and hue separately).  Since the human eye is more sensitive to brightness than hue, you can afford to discard more information about an images hue, especially the higher frequencies.

The image is broken up into 8X8 blocks which are then processed independently.

The image data in each block is processed by using lossy — a data compression algorithm that actually reduces the amount of information in the data, rather than just the number of bits used to represent that information.  The lost information is usually removed because it is subjectively less important to the quality of the data or because it can be recovered reasonably by interpolation from the remaining data.  In other words, the image is crunched down smaller, but image quality is thrown out in the process.  Most of the time, however, you can’t see the difference in image quality unless you try to print the graphic on a professional imagesetter.

TIFF images (Tagged Image File Format) is a very flexible image format which was created by Aldus Corporation, who later merged with Adobe Corporation.  TIFF uses an 8 to 16-bit per color channel storage method without losing image data.  A disadvantage is that it creates very large files.

Sadly, once an image is produced as a JPEG file, there is no turning back.  In other words, re-saving the file as a TIFF file won’t improve the quality of the image as the data lost during compression can’t be recovered.  In other words, there is nothing an editor can do on his/her end to improve an image once it is sent to them as a JPEG file.

For those with digital cameras, it’s my understanding that you have no option to change image format, it’s going to be JPG by default.  But when taking photos — you do have an option of changing resolution.  My advice before snapping that photo is to think first: what is the ultimate end use for this photo?  If it is print such as for the PC Alamode, a church newsletter or some other print media, then set the resolution as high as your instruction manual says you can set it.

That way, when the photo credit appears with your name next to the photo, you won’t have to wear a disguise in public to hide your identity.

If you are scanning an image to be used in print media, set your scanner resolution for 300-600dpi and if it is a color image, set the selection to “millions of colors.”  After the image is scanned, save it as a TIFF file.  The resulting file may be larger than the average home PC user is used to but the results will make recipient of the image a far happier person than should you send him a typical 72dpi image.

Remember this: JPG good for attaching to e-mail.  TIF good for high-end publishing.