Understanding the digital image  |  Image size dialog box  |  Resizing - no resampling  |  Resampling & interpolation
Resizing - with resampling checked  |  Sizing for print (inkjet)  |  Sizing for web or email  |  Sizing for email (Elements only)

Resolution and Resizing Demystified

image dialog box

How to use the image dialog box in Photoshop and Photoshop Elements to properly size your photographs

   for printing

   For viewing on the web and via email

   for digital photo competitions

Understanding the digital image

A quick review of the basics

A digital photograph consists of a rectangular grid of tiny squares called pixels similar to a needlepoint or a tile mosaic.

The small size of each pixel creates the illusion of continuous tone.

Enlarging the image reveals the actual pixels.

pixels

3 Ways to describe a digital image

file size

Canon and Nikon camera menus

canon menunikon menu

1) Pixel dimensions

This is simply a counting of the number of pixels along horizontal & vertical axes. This is determined by settings you choose in camera.

Pixel dimensions are the best way to describe your image. They tell you exactly how much information you have to work with.

2) Total number of pixels

Length times width. Measured in ‘megapixels’.

The greater the number of pixels in your image the larger size you can print and maintain fine detail.

3) File Size

Indicates how much space your image takes up on a hard drive or in your computer’s memory. Usually measured in megabytes or kilobytes.

If you save your image as a JPEG, this file size will be affected by the quality setting you choose (more later on this).

 

Resolution

In your camera or on your hard drive, a digital image has no inherent physical size ... it’s just a series of numbers.

It only gains physical size when you display it as a print, on a monitor, or project it.

The way you determine this size is through setting the resolution; by saying how many pixels will fit into an inch.

Resolution is the density of pixels in a displayed image.

The unit of measurement for resolution is pixels per inch (ppi).

The greater the pixel density (a higher ppi number); the smaller each pixel must be to fit into the given measurement and the better illusion of continuous tone will be.

Referring to your image only by the ppi resolution is almost meaningless. It's like saying, "I have 6 inch tiles on my kitchen floor. How large is my kitchen?" You must know the pixel dimensions in addition to the resolution to make any sense of the image.

The Image Size dialog box

Where you change the size of your image

Can be found in Photoshop:
photoshop

In Photoshop Elements:
elements

 

image dialog box

In this example we know the height is 3000 px and we want a 10" high picture so 3000 / 10 = 300 ppi

The document size (print dimensions) are a function of the pixel height / resolution and pixel width / resolution.

There are two ways to change the size of an image:
1: No resampling or 2: resampling

The most important feature in the image dialog box is the resample image check box.

image dialog box

Resizing Without resampling

Number of pixels stays the same; only their size changes. They expand or contract to fit the desired dimension.

The 'safer' way to resize.

The 'Resample Image' box is UNCHECKED. Note that the pixel dimensions cannot be changed (they are greyed out).

No matter how large or small you display your image it will always have the same pixel dimensions & same number of pixels. No pixels will be added or subtracted.

You are only making the pixels larger or smaller

When you change the numbers in the resolution box (how many pixels there are to an inch) the size your image will be displayed changes accordingly. The higher the ppi number, the tinier the pixels. If you make the ppi a smaller number, each individual pixel grows larger to fill the unit of measurement.

giant chess   chessboard   medium chess    mini chess

These 4 chessboards have vastly different sizes, yet they are all 8 squares by 8 squares.
They range in resolution from 1 square per foot down to 8 squares per ft.

The danger in resizing: Pixelation

There is a practical limit to how large you can resize an image. If you enlarge it too much, the pixels become too large and the illusion of continuous tone is lost.

(Trivia: When this effect is done deliberately for artistic reasons it is called pixelization).

The good news:

With no resampling you are never in danger of permanently degrading your image because you have not changed it. You have just temporarily changed the size of the pixels.

If enlarging your image to the desired size without resampling results in a pixelated image, then you are forced to use resampling to achieve that size.

pixelation

Resampling

Resampling involves either adding new pixels (resampling up) to enlarge an image or removing existing pixels (resampling down or downsampling) to make your image smaller. You specify a method to determine how these pixels are added or deleted.

Resizing - with resampling checked

Checking the resample image box makes all fields accessible.

First change the resolution to the desired amount.

Then you can alter either the pixel dimensions or the document size to you preferences. Note how changing one, automatically changes the other.

Always leave the constrain proportions box checked to avoid distorting your image.

The scale styles checkbox only applies to images that use layer styles, usually not a concern for photographic work.

In the bottom field, choose 'Bicubic Sharper' if the new size will be smaller than the original or 'Bicubic Smoother' if larger.

resample dialog

The difficulty with resampling up: Interpolation

 

interpolation

The problem with adding new pixels is how do we determine what color should they be?

Different formulae are used to 'interpolate' what new pixels should look like based on surrounding content. Unfortunately interpolate is just a fancy word for guessing.

interpolate2

Bottom Line:

Too much resampling up: Edges become blurred; colors muddied. Resampling down (making your images smaller by eliminating pixels can usually be done without too much image degradation -- there's less guess work involved).

Danger: If you resample your image you have permanently changed it.

There's no going back; unless you're smart enough to work on a copy and not your original image!

Sizing for print (inkjet)

resample dialog

In the document size box set the unit of measurement to inches (or cm if you prefer).

Starting with the resample box unchecked, set the resolution to 300 ppi.

The height and width shown indicate the uninterpolated size print you now have.

If you prefer a different size, change either the height or width to your preferred size. Always leave the 'constrain proportions' box checked.

The rule of thumb is enlarging a digital image more than 150% will produce a noticeably degraded image.

In the bottom field, choose the appropriate resampling algorithm.

Use the 'Save As' option under the file menu so you don't overwrite your original file.

Sizing for web or email

Before you go to the Image dialog box, choose Edit / 'Convert to Profile' from the pulldown menu. In the destination space choose sRGB.profile

If the source space is already sRGB then this step is unnecessary.

Check 'OK'.

 

Open the image size dialog box, check the resample box and set the resolution to 72 ppi. Some applications may require a different resolution. For example, if the image is to be used in a PowerPoint presentation, a resolution of 102 ppi works best.

On a vertical image set the height to 600 pixels max.

For a horizontal image set the width to 800 pixels max.

These sizes match the monitor resolution of most computers so that when viewed, the picture will fit on the screen

In the bottom field, choose the appropriate resampling algorithm.

Apply output sharpening if you desire.

In Photoshop choose File / Save for Web (CS3: Save for Web and Devices).

Choose JPEG, the quality you wish (I usually use medium) and check the ICC profile box.

save for web

A note on JPEG quality settings

The term 'quality' refers to the amount of compression applied to your image in order to make it smaller. Think of your file being written to disk using a form of shorthand. Smaller images are desirable because they take less time to transmit over the internet. But smaller images with greater compression can show degradation. So there is often a trade off between image quality and file size. In each instance, depending on the purpose of your image, you make the decision which is more important.

compression 10   compression 60   compression 100

Low quality (1 or 10)          Medium Quality (6 or 40)          High Quality (11 or 100)

Sizing for email (Elements only)

email in elements

Select the image(s) you want to send.

Choose Email from the File pulldown menu.

Choose 'individual attachments' from the format box.

Select size and quality. Medium (800 x 600 px.) is a good choice.

Click the 'next' button and send your email.

 

 

Sizing for Digital Competitions

Digital competitions usually require you to upload your image as a JPEG and each organization has specific requirements for maximum pixel dimensions and maximum file size.

Start with a high resolution image. Don't begin with one that has already been downsized for email.

Downsample your image to fit within the pixel length and width constraints required by the competition rules.

Save your image in the JPEG format using the 'Save As' option from the file menu. In the JPEG options dialog box move the slider to the right to set the image quality. As the slider moves, the file size which is shown to the right will change. Move the slider to the highest possible quality setting (as far right as possible) while keeping the file size within the requirements set forth in the competition rules.

jpeg options
 
© 2007 George Garbeck 
 
Understanding the digital image  |  Image size dialog box  |  Resizing - no resampling  |  Resampling & interpolation
Resizing - with resampling checked  |  Sizing for print (inkjet)  |  Sizing for web or email  |  Sizing for email (Elements only)