The 72 dpi myth

A couple of decades ago, some cruel programmer at Adobe decided it would be a funny joke to put in 72 dpi in the resize window for web images. 20 years later, we are still struggling with a large percentage of “photographers” who have no clue that dpi/ppi is irrelevant when creating output for web display.

I’m going to first state some facts and will go on to explain more in detail:

As it relates to web images that do not include embedded text,

Dpi is irrelevant for web output.
Dpi does not affect file size in any perceivable manner.
Dpi does make a web image appear more dense or make it weigh more.
Dpi does not prevent or encourage image theft.
Dpi does not affect colors, artifacts, compression, or noise.
Dpi settings from your digital camera is irrelevant and arbitrary.
Dpi expressed without inches is meaningless.
72 dpi is not better or worse for printing without knowing the inches or pixel dimensions.
Dpi does not make an image load faster or slower on the web.
Dpi will not make your images more fragrant, may not be used as effective contraception, and will not cure erectile dysfunction.

Let’s begin with a demonstration. The following images have varying dpi settings with the corresponding file sizes below it. There are no differences between the images. They are identical. Any differences in file size is due to differences in jpg compression.

15.1 K

15.2 K

15.1 K

15.6 K

As you can see, the differences in file size is negligible. The images are identical and if you think you see a differences, get your eyes checked.

Dpi does not prevent or encourage image theft. Why? Because dpi does not affect image dimension. If you post a 3,000 x 2,000 pixel image on the web, it is a high resolution image regardless if the dpi is 1 or 20,000. If you post a 200 x 300 pixel image like the ones above, it’s a low resolution image. Yes, even the 20,000 dpi version. So why would having 72 dpi be better than 20,000 dpi? It’s not better. The only thing that matters is pixel dimensions.

Dpi settings from your digital camera is irrelevant and arbitrary. Ignore the default dpi setting you see when you import your photos into your computer. It means nothing.

Dpi expressed without inches is meaningless. Dpi is used with print dimensions so you would say 8×10″ at 300 dpi or 2,400 x 3,000 pixels. If you were to print this image at 150 dpi without resampling the dimensions, you will get a 16″ x 20″ print. It’s just simple math. I’ll give you an analogy for printing images. DPI=Miles per gallon. Image dimension=size of the gas tank. If someone wants a car with a large gas tank, they don’t say I want a car that gets 40 miles per gallon. They say I want a car with a 25 gallon tank. So if someone wants a high resolution image, they should ask for an image with large pixel dimensions, not an image with 300 dpi.

72 dpi is not better or worse for printing without knowing the inches or pixel dimensions. I often hear someone say “my client requires 300 dpi images”. That is a nonsensical statement. The correct statement would be “my client requires 8×10″ images at 300 dpi. The following are all the same:

8×10″ @ 300 dpi = 2,400 x 3,000 pixels
33.3 x 41.6″ @ 72 dpi = 2,400 x 3,000 pixels
2,400 x 3,000″ @ 1 dpi = 2,400 x 3,000 pixels

So when someone complains that the 2,400 x 3,000 pixel image you gave them is only 72 dpi, tell them to trust you and print it at 8×10″ anyway.

Michael Beach explains it well here:

Let’s see if this clears anything up for anyone. Note, I’m using CS2 but I think later versions are pretty close if you want to follow along.

File.. New
Create a 500×500 image at 100dpi

Once it is open, go to Image.. Image Size (or Alt+Ctrl+I)
You will see that there are 2 sections: Pixel Dimensions and Document Size. Pixel Dimensions refer to monitor display. You’ll notice a lack of any dpi or ppi reference in this section. That’s because it is irrelevant.

Document Size refers to the image printed on a page. By default it shows you how large it will print in inches and what print resolution it is set at in pixels/inch.

Through basic math and the image below you can see that if you have 500 pixels on a side, and you print 100 pixels to every inch, you will get 5 inches out of those 500 pixels.

The last checkbox at the bottom is the only one of the 3 checkboxes that really concerns us right now… Resample Image. When checked, it tells the program to make sure that when you change anything the size of the printed image stays the same.

Change the Resolution to 200 pixels/inch and hit ok. Now that we want to print 200 pixels to every inch instead of 100 we’ll need twice as many pixels along each side to get our same 5 inches. That is reflected in the Pixel Dimensions changing from 500×500 to 1000×1000 as seen below.

You’ll also notice the file size quadrupled, as we went from 250,000 total pixels to 1,000,000. The image got bigger on the screen because the pixel dimensions increased..not because it is set at a higher dpi.

A critical point is that the image is not a 5″ x 5″ image. It is a 1000 pixel x 1000 pixel image. It can be printed at any size we want, with varying degrees of clarity. Open the Image Size dialog back up and uncheck Resample Image. You’ll notice the Pixel Dimensions section grays out and is locked in.

Now change the Width to 10 inches. The program tells us that to print our 1000 pixel x 1000 pixel image at 10 inches by 10 inches, it will have to space the pixels out so that there are 100 of them along every inch of each side.

In this process the pixel dimensions remained the same at 1000×1000 so the file size did not change at all, as you can see below.

When dpi does matter when not talking about print images. The one instance when dpi matters is scanning images. When you scan a negative, slide, or print, you will have a choice of dpi. The dpi is based on the input size of the image. So while the Nikon Coolscan V ED scans at 4,000 dpi, you’re talking about scanning a 35 mm image that is roughly 1 inch so a 35mm negative yields a 21 megapixel image.

When dpi does matter for web images. If you are embedding text into an image, dpi does matter because Photoshop expresses text in terms of point size which is a physical measurement. Here are two identical images. One is 72 dpi and the other is 300 dpi. Text has been entered at 12 points. As you can see, the resulting text is different depending on the dpi of the image. 12 points on the 72 dpi image appears smaller because the physical print of a 450×300 pixel image is approximately 4″ x 6″. The same 12 points on a 300 dpi image appears bigger because the physical print of a 450×300 pixel image is only 1″ x 1.5″.