This page began as a point of view by Carl House. Sasha Mitelman made his comments in indented text. I might do more blending of this at some point, meanwhile, we hope it's helpful. (Carl) On 2/1/01 I added a new summary.
When photographs or other source art first become digital images, a decision
is made as to the level of quality desired. Four color printing is the
most demanding probable use; digital files prepared for four color printing
are likely to be in .pdf, .tif, .bmp, or .eps formats and are likely to
be 5mb to 75mb in size. I call this "source art" because you can do just
about anything you want to with it. You can't use it on the Internet, but
you can convert it for use on the Internet.
Actually you can use any file size or type you want, and if someone has a fast enough connection (ISDN, DSL, T1 or better), moving around 75Mb files is no problem. In practice, however, modem connections limit reasonable file sizes to a few Mb.
Other file formats commonly seen on the Net include JPG (also written JPEG), JFIF (a derivative of JPEG), GIF and PNG.
Images for the Internet
Images for use on the Internet must be as small as possible while preserving
quality at an acceptable level. I tend to think of photographs for use
on the web in two categories. A "webpage image" is intended to be the primary
image on a page; it is fairly large and should be of high quality. I tend
to make them about 300 pixels in height and try to get files 10kb to 40kb
in size. Two large photos are shown below. Tracy Grilli (smiling) is 350
pixels tall and 14,582 bytes in size.
Be very careful about whether or not you are referring to compressed or uncompressed file size. In this case, the uncompressed size is (292 pixels wide)(351 pixels high)(3 bytes/square pixel) / (1024 bytes/Kb) = 300Kb. The 20.59x compression is what you get from using JPEG. The "3 bytes per pixel" comes from the fact that you have to specify three numbers (Red, Green, Blue) to determine the color of each point in the image.Tracie Moll (grabbing air) is 300 pixels tall and 51,830 bytes in size.
Similarly, this one actually has an uncompressed size of (662)(300)(3)/1024 = 582Kb. The size you give (51.83Kb) is that of the compressed file and is smaller by only about 11.23x, which probably means it was done at a higher quality setting within Photoshop than the image described above."Thumbnail" images are much smaller and a webpage might include several thumbnail images. They should be 80-120 pixels in height (I use 100 pixels of height) and I hope for file sizes of 3kb to 12kb. The smaller image of Tracie below is 100 pixels tall and 8889 bytes in size.
(Similar comments here...)More tests and discussion of file image quality is presented at JPEG Image Quality
File Compression & Bundling
Files are often compressed & bundled for transmission or for archiving.
"zip" is the file name extension used for files processed via PKZip, WinZip,
and similar software. ".sit" is the file name extension used for files
in "mime" format made available by Alladin Systems.
".sit" refers to StuffIt, which is a file compression format (as opposed to something geared specifically for compressing images) developed by Alladin. "MIME" stands for Multipurpose Internet Mail Extensions, which specifies a standard way of including different types of content (plain text, images, sound, etc.) into a single email. It has nothing whatsoever to do with the compression itself. Take a look at this link for an explanation.
File names to be used on the internet must not have spaces and I
prefer them to be short, like 8 characters, or maybe 10 or 12, plus of
course, the extension. It is OK to mix upper/lower case to help them be
more recognizeable. For example, I might name a photo of Dwight Eisenhower
"DwtEsnhr". That is 8 characters and fairly recognizeable. Using upper
and lower case has an important implication if you are using a UNIX server
because then the name is case sensitive. That means "JoeSmith" is not the
same as "joesmith". NT servers are not case sensitive, so for NT "JoeSmith"
is exactly the same as "joesmith". It might be easier to always use lowercase
for naming files, paths, and URL's for use on the internet.
I would make this last recommendation forcefully and in big letters. A large chunk of the Net is run on Unix servers and routers (even though many end users use various flavors of Windows.The thumbnails and the web images, as I use them, must have identical names, but therefore must be stored in different directories. For example, I will create a directory like this: c:\yamadabaske\images\t\ The thumbnails will go into the \images\t\ subfolder while the web images will go in \images\. I would keep my "source art" tiff images in an entirely different place because they are much larger and present an entirely different problem for backup.
Mac vs. PC conventions
There is a very important difference in images for use on a Mac vs. images
for use on a PC.
PC's always use the file name extension (".tif" or ".jpg" or whatever) to identify the file format. If PhotoShop can't identify the file format by the extension it does not even try to open the file.
I'm not 100% sure if this works on PC's, but sometimes you can force Photoshop to open a file by doing an "Open..." and specifying the file type (it should be in a pull-down menu in the dialog box). I know this works on Macs for files that Photoshop has trouble recognizing.Mac's, as I understand it, look at the file information to identify format and therefore give you the freedom to make the extension be whatever you want it to be.
My basic process is:
1) Save in tif or pdf (Photoshop) format with adequate resolution, not jpeg. Create a jpeg when you want it. You are going to want to choose size of image and compression quality anyway when you make a jpeg, so let your original be a non-compressed image. The advantage of tif is everybody can read it. The disadvantage is it does not provide for layers. pdf is the native Photoshop format and it does provide for layers. Paint Shop Pro (software from Jasc) only costs $60 or $80 and people get very good results with it (processing images).
2) If you want to put an image on the web, height should not be more than 360 pixels.
3) I've recently acquired a Nikon Coolpix 990 camera and it is superb, even using low quality image settings. I'm using the lowest quality image settings and getting very good results for family photo album. Cost is $950 or so, but well worth it. Cost for a Nikon digital back that would use all my old Nikor lenses is now down to $5000, so I'll probably get that sometime soon.
4) I have no desire to acquire a scanner because I won't use it enough to justify the quality I would want. I rent one at Kinko's when I need one. Costs $18/hour and I scan scan 30 or so images in an hour. Cropping and resizing and any image adjustments are done on my own computer using any of several image processing programs. Each has different advantages, so I use whatever software will do what I want with the least effort and chance of error. Photoshop really is, in my opinion, much better than all others for preserving good image quality.
5) I get all the photo artwork from Swim Magazine on a CD. These scans are 25mb to 75mb in size and were done on a scanner that cost $300,000 or so. Some are superb, but often the source photo was not very good. I've also had work done on a $100,000 scanner and because the person doing it was careless I was disgusted. I had work done on a $5000 scanner and the person doing it was very careful, but the results were not better than what I get at Kinko's on a scanner that cost under $1000. People doing scanning always ask you what you want to do with it and then they give you what they think is best for that purpose. The problem with that is you don't have any flexibility. They give you web quality and you can't make a nice print from it if you want to. The images that I save as my personal artwork are typically 500,000 bytes in size. Images I put on the web are usually about 40,000 bytes if 360 pixels high or 6-10k if it is a thumbnail image of 100 or 120 pixels of height.
6) I've done a lot of work with images over the recent several years, but the range of my skills is rather small. I go to a skilled Photoshop person when I want special work done on an image. My skills include cropping, resizing, very limited image adjustment, and great concern for naming and cataloguing and documenting images.
7) If you care about photography and aren't working personally with digital images, that should be your next step. It doesn't have to cost much. There will be some pain in learning; I have no patience for struggling to learn; I always find a teacher and limit what I want to learn to what I will do often enough to remember. Actually digital work is far less costly than SLR/film work because there are no variable costs once you have the setup. I'll save enough in processing costs in a year or so to pay for all my investment so far.