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Image resolution – What it is and what it’s not

Av R Nilsson ⋅ ⋅ Kommentera

(This is a translation from my swedish text. Please excuse my English)

Sometimes I run into professional photographers or even other image professionals that claim to deliver images to me in high resolution 300 dpi. Why it is wrong to say this, I try to explain here.

To say that an image has a resolution of 300 dpi (dots per inch) is to say that the distance between Stockholm and Gothenburg is 90 km per hour. It is relative. Sure dpi is per definition a measure of density and in that meaning a resolution. But it is rarely relevant in this case.

For a start, dpi, dots per inch is term used in printing and cannot be directly translated to digital imaging. Instead we talk about ppi as in pixels per inch (where a pixel is the smallest element of an image).

The problem with DPI (and/or PPI in a way) is that it is only relevant if you at the same time states the size it’s going to be used for in inches/cm. You can express an images resolution/quality if you say this image is 300 dpi at 12 cm. Compare it with you can say the distance between to cities is 90 km per hour in 5 hours. Sure you can say that but it’s not very practical. It’s easier to say just 500 km.

That is why an image’s resolution shall be stated in pixels. The term megapixels is familiar to all who has bought a digital camera (or even a cellphone these days). So why does the term dpi shows up almost every time you here about resolution? Last example I found was from the large and well-known image bureau Corbis who organized a photo contest. When reading the technical requirements, tada there it is: high resolution = 300 dpi (and also they stated at least 10 MB filesize).

To prove my point that this is nonsense I fire up Photoshop. I create an ugly image that the standard most-used JPEG-format has a bit trouble compressing (hard contrast with hard edges).

This work of art is 500 x 500 pixels. It’s not pretty and the resolution is not much to brag with. Okay now lets make this image 300 dpi (or rather ppi but we leave that out for now). In Photoshop select the meny Image -> Image size.

Click Ok and voila, the image is 300 ppi. Just to flip out, try enter 50 000 ppi now. What happens? Does the image magically becomes super-duper-hi-res? Of course not. What happened was that the image in printed size shrunk from 18 cm down to 4 cm. Or with 50 000 ppi down to 0.03 cm.

When working with digital images, you can never know in what size the image eventually will be printed. Will it be used for a poster or a stamp? Or on the web? The only thing you can be sure of is the actual size of the image. In this case 500 x 500 pixels, or 0.25 megapixles. That is the only thing that matters.

And about actual file size in megabytes, this is determined by a lot of factors. If I save the above image in 50% JPEG the file size becomes 0.14 MB. With 100% quality the image is 0.52 MB. If we overdo it again and convert it to a 32-bit HDR in uncompressed TIFF we have a 3 MB image. For the exact same image.

So next time someone asks you in what resolution you can deliver an image you say 10 megapixels if your camera can capture that (and you haven’t cropped it). If they then a bit confused asks you to say it in dpi, when you can (without sarcasm of course) say: whatever dpi you prefer.

Have fun!