When discussing the resolution of digital images, far too often people get hung up focusing on DPI (or PPI) as a way to measure the size and quality of a digital image. This may sound completely wrong to you, but the DPI of an image has nothing to do with digital image quality. The reason? A digital image’s resolution is expressed as it’s pixel dimensions, either as pixels by pixels or the often heard keyword, megapixels. So why do people get hung up on DPI? The simple reason is that when it comes to printing, DPI actually IS the measure of quality. Confusing, right?
10 DPI vs 1,000 DPI
Click on each image below. Can you tell the difference?
You should be saying to yourself, “They look exactly the same.” Both are 600 x 900 pixels, both saved at the exact same 20% compression rate but they have vastly different DPI values. So why isn’t there a visual difference? As noted above, digital image quality is measured by the number of pixels in an image, either by the pixel dimensions or the megapixel value. In this case, they are both 0.5-megapixel images since they have the exact same pixels dimensions (600 x 900 pixels).
Now, if you want to discuss the DPI of a digital image, YOU MUST ALSO assign a physical size to it. You can’t simply say, “This image is 200 dpi.” You have to say something like “This image is 200 dpi at 4 inches by 6 inches.” It is important to remember that a digital image has no absolute size or resolution. Think about this, when do you typically discuss DPI? The answer you should be thinking about is “When I want to print the image.” This is where DPI comes into play because a printer may output at 150 dpi, 200 dpi or 300 dpi and each would require a different file size to print the optimum print. You may also be thinking that you discuss DPI when you scan a photo, but we will get to that later. For uploading to social media, 150 dpi is adaquate.
The issue that causes all this confusion is that many users interpret a photo editing program’s reference to DPI as a measure of “resolution” but this is actually the displaying the OUTPUT or printing resolution, not the resolution of the digital image. Has this happened to you: You had your photos scanned by ScanMyPhotos but when you open the file in Photoshop, it says that it is 72 dpi and you clearly paid for 600 dpi? Before you start writing a strongly worded letter to ScanMyPhotos, look at the width and height? Does it show that the photo is 50″ by 33″? We all know you did not send a 3 foot by 4 foot photo for scanning so what happened? Nearly all monitors can only display 72 dpi so most programs default to showing 72 dpi.
So, when working in Photoshop for example, the first thing you want to do when looking at File->Image Size is to TURN RESAMPLE IMAGE OFF. If you then turn your attention to the rest of the dialog box, you can see it connects Resolution, Height, and Width together while Pixel Dimensions is separate and uneditable from the Document Size. As long as you have Resample Image turned off, if you change any one of the values for Width, Height or Resolution you simultaneously change the other two, but the Pixel Dimensions will always remain the same. As the resolution goes up, the width and height go down, and vice versa, because a digital image has no absolute size or resolution. All it has is a certain number of pixels in each dimension that will be displayed on a monitor or screen. Click on the image below for a detailed look at the relationship between size and DPI and how it doesn’t affect the actual pixels in the image.
So why do people ask for DPI and when should you care what the DPI of an image is?
When someone asks you for a 300 dpi photo, what they really are asking for is a digital image of sufficient pixel dimensions that meet their pixels per inch requirement for the printed image at the requested size. Many years ago when we used to print photos, we required images to be 300 dpi at the requested print size.
“But what about my flatbed scanner?”, you may ask, “I use DPI when I’m scanning. Your company even says DPI regarding scanning.” This is true, however, what you probably don’t realize is that when scanning a photo or document on your scanner, the scanner is automatically assigning the size to whatever it is in the physical world. Therefore, you may be telling it to scan at 600 dpi but what it sees in its computer brain is that you are scanning a 4″x6″ photo at 600 dpi, thus resulting in a physical size along with a resolution. Let’s use ScanMyPhotos as an example. We will scan your photos at 300 or 600 dpi. Now, we can’t know what size your photos are going to be but it is assumed that whatever size they are, they will be scanned at the selected resolution for that size. This is why scanning companies tout resolutions, not pixel dimensions or megapixels because a 4×6 scanned at 600 dpi is different than a 5×7 scanned at 600 dpi. Different size photos will result in different size digital images.
I have personally found this website very useful and you can play around with values to see what the optimum print size is for any digital file. Megapixel Calculator
To find the optimum print size for any digital image
Required information: Pixel by pixel dimensions of the file and the printing resolution of the printer
Step 1: Divide the pixel height by the required DPI
Step 2: Divide the pixel width by the required DP
Step 3: Put those numbers together and VOILA!
Example: You have a file that is 2000×3000 pixels, what is the optimum size if the printer prints at 240 dpi?
Show your work: 2000/240 = 8.3, 3000/240 = 12.5
Answer: The optimum print size for a 2000 x 3000 pixel image printed at 240 dpi is 8.3” x 12.5”
To find the final pixel by pixel dimensions of a scanned image
- Required information: The size of the item being scanned and the resolution it is being scanned at.
- Step 1: Multiply the height of the item by the DPI
- Step 2: Multiply the width of the item by the DPI
- Step 3: Put those numbers together.
- Example: What will be the digital size of my 5”x7” photo when scanned at 300 dpi?
- Show your work: 5 x 300 = 1500, 7 x 300 = 2100.
- Answer:The final image size will be 1500 x 2100 pixels.
To find an image’s Megapixel equivalent number:
- Required information: The pixel by pixel dimensions.
- Step 1: If you don’t have the pixel by pixel dimensions, use the above steps to find the final pixel by pixel dimensions of a scanned image.
- Step 2: Multiply the pixel width by the pixel height
- Step 3: Divide by 1,000,000
- Example: How many Megapixels will my 4”x6” photo, scanned at 600 dpi have?
- Show your work: 4 x 600 = 2400 px, 6 x 6000 = 3600px, 2400 x 3600 = 8,640,000, 8,640,000/1,000,000 = 8.64
- Answer: A 4″x6″ photo scanned at 600 dpi will result in a 8.64 megapixel file.
Why Should I Add 600 DPI Scanning?
The quick answer is that higher resolutions lead to better scans for reproducing your images. 600 DPI scans produce much larger files but help ensure every detail in your print is recorded in digital form. If you want to be sure as much detail as possible is captured during scanning, add 600 DPI scanning for an additional 13¢ per scan. If you want files that are easier to work with, 300 DPI scans would be a better choice. And if you are just uploading to coail media for sharing 150 DPI is often fine.
600 DPI Scans Allow for Double the Enlargement
Part of the benefit of scanning your old photos, in addition to the added security of backing them up in a digital format, is the ability to create new prints, enlargements and photo products such as photo books. The quality and size of the prints you will be able to reproduce will depend upon both the quality of the original photo and the resolution the photo is scanned at. See the charts below for a guide comparing original print size, resolution and reproduction size. Scanning at a higher DPI will give you more options for reprints and copies to be passed down for future generations.
300 DPI Scans and Reproduction Size Recommendations*
|3×5 Reprint||4×6 Reprint||5×7 Reprint||8×10 Reprint||11×14 Reprint||16×20 Reprint|
600 DPI Scans and Reproduction Size Recommendations*
*Our recommendations are based ONLY on resolution. Final reproduction quality is based on overall quality, clarity, and exposure of the original image. Fair Rating based on 150 DPI image resolution/size. Good rating based on 225 DPI image resolution/size. Great rating based on 300+ DPI image resolution/size. NR is not recommended.
For more, please visit ScanMyPhotos.com