Resolving Resolution

resolving resolution

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.

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. We've scanned a 4x6" 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. Learn more about the terms you'll encounter when you go digital.

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, let's 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!

resolving resolutionCool! 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, it's 9 pixels x 3 bytes big, or 27 bytes. One Megabyte is 1 million bytes. Our 4 x 6 inch print is 2,160,000 pixels so if we do the math, we find that the image is (2160000 x 3 bytes) 6,480,000 or simply 6.4 Megabytes (MB) in size.

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Give me lots of resolution! More is better, right?
Hold on now. More is not always better. Some firms are marketing 3000 DPI for film and 600 DPI for prints so they can make claims about "higher quality" but science doesn't support the marketing rhetoric. 2500 dpi for slides and negatives is usually sufficient and best for most film. However, if your film is of a low ASA and of high enough quality, 3200 dpi could be a reasonable option.

4000 dpi or higher is usually not 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 higher DPI scanning can work well on that type of film. Paper photos are only printed at 150 to 300 dpi so when scanning prints, there is no more than 300 dpi of information to recover. Scanning at a higher resolution gives you more pixels 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. That only happens in Hollywood.

In fact, there are some downsides to scanning prints at a higher DPI. Prints have usually been stacked and touched and left in boxes. That handling leaves minute scratches and fingerprints on the surface which get picked up and magnified by scanning. The higher the DPI, the more prevalent the defects. We will, occasionally, recommend scanning tiny photos at 600DPI just so the image dimensions will more closely match those of the other photos we're scanning.

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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.

resolving resolutionA TIFF file is an example of a lossless file format. It manages to make the file size smaller the same way that zipping works. Let's try to compress our 3x3 image and 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.

resolving resolutionJPEG 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 then save the file over and over again.

It suffers from 2nd generation degradation as each successive save recompresses and therefore reduces the detail of an image that already lacked some detail. It's much like a photocopy of a photocopy of a photocopy. Each successive copy of the previous copy gets worse.

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Does that mean that my JPEG files degrade over time?
No, of course not. That's the beauty of going digital! JPEG files don't degrade over time. The only time you'll encounter the aforementioned loss of quality is when the file is recompressed and saved on your hard drive by editing it in an editing software like Photoshop.

Simply viewing the file does no harm and the original files will always be on the read-only discs that we send you so you can always go back to the source no matter what happens to any copies you may make.

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How many images can fit on a CD or DVD?
Below, we have created a few charts to help you get an idea of how many images can fit on a CD or DVD and how big your files will be at the different resolutions we offer. We have also included the differences in size between TIFFs and JPEGs as well as how many Megapixels (MP) each resolution produces.

Print Sizes vs. Resolution
Print Size DPI Pixels MP TIFF JPG
3x5 300 900x1500 1.3 4 MB ~1 MB
600 1800x3000 5.4 16.2 MB ~3.2 MB
4x6 300 1200x1800 2.1 6.3 MB ~1.3 MB
600 2400x3600 8.64 26 MB ~5 MB
5x7 300 1500x2100 3.15 9.45 MB ~2 MB
600 3000x4200 12.6 37.8 MB ~7.6 MB

Slides & Negatives vs. Resolution
2500 3250x2250 7.3 21.9 MB ~5.5 MB
3200 4160x2880 12.0 39.5 MB ~9 MB
4000 5200x3600 18.7 56.1 MB ~14 MB
Based on 35mm Film being approximately 1.3"x0.9"

Please Note: The JPEG file sizes are approximate and will vary depending on the image content. Some images compress in JPEG better than others. Now you're probably wondering how many images fit on a disc. That's easy to figure out, too. A CD can hold 650MB while a DVD can hold 4700 MB (4.7 GB).

Amount of Prints that would fit on a CD or DVD
@ 300 dpi @ 600 dpi
3x5 162 650 1175 4700 39 185 284 1342
4x6 100 430 746 3100 25 130 180 940
5x7 68 325 490 2350 17 81 123 587
These numbers are estimates. Actual results may vary.

Amount of Slides that would fit on a CD or DVD
2500 30 118 213 885
3200 16 72 130 552
4000 11 46 83 335
These numbers are estimates. Actual results may vary.

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Would you like to know more about going digital?
Visit our Scanning 101 page for a primer on common terms you'll encounter!


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