Every now and then you need to resize a photo for different reasons. Being an eager hobby photographer I like to be able to print some of my photos. Sometimes I want to make smaller prints and sometimes I want to make large ones. Storing different copies of my images would not be a space saver, so instead I use a different method: Scaling using DPI.
[disclaimer] This guide is written using Photoshop, but there are several other programs available with the same capabilities as described here (like Paint.net). This guide will not focus on how to create photos for use on the web where pictures need to be downsized and data removed.
To be able to resize a photo (making it larger or smaller) without removing data you need to know what a digital photo is and you will also need to know what DPI is and how it works. So let’s start with that.
All photos stored on your computer are what we can call Bitmap images. The Bitmap is simply explained a grid (or a mesh) that we fill with pixels (or dots) of various colors. Up close you will only see colorful pixels but zoomed out you will perceive it as a photo. A little trick of the eye as it were. And this is where the DPI comes in.
DPI – Dots Per Inch
As you may know, an image has two dimensions (height and width) but there is also a third one; called DPI (Dots Per Inch). The DPI is what tells you how much information is stored in your photo. The more pixels (or dots) we are able to squeeze into our photo, the more information we store and the larger the file. We measure this in inches.
Lets break this down a bit. Lets say you’re at a stadium where each row is a line in your photo. Each person is a pixel. The more people you can squeeze into each row, the more people will get to see the match. Hence, the more pixels you can squeeze into an inch, the more information is stored in the photograph, and the better it will look.
Take a look at the illustration above that demonstrates the difference between 300 DPI and 72 DPI.
Different DPI = Different usage
Your monitor uses a DPI of 72 or 96, which plainly speaking means, 72 (or 96) pixels spread out on an inch long line. A newspaper will traditionally use a DPI of about 150 DPI, and a magazine will normally have a setting of 300 DPI. High Resolution Photos can use from 600 DPI and up to the thousands. Meaning that what you are going to use the photo for will decide how large DPI you should use. Some Printers will only print a DPI up to 600 so anything above that will just be a waste of space.
Also know that the larger the DPI, the more hard drive space and memory the photo will consume.
Scaling using DPI
We may convert the image physical size (height and width) to DPI and vice versa. Any normal digital camera will use a DPI-setting of 72 (or 96), making the images large in size. My camera takes pictures that are about 3000×2000 pixels with a DPI of 72, which is equal to 105×70 cm. Which of course is way to large to print.
If I were to just strip off the excessive data I would be stuck with a pixellated, unsharp photo. Instead I want to keep all the information, while still be able to print High-Res photos on my Photo paper. This is where I open my photo in Photoshop (but by all means use Paint.net or similar).
- Scaling the photo down to 40 x 30 will give me a DPI of 177 DPI
- Scaling the photo down to 15×10 cm gives me a DPI of 500 DPI
The great thing about this kind of scaling is that should I decide to print this image in a larger format later I’m still able. Because no data is removed only stacked together tighter, there is no loss in data or image quality. That being said, if you want to double the image size (from 40×30 cm to 80×60) the image will not be sharp to look at up close, but zoomed out it will be. Take a closer look at a large poster next time you walk by it, up close you will see the pixels, but from a far …
In Photoshop you are able to resize an image and save it without the loss of data. If you are using another program try not to save the changes after scaling.
- Open your photo
- Choose Image Size from the Image Menu
- To Resize an image using the DPI information, uncheck “Resample Image”
- Reduce the print size by entering a new width or height (to suit your Photo paper).
If you decide to use paint.net, be advised that if you are not careful it will remove data that are lost to you if you save the image after printing it.
- Open your image
- Choose Resize from the Image-Menu.
Make a note of the file size in the top right corner, and the print size at the bottom of the resize-window.
- Change the resolution (DPI) from 72 to 350.
Notice how the file size is unchanged, but the printsize has changed.
You may if you like change the printsize instead of the DPI, in paint.net this will set the DPI to a default of 180. My recommendation is to keep the DPI at least 300 to keep the printed image sharp and crisp on a High-Res photo paper. Read more about resizing using paint.net here.