spacerlogo
HOME CLASSES NOTES CONTACT
spacerRobert Jones
Associate Professor
Delaware County
Community College
 
 
 
 
 
spacer spacer spacer spacer spacer spacer spacer spacer spacer spacer spacer spacer spacer  
     

Image Size Basics

   
     
Resampling    
     

Understanding resampling in Photoshop's Image Size dialog box

What is Resampling?
Resampling refers to changing the pixel dimensions of an image.
Downsampling or decreasing the number of pixels deletes information from the image. While this may
sound bad, Photoshop actually handles downsampling very well and the loss of information is not noticable
to the eye.
Upsampling or increasing the number of pixels adds information to your image. This technique actually
causes your image to appear blurry, not a good thing. There is a small amount of room for upsampling,
but should be avoided most of the time.

Bicubic interpolation is the highest quality of adding or removing pixels that Photoshop uses.

    
       
Original Image
This Image Size dialog box illustrates an original
image measuring 5" x 7" @ 300 ppi and a file size
of 9.01 megabytes.

 Downsampling
This Image Size dialog box illustrates a Downsampled image in which the resolution was changed from 300 ppi to 150 ppi.

    
   
Notice that when you downsample by changing either the Resolution or the Document Size that the Pixel Dimensions(File Size) changes to reflect the discarding of pixel information.    
       

Upsampling
This Image Size dialog box illustrates Upsampling
an image from a resolution of 300 ppi to 600 ppi.
The Pixel Dimension(File Size) has increased from
9MB to 36MB. Where does the additional pixel
information come from? Photoshop tries to do its
best job by looking at two pixel's colors and values
and averages the needed pixel information between
them. The result: a blurry image.

Lesson learned: Feel free to downsample an image, Photoshop does that very well. Beware of Upsampling
because you will most likely end up with a poor quality image.

This technique is particulary useful when preparing images for the web where small file sizes are all important.

    

Another option to consider if you're main intention is not to change file size(total number of pixels in an image measured in kilobytes, megabytes, etc) is to uncheck Resampling which I'll refer to as Resizing.

 

    
     
   
Resizing

Resizing retains the original file size while trading off an indirect proportion between an image's dimensions and its resolution. If you start with a 5" x 7" image @ 300 ppi and change the resolution to a lower number, say 150 ppi, the image's dimensions will double. Conversely, if you resize an 300 ppi image to 600 ppi the dimensions will be cut-in-half. One increases when the other decreases. See the examples below.

In all the examples below the File Size remains the same at 9 MB.

    
Original Image Lower resolution/larger dimensions    
   
     
Higher resolution/lower dimensions      
     
     
   
Crop Tool    
   
Another tool for adjusting the size and resolution of your image is the Crop Tool.    
   
Pictured here is the Crop Tool and it's options. Use the Crop Tool with it's options blank and
simply click and drag around the area of your image you want to retain.		    
     

Original image and image showing Crop Tool bounding box. Area outside selection is dimmed
in order to highlight the area of the image which will be retained after cropping. If you're not
pleased with the bounding box you can still resize it by dragging the corner handles or side
handles indicated by the sample squares.

 

    

   
 
   
   
   
   
     
   
You can also enter dimensions and/or resolution in the Crop Tool Options.