The formula below illustrates how when printing an image, the image resolution is divided by the number of inches you want your printed image size to be. The quotient of that division is the number of dots you will be able to print per inch. The more dots, the better the image quality.
A very high quality print has 300 dots per inch, so we say it’s “300 DPI”
If your image file resolution is 3000 x 2400 and you want to print with 300 DPI, you can get up to 8×10″.
Resolution inches DPI
3000 / 10 = 300
So, when printing image files from your digital camera, The resolution of your image determines DPI, and so image quality, based on the desired print size in inches.
To print 300 DPI at 8×10″ your camera needs a supported resolution of 3000x 2400 or larger, and you need to be sure you are using that setting.
However, if you wanted to print a 16 x20″ with the same 3000 x 2400 image file:
Resolution inches DPI
3000 / 20 = 150
You would then only be able to print at 150 DPI, and so your print would not look as good.
If you do the formula, you find you would need an image file of 6000 x 4800, and perhaps your camera may not have the ability to get such high resolution:
Resolution inches DPI
6000 / 20 = 300
Editing image file resolution with a specified DPI
An easy way to know if your image can be printed at 8 x 10″ with 300 DPI is to simply always use the 3000 x 2400 setting on your camera, and don’t try to print bigger than 8 x 10″.
However, you may have a need to be sure your image file does not exceed a certain size in resolution, but still maintains a specified DPI, such as when you are submitting image files on line for a juried art show. Perhaps they are printing brochures with small images, and so only require 180 dpi, and don’t want to have to work with a 1000’s of larger files than necessary.
To edit image size and DPI, you will need to use some kind of image editing program.
In this demonstration we will be using the free image editing program “IrfanView”. This program works for Windows PC only, it will not install on a Mac.
If you are using another program, or use a Mac, the same image settings (step 5 below) will be adjusted, it’s just a matter of finding them in your image editing program. Chances are it will be laid out essentially the same way.
You may download the IrfanView program here:
- Open IrfanView
2. Then, go to “File” then “Open”
3. Browse for and select your image file, it will open in IrfanView
4. With your image open in IrfanView, click “Image” then “Resize Resample”
(The following instructions refer to the image below)
5. A new box will open, in the field for DPI (at the bottom), type in “300”.
Next, adjust the image size. Select “Inches” as the unit of measure, then adjust size to the desired width. The image height will auto adjust to keep the original image ratio. You may also scale the width to the height by adjusting the height first. Notice that the DPI has now been reduced automatically to the new reduced size. Change the DPI back to 300 to apply it to the new image size.
Notice at the top, the image size in pixels is displayed as “Current size” and “New Size”. Be sure your new image pixel size does not exceed it’s current pixel size. If this is the case, it means your original image did not have a high enough resolution for your new size.
Click “Ok”.
6. Save the edited image. Go to “File” then “Save as”
When you “Save as” You will have an opportunity to rename the file, and or change the format type (JPG, PNG, RAW, ect.)
For quick and easy cropping, straightening and color adjustments I recommend Picasa, ( now called “Google Photos”) https://photos.google.com/ , a free image management program by google.
You can of course crop and adjust color in IrfanView also, but because IrfanView gives many more adjustment options as well, it may be more complicated.