Help for the Computer User

February 16th, 2004

Understanding Digital Photos - Part 2
Joe Margiotta (flyfisherjoe)

Pixels are the best way to look at the dimensions of a digital photo to relate to it's size in different presentations. They are expressed as pixels per inch or PPI. The easiest medium to relate this to is the computer monitor screen or display. It's resolution is also measured in pixels. The most common monitor size today is a 17", that size refers to the diagonal dimension of the screen. In the case of a CRT monitor (one like a TV) it will measure less because the housing overlaps the edges. With LCD (Liquid Crystal Diode), which are the flat thin panel displays, the diagonal will measure the actual size. Most people use a 17" monitor at a setting of 1024 X 768. Some prefer to have everything look bigger and use a lower resolution (800 X 600) to accomplish this. That's right, I said lower. This illustrates the relationship of PPI to a pictures actual size on a monitor screen. To see what your screen setting is, right click your mouse on the desktop, select "Properties" and open the "Settings" tab.

Let's say you have a picture from your camera and using your photo editing software you determine it is 1984 X 1488. If you want to display it on your monitor as a full background image you need to reduce it in size. If your monitor is set at 1024 pixels wide and 768 pixels high, you would reduce one dimension of the picture and let the other stay so it remains at the proper proportion (this camera's pictures are the same proportion as the screen). Changing both dimensions without being proportional would result in a distorted picture. In this case, changing the width to 1024 will result in a 1024 X 768 picture and fill the whole screen when used as a background on this monitor. If you change it to 800 it will end up as an 800 X 600 size and will only fill that portion of the screen, unless you have your monitor set to 800 X 600, in which case it will fill the screen

What if you want to send someone the picture in e-mail or post it on a bulletin board? A picture that will fill the whole screen is too big! let's look at some relative sizes on your display:

This line is 200 pixels long:


This line is 400 pixels long:


This line is 600 pixels long:


It should be clear that a picture with a dimension of 600 pixels is big for posting on a bulletin board. 300 to 400 would be much better. There is also another consideration when e-mailing, placing on a web site or posting. That is the file size, which is not the measure of a picture in pixels. File size is measured in bytes of required storage. This is usually more than a thousand and referred to as kilobytes or K. When viewing pictures on a web site your computer must transfer and store the picture to it's memory and possibly store it as a temporary file on your computer. The larger the file size and the slower your Internet access speed is, the slower the process. I'm sure you've all visited web sites that took a long time to load. Pictures and/or graphic with too large of file size is usually the cause. I'm also sure you've experienced problems in bulletin boards loading because someone has posted a picture too large in pixel dimensions and/or file size.

Now let's see the process in preparing pictures for use on the Internet by resizing and compressing file size. The first step is to resize them to a manageable size. Between 300 and 400 pixels is small enough to have a small file size and large enough to have detail. Look at the lines above to get an idea of this.

What about PPI? On a monitor display it won't effect the physical size. The size is determined by the number of pixels displayed (determined by the resolution setting) and the relative amount of the pixel dimension. A 512 pixel wide picture will be half as wide as a 1024 wide screen display and about 64% of an 800 wide screen display.

NOTE: Photo editing software will allow you to "enlarge" pictures as well as reduce, but I advise NOT doing that. As you enlarge, you reduce the quality. Only use it to make them smaller.

As an example to illustrate resizing and compression, the photo below is resized from a larger picture (2560) to 300 pixels wide, the photo on the left has a file size of 119K, the one on the right is compressed to 16K. They both are 72 PPI and have the exact same dimensions. Even though the right picture has a file size that is less than 14% of the one on the left, it still looks good on your screen.

Crappie Photo Compressed Crappie Photo
This photo is 119K in file size.
load with 56K modem = 22 sec.
This photo is 16K in file size.
load with 56K modem = 3 sec.
*Photos are property of Robert Munoz, robertsfishmounts.com
and are used here with his permission.

This Crappie photo shows that pictures the same physical size can have very different file sizes. Comparing the estimated load times illustrates the benefit to those with slow Internet access and shows that compression with good software gives quality results. This particular compression was done using Adobe Photoshop Elements with a quality setting of only 35. The original photo, out of the camera, is 2560 pixels wide and has a file size of 2.28 MB or 2280K. If this large photo were posted on a bulletin board it would take at least 11 minutes to load with a fast modem and about a 40" monitor to view it all at once.

Good photo editing software allows you to compress with options like "save for web" or just saving as a jpg type with the ability to choose the amount of compression and usually a preview and an estimated "load time" at various modem speeds. With some experimenting you will see how much you can compress without losing too much.

When it comes to printing a picture, you must look at things differently. Printing quality is evident when comparing a newspaper picture with a magazine. If you look close you can actually see the dots that comprise the newsprint and the spaces in between. The dots can be thought of as pixels. They are so many in the magazine picture you can't see where one begins and another ends. The high number of dots give the picture higher quality. The PPI required to have a nice looking picture for monitors is as little as 72 PPI. Printers, however, will be seriously effected by the PPI. It will be a factor in the size and quality. A higher PPI will produce a better quality print. The large Crappie picture that I talked about would be great for printing. Realize that you will need more pixels to make the picture because the desired PPI is a larger number.

Inches X PPI = total number of pixels

A 4" wide picture at 72 PPI requires the pixel dimension to be 288.
72 PPI would not produce a good quality print.

A 4" wide picture at 300 PPI requires the pixel dimension to be 1200, but would be a better quality print. It would also have a larger file size

Number of pixels PPI = size in inches of printed picture.
Keep in mind the importance of small file size for web use (fast loading) and large file size for printing (high quality and size), and the same applies for pictures that are scanned in.

Tip of the Week

Finding what your're looking for on a web site page can be easy with the "Find" function of youre browser. It is accessed by pressing the "Ctrl" key and the "F" key at the same time or in the top menu under "Edit." Simply type in the word you want to find and click "Find Next" button or press the "Enter Key" This is for sites that do NOT use frames. Sites that do use frames (two or more sections that comprise the whole page), like FAOL, will require you to select the frame first. For example, if you go to the "Fly Tying" section and select "Fly Archives," you will be at a page with frames. the one on the left has the names of flies listed. Select this section by clicking anywhere in an empty spot. Then type in caddis, press "Enter" or click "Find Next" and you will find the first caddis pattern. Keep pressing "Enter and you will keep finding all the caddis patterns on that page. Note: Sometimes frames are not clearly visible so if the search isn't working, try click selecting the area you want to search.

I hope this helps! ~ JM

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