I’m sure we’ve all received an email with lots of photos attached that seemed to take forever to download – especially over a dial-up internet connection. Or perhaps you tried making a large print of the photo you found on the internet and, besides seeing copyright stamped on the print, the print came out really bad quality. Or how about that little photo from either internet or email that looks good small, but not so good at full screen size? These are three different applications of photo files that require very different size files; the photo file needs to be tailored for the function for which you intend to use it.
A print file needs a lot of information in the file if it is to print at high quality. An internet file needs a limited file size so it can download quickly, and the full size screen view needs a file size in between the first two. So – how do we take a picture that is going to be used for all of these functions. When I don’t know exactly how a photo file will be used, I set my camera to take the largest possible file size; then I use software on my computer to adapt that huge file to the application needed. Before resetting your camera for file size, let’s examine what each application will require.
If I intend to print a file at high quality, I want a file size that will provide the printer with at least 250 pixels for each inch on the print (some print services will require 300 pixel/inch – or more). If the print will be 10” wide by 8” high, the pixel dimension needs to be at least 2500 pixels wide by 2000 pixels high. Some print services may tell you they can print an 8x10 photo using a file that’s 100 pixels by 125 pixel, but they’ll have to use software to guess what the missing pixels should be and unless the picture is of a solid white wall, it most likely won’t have the quality that you would like. As a reference, a newspaper photo is printed at about 72 dpi.
The amount of compression of the file is important for quality prints. With the first computers that could store an image, each pixel would require a byte of memory to store it – or 1 million pixels needed a Megabyte of memory for storage – and that’s a lot of memory if a very large home computer could only store 64 Kilobytes. Compression algorhythms were developed that allowed the program to save (for example) 10 pixels of the same description instead 10 unrelated pixels – end result – less memory was used. If this is confusing, don’t worry about it - the how isn’t that critical – just be aware that compression can hinder the quality of your photo file - especially if it’s for print. Typically, if I’m taking my files to the print service (or printing myself) I use files saved as JPG format with no compression; if I’m uploading to a print service, I’ll save the files as JPG with a little compression to decrease the file size. If you’re using a graphics program to develop and size your pictures, you’ll need to do a little experimenting to determine how much compression can be used before it starts showing in your prints.
Files that are to be displayed on a computer monitor will need different consideration – specifically, how large a picture will a typical monitor display. (Be careful not to equate your monitor dimensions with the file size needed.) While a monitor is sometimes measured by diagonal screen size, the more important measurement is the pixel size it will display – and that is traditionally 800 pixels wide by 600 pixels high. Depending on the software display settings, a file larger that 800x600 may not fit on the screen and may require you to scroll to see the whole picture. If your monitor and the software settings will allow a higher setting, that 800x600 image will appear as a less-than-full-screen image.
If you’re emailing an image for viewing on a monitor, you’ll need it small enough to display on the monitor AND compressed to a small file size. Keep in mind, some email services limit the file size that you can send. If you send an image of 640x480, it will display as about a 9”x7” image on an average monitor and can be compressed down to under 100K and still look good – with a dialup connection, this should only take a couple of seconds to download. This picture should look good on a monitor or digital slide show.
If that photo is intended for a website, keep in mind all of the picts on the web page have to download; if there are 20 – 100K picts to download, that’s a lot of info and could take a while. In this case, I would size those picts to a max size of about 150x120 pixels with a really heavy compression to about 35K; I could link this “thumbnail” to a page that would show a larger pict.
My rule of thumb for emailing images is: if it will take longer to download than it would to hand deliver by dogsled with 3 lame dogs, don’t email it; burn it to CD and mail the CD; you’ll keep your friends longer.
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Tip #1: Plan for the original pict file to be large enough for the most demanding application for which you’ll use it.
Tip #2: Print picture files – aim for 250 pixels / inch.
Tip #3: Email picture files – aim for a display of about 72 pixels / inch AND compress the file.
Tip #4: Have lots of picts to share, consider setting up a simple website and emailing the web address to your friends.
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