Forestry Images: Forest Health, Natural Resources & Silviculture Images

Guide to Image Resolutions

Deciding what resolution to select when downloading images is based on largely upon how you plan to use the image.  This is all about pixels…if you do not have enough pixels to cover the space that you plan to put the image, you are out of luck!

Let’s start at the beginning…resolution is listed as [the number of pixels in width]x[the number of pixels in length]. 

For instance, the 5 resolutions we offer on our site are:

192x128
384x256
768x512
1536x1024
3072x2048

Many digital cameras express resolution in term of Megapixels (MP).  For example, a Nikon D70 is a 6.1 MP camera; when taking a picture at the highest resolution setting of the camera the image will have about 6.1 million pixels in it.  To determine the MP in an image, multiply the number of pixels in the width by the number of pixels in length.

For instance, the 5 resolutions we offer on our site have the following MP

Now that you know how many pixels you have, how do you know what size will it be when it is used?  Every output has a certain resolution associated with it, often expressed as pixels per inch or PPI.  PPI is different than DPI or dots per inch since DPI refers to the amount of toner or ink used per inch of paper.  Despite this difference, many computer programs use DPI is as a crude guideline to approximate the PPI.  Here is a brief list of PPI outputs:

To find out the maximum size that a given image can be displayed or printed at without distortion, divide the dimensions in pixels by the PPI for the output.  Here are 2 examples for our resolutions:

This gives you guidelines for the maximum size if you plan to use the entire image.  You can always choose to make the image smaller than the maximum size with out worrying about distorting the image.  If you choose to make the image larger, you run the risk of reduced quality in your final product.  It is also important to note that if you plan to zoom in on a portion of the image and enlarge it to show a certain feature, you will need a larger image to start with since you will be ultimately reducing the number of pixels in the image when you crop it.