File Formats and File Size

When you take photos with a digital camera or when you scan a photo on a scanner your images have to be stored in the computer using a particular coding system called a file format. There are a number of issues that we concern ourselves with when we select a format. They include file size, image quality of format (lossy vs. lossless), proprietary vs. non-proprietary.

File Size: For storage of images file size is becoming much less important than it was just a couple of years ago. We can now store our files on writable CDs or even onto DVDs at a very low cost. However, for some aspects of using images file size is still very important. For example, you would not want to e-mail a 12 megapixel image stored in the PSD format. The file size for this file would be about 30 Mb which would take a very long time to send or receive even on a high speed connection. And an image that was about 500 x 400 pixels stored in JPED would take only a few seconds to send or receive even on a slow dialup connection. A comparison of file sizes using different file formats is given at the table at the end of this section.

Image Quality: When an image is saved in the computer it is frequently compressed to save storage size. Some of these compression methods (such as zip and lzw) are what is called “lossless” compression. When the file is uncompressed the result is exactly like the original. The uncompressed file is exactly like the original. However some compression methods (jpeg) are “lossy”. When the file is uncompressed it is nearly like the original but not exactly like the original. When a lossy compression is used repeatedly on the same image there is a risk of the file changing in a noticeable way.

Proprietary: Most image editing software, including Photoshop and Photoshop Elements, have a file format that takes advantage of the features of the software. The Photoshop proprietary format is the PSD format. However these proprietary formats normally cannot be transferred between different editing programs nor can they be viewed in many of the standard viewing programs. If you wish your files to be transportable you should use one that is in general use. The two that are discussed in this section are jpeg and tiff.

There are a number of formats that might be used but for purposes of general use in Photoshop Elements (or Photoshop 7) there are three that are most important. They are discussed here.

PSD

The Photoshop (PSD) format is the proprietary Photoshop that is native to both Photoshop and Photoshop Elements. Because the PSD format is proprietary it will save all aspects of a Photoshop file. An image saved in the PSD format then retrieved by Photoshop will be exactly like it was when saved. It will not lose any pixels. It will maintain all of the layers in the image. The Photoshop format is completely lossless. But because it is proprietary the PSD format will not be readable by non-Adobe programs. It is not as portable as public domain formats. Also a file saved using the PSD format will likely be much larger than most other formats (see the table at the end of this section.)

This format should be used if you might want to continue working on an image or if you would like to save a layer for use in other images. If you can handle the space size it is also useful for permanent saving of edited images.

TIFF

TIFF is an acronym for Tag(ged) Image File Format. It is one of the most popular and flexible of the current public domain image file formats. Some digital cameras use tiff as one of the file options when taking photos. One of the primary advantages of the tiff format is that it is very portable. Most image editing software can read the tiff format. And the tiff format is completely lossless in that it preserves all pixels in an image. You can save layers within the tiff format.

The tiff format can be saved via Photoshop and Photoshop Elements in either compressed or uncompressed modes. The uncompressed mode produces files that are approximately the same size as the PSD files. The compression method recommended for tiff files is the LZW compression which reduces the file size to about a third of the compressed size (see the table at the end of this section.) Both the compressed and uncompressed versions are lossless.

JPEG

JPEG (pronounced "jay-peg") is a standardized image compression mechanism.  JPEG stands for Joint Photographic Experts Group, the original name of the committee that wrote the standard.  JPEG is "lossy," meaning that the decompressed image isn't quite the same as the one you started with.  JPEG is designed to exploit known limitations of the human eye, notably the fact that small color changes are perceived less accurately than small changes in brightness.

Digital cameras almost always can store their photos in the jpeg format.  And normally you are given a choice of the quality and size of the jpeg file.  (The larger the file the higher the quality.)  This might be designated as fine, normal, and basic or a similar designation.  Most people prefer to use the best quality for their photos, but you might use a lower quality if you wish to get more photos on your memory card.

When you save an image in Photoshop Elements using the jpeg format you will be given a choice of what quality you wish to save your image.  These choices are numbered from 0 (lowest quality, smallest file) to 12 (highest quality, largest file).  In the File Size table below the values 10, 7, and 3 were used for JPEG high, medium, and low respectively.  For comparison a 12 megapixel image saved at the 0 setting takes up 0.278 Mb compared to 0.447 Mb for the 3 setting.  And the same image saved at the 12 setting takes up 5.1 Mb compared to 2.1 Mb for the 10 setting.

Even with the lowest quality settings one would visually see very little difference from lossless formats.  However with repeated opening and savings of an image visible differences will occur.  And this will occur much more rapidly with the lower quality settings.

One should note that copying a file from the camera to a hard drive of from a hard drive will not effect the quality of an image at all.  Nor will opening an image in Photoshop change the image if it is not then saved.

File Size Comparisons for Different File Formats

Megapixels	Tiff	PSD	Tiff LZW	JPEG high	JPEG med	JPEG low
12	35.2 Mb	30.0 Mb	13.3 Mb	2.073 Mb	0.865 Mb	0.447 Mb
8	23.5 Mb	20.9 Mb	9.9 Mb	1.534 Mb	0.617 Mb	0.331 Mb
6	17.6 Mb	16.0 Mb	7.9 Mb	1.214 Mb	0.474 Mb	0.255 Mb
5	14.7 Mb	13.6 Mb	6.9 Mb	1.048 Mb	0.408 Mb	0.224 Mb
4	11.7 Mb	11.0 Mb	5.7 Mb	0.856 Mb	0.341 Mb	0.191 Mb
3	8.8 Mb	8.4 Mb	4.6 Mb	0.674 Mb	0.274 Mb	0.156 Mb
2	5.9 Mb	5.6 Mb	3.1 Mb	0.463 Mb	0. 202 Mb	0.119 Mb