My Nikon camera takes photos with a resolution of 300 dots per inch (dpi for printing), but not all cameras have this default setting. 200 dpi to 300 dpi is usually sufficient for most photo printing, although you can go higher if your software, printer and photo image have the capability.
If you saved an image from Facebook for printing, for example, you may be disappointed because the 72 pixels per inch (ppi for monitors) of most web-based jpeg images is not sufficient for printing clear color photos. Web-based images are usually optimized at a lower resolution to make them load faster, which is the opposite of what you want for printing. Downsampling is a term that applies to reducing the number of pixels or dots in order to make a smaller image. (Note: Sizing a picture larger, or upsampling, is another discussion!)
Your printer may allow you a choice of print sizes, but there may be times when you want to edit a photo to exactly fit the dimensions of a frame, and you don't want to lose any digital information available to you. To find the original dimensions and size of your photo:
Hover your cursor over an image on a Windows device, it will tell you the file type, dimensions, and size in kilobytes. However, to figure the dots- or pixels-per-inch resolution, you may need to open the photo in an image viewer such as the quick and useful free conversion software IrfanView.
click for larger image
As you download and install IrfanView, make sure all images are associated with it so they open quickly. Click on your image so it comes up in IrfanView (if it doesn't, drag your image file to the Irfan shortcut) and go to Image/Resize/Resample.
You will be shown the width and height in pixels or inches as well as the dpi resolution. If you like, you can resize by measurement or percentage to save in a number of different file formats. IrfanView does not automatically refigure the DPI input number as Photoshop does, although you can input the dpi yourself, if you have an idea of what it should be.
To make the best use of your photo's digital information, open in Photoshop, then go to Image/Image Size, or right-click on the top blue margin of the photo and choose Image Size. You will be shown the image size and resolution. The photo above shows a size of 22.222" x 16" with a resolution of 72 dpi. The dimensions are far too big, but I would like a better resolution as I decrease the size.
Unclick the "Resample Image" box, which will enable Photoshop to recalculate the resolution to make use of all available digital information as you size down. In this case I have a 5" x 7" frame. You may need to crop in order to get the dimensions right. As the image (document) size decreases, the resolution, or the dimensions of each pixel or dot, increases.
(The Pixel Dimensions indicator at the top of the dialog box stays the same since it is actually measuring the total dimensions of all pixels.) Downsampling (checking the Resample Image box) will cause the Pixel Dimensions to show a smaller number alongside the original number.
Resizing images this way also applies to making small pictures larger, if you have sufficient resolution to begin with. This way you will not be upsampling, or interpolating, by letting Photoshop guess what the pixels ought to look like in order to keep the original resolution of 300 dpi. Photoshop will only use the digital information that is already available.
In this old black and white photo scan, I want a small picture of each of the girls' faces for a hinged frame.
Cropped and needing a little restoration help.
The cropped image is about 3/4" x 1" with a resolution of 300. My frame openings are 2.5 x 3.5", so I replace the width and height input in inches, making sure the Resample Image box is unchecked.
Although this lowers the resolution to 87, it will be sufficient for the very small frames,especially since it will be black and white.
Here the sepia is turned to black and white by adding a Channel Mixer adjustment layer, checking the Monochrome box to take out color, and adjusting the Constant sliders for contrast and the Red, Green and Blue sliders for the right mix of shades of gray. Ready for printing.
Understanding Resolution and Resampling
Changing an Image's Resolution and Size
Digital Photos and Ideal Print Resolution
All About DPI, PPI and Printing
Print Size and Viewing Distance
Image Size Calculator
Best tools to crop and resize images online
Best online photo editing websites