Information on Image Resolution

You're probably wondering how big your files will be if you scan them at different resolutions, or DPI. DPI is a the term normally associated with scanning and it means Dots Per Inch. DPI is a term that's been carried forward from the early days when scanners could only capture black and white and black was represented with a dot. Today we really scan in Pixels but the term remains and is now used interchangeably with PPI or Pixels Per Inch.

Here is a simple example to show you how you can determine how many pixels you'll get from your photographs scanned to CD.

Suppose you want to scan your 4x6 inch prints at 300 dpi. You'll get an image that is 1200 pixels by 1800 pixels. We arrived at those numbers by doing some simple arithmetic. Our print is 4 inches high and we are scanning at 300 Dots (pixels) Per Inch. Each linear inch of the print will produce 300 pixels. 4 x 300 = 1200 pixels. We do the same math against the width of the picture and get 6 inches x 300 DPI = 1800 pixels. Our print yields 1200 x 1800 pixels.

Ok, so how many MegaPixels is that?
Everyone is talking MegaPixels these days and we can credit the Digital Camera manufacturers with that. When you buy a digital camera, the price is usually related to how many MegaPixels it can produce. The higher the number, the sharper your images and the more expensive the camera. More pixels generally means more detail. Back to our example of the 4x6 print. We've scanned the photo and we now have a digital image that's 1200 x 1800. That means we have a matrix or grid that is 1200 in one direction and 1800 in the other.

•	•	•
•	•	•
•	•	•

Imagine this is our image. It's only 3 pixels x 3 pixels so its not much of an image, but it can help exemplify how we compute MegaPixels. This image contains 9 pixels. 3 pixels in height by 3 pixels in width. Now, lets just build from there.

If this simple 3x3 pixel image contains 9 pixels, our 1200 x 1800 pixel image must contain 2,160,000 pixels. 1200 x 1800 = 2,160,000. That's 2.16 million pixels or 2.16 Mega Pixels.

Cool! So how big is my file?
The answer to this is, it depends.... I know, just when you thought you had a handle on this stuff, somebody had to mix it up. The reason it depends is because of compression, but more on that later. If we take compression out of the equation, it's simple to compute file size. A pixel is made up of the three primary colors of Red, Green, and Blue or simply RGB. The computer stores the representation of those colors in bytes. One byte for each of the three colors. Therefore, 1 pixel = 3 bytes. If our image is 9 pixels, like in the example above, its 9 pixels x 3 bytes big, or 27 bytes. One megabyte is 1 million bytes. Our 4x6 inch print is 2,160,000 pixels so if we do the math, we find that the image is (2160000 x 3 ) 6,480,000 or simply 6.4 Megabytes (MB) in size.

Give me lots of resolution! The more is better. Right?
Hold on now. More is not always better. 2000 dpi for film is usually enough for most people but if you think you might want to crop, 3000 dpi may be a good option, too. 4000 dpi or higher is not always the best option for consumer level film and/or older film. Modern electronics exceed the resolution that's available in most film so by scanning at a higher dpi, you actually start to see the chemistry of the film as flecks or grain. Film like this actually looks better when scanned at a lower resolution. If you need to crop or print large, you can use software like PhotoShop to increase the resolution using special software techniques to get the desired size with better quality results and less grain. Higher end film or some of the more modern consumer level high resolution film exude resolution and 4000dpi works well on that film.

When scanning prints, there is usually no more than 600 dpi of information in a print so scanning at a higher resolution gives you more data without any extra information. That means, for instance, if you can see the face of a watch on someone's arm in the print but can't read the time, scanning at a higher resolution won't bring the time into view.

Ok. I think I've got it. But what about compression?
Here's where it gets tricky. There are two types of compression. The first type is called Lossless because it results in no loss in quality or degradation to the image. It manages to make the file size smaller the same way that zipping works. Lets try to compress our 3x3 image. Lets suppose that all of the pixels in that image are the same color. We could store that image uncompressed and it will take up 27 bytes. But since all the colors are the same, we could just store it in such a way that describes the image as 9 black pixels. Since the pixel takes up 3 bytes, we need three bytes for that then we need one more byte to indicate how many of those black pixels we have, 9 in this case. We've just compressed our 27 byte image down to 4 bytes. Of course, this is over simplified, but that's the basic concept. We can now uncompress the file when we open it and get an exact representation of the image with no loss in quality. Pretty clever, huh?

The other type of compression is called lossy because it results in the loss of data. It works on the assumption that the human eye can't detect certain details in an image so it strips those details out. This type of compression is much more complex and also very configurable. JPEG is considered a lossy compression file format but its degree of loss is configurable. You can vary the quality factor from 0 to 100%. As you lower the quality factor toward 0, your file size gets smaller and your picture loses more detail. The biggest problem with lossy compression comes up when you open and the save the file over and over again. It suffers from 2nd generation degradation as each successive save reduces the detail of an image that already lacked detail. Its much like a photocopy of a photocopy of a photocopy. Each successive copy gets worse.

Here is a table of various sizes you can expect from your print images.

Print size	300 DPI	600 DPI
3x5	900 x 1500 1.35 MegaPixels 4 MB TIFF ~1 MB JPEG	1800 x 3000 5.4 MegaPixels 16.2 MB TIFF ~3.2 MB JPEG
4x6	1200 x 1800 2.1 MegaPixels 6.3 MB TIFF ~1.3 MB JPEG	2400 x 3600 8.64 MegaPixels 26 MB TIFF ~5 MB JPEG
5x7	1500 x 2100 3.15 MegaPixels 9.45 MB TIFF ~2 MB JPEG	3000 x 4200 12.6 MegaPixels 37.8 MB TIFF ~7.6 MB JPEG