Understanding Image Resolution

Understanding Resolution and DPI

I don't know how many times I have had this conversation only to see eyes glaze over with that deer-in-the-headlights look.

It really is not that complicated.

There are three things to consider.

1. Cameras


2. Monitors


3. Printers
   

Cameras are marketed by the number of pixels on their sensors . A pixel is the smallest element of an image.

Digital cameras range from 1 or 2 Mega Pixels up to 40 Mega Pixels or even larger.

A typical digital camera would be 8 or 10 Mega Pixels. For this discussion we will use the Canon EOS Rebel Xti 10 Mega Pixel Camera.

The Rebel Xti produces an image file that is 3888 pixels x 2592 pixels for a rough total of 10 million pixels.

For comparison we will use the Canon Powershot A590 8 Mega Pixel Camera that produces an image file 3264 pixels x 2,448 pixels. Multiply these two number and you get roughly 7.9 Million Pixels.

So why is having 2 million more pixels better?

Well, It's sort of like painting a wall. The more paint you have, (pixels) the more wall you can cover. Or, trying to cover a large wall with little paint will result in a splotchy paint job…

How does this apply to digital photography you ask…

Well, first we need to talk about DPI, (We should be talking about PPI or pixels per inch but for the sake of conversation and the fact that DPI is more commonly used we will stick to DPI or dots per inch even though technically we are talking about pixels). So, understand that a “pixel” is a “dot”. If you are a tech-head don't bust me out on this, trying to keep it at a tenth grade level here.

The human eye has its limits. Even the most near sighted person with his or her nose held directly up to a printed photo cannot resolve any more than 300 DPI. For this reason 300 is used as sort of a benchmark in photography.

Simply put, a human being can see the difference between 200 dots per inch and 300 dots per inch but cannot see the difference between 300 and 400… (Again, don't bust me out tech-heads… I know this is largely dependant on the content and detail of the image. We are working with generalities here today.)

So 300 DPI is our benchmark . Using the two cameras mentioned above we can now calculate the size of the image we can print using these cameras and maintaining our benchmark.

With the Rebel Xti 10 Megal Pixel camera we can print,

(3888 / 300) x (2592 / 300) or 12.9 inches x 8.6 inches

With the Powershot A590 we can print,

(3264 / 300) x (2,448 / 300) or 10.8 inches x 8.1 inches

So you see, The Rebel Xti puts more paint in your bucket…

As the printed image size increases, the 10MP camera will hold up longer.

Now, keeping things at a 10th Grade Math level you should be able to deduce that if we print duplicate photos from both of these cameras at 5”x7” we will se no difference as far as resolution is concerned. Both of these cameras have enough pixels to make decent 8X10 and smaller printed photographs.

A note about our Benchmark.

The 300dpi Benchmark is based on a person viewing a photograph at some standard distance, something in the neighborhood of 18 inches. So, for images in books and magazines 300 is a good number to shoot for.

For portraits and art that will hang on a wall or otherwise be viewed from several feet, a much smaller dpi can be used. In other words, you can print much larger for these images and still get away with it.

As mentioned above, the amount of detail in an image also plays into mix. A photo of a smooth surface takes little information to reproduce whereas a macro image of a bee or other insect will take allot of information or resolution to reproduce those details on the printed page.

But you said Monitors too…

Yes I did. You ever get an email from a friend with this image attached that is several times larger than your monitor?

Computer monitors generally display images at 72 DPI (Back off tech-heads!) You can do the above math using 72 instead of 300 to find out why that image is so big on your screen…

So, be nice to your friends and re-size or downsize that photo before you send it in an email.

Here are the top-ten screen resolutions used by visitors to weisd.com over the last year,

• 1024 X 768 (75% of the total)

1. 1280 X 1024


2. 1280 X 800


3. 1440 X 900


4. 800 X 600


5. 1680 X 1050


6. 1152 X 864


7. 1920 X 1200


8. 1300 X 2300


9. 1280 X 768

So you can now see that if you take the Rebel XTi image file and email it to you friend with a 1024x768 screen resolution without resizing it first, the image will be near 3 times larger than his or her screen.

My preference is to send images that are 800 pixels wide for horizontal or 600 pixels tall for vertical compositions.

Some programs like Microsoft's PictureViewer or Windows Explorer and others will automatically resize images so that they fit on your computer screen. This is convenient but these programs use simple and fast algorithms to do this and many times will result in an image that is degraded and just looks bad.

For best results, resize images for use in computing.

One more thing…

If you are shopping for a new printer you will see resolution specs that look something like, 4800 x 2400 dpi

Not going to go into a long discussion on this but I will say that everything said above has very little to do with this spec.

Simply said, most all printers that one would use to print a photograph are capable of putting 300 dots, (pixels) in one linear inch of paper. In keeping with the dummied-down 10th grade, (not picking on you Sophomores) explanation, one can think of printer resolution specs as how many dots a particular printer can fit inside one pixel.

This has more to do with reproducing good tones and colors than it has to do with the actual resolution we discussed above.

In Closing,

This is a very simple explanation of what can be a complicated subject. Visit any photographer's forum on the Web and you will find plenty of debate and discussion. However, I think this covers the basics…