Altering the Dimensions and Resolution of an Image

If you do not know the final dimensions and optimum resolution for an image when you are scanning, you can alter both attributes when you decide what they should be. In Photoshop 4.0, the control is split between print and multimedia attributes.

To alter the image size for print images

• From the Image menu, select Image Size....

The Image Size dialogue box is displayed.

• Check Constrain Proportions if you wish to maintain the image's current proportions. Uncheck Resample Image if you wish to maintain the size of the file
• Under Print Size, enter new values in the dimension fields (if Constrain Proportions is checked you only need to enter one value)
• Under Print Size, enter a new value in the Resolution field (if Resample Image is unchecked, any alteration will affect the dimensions)

Note the revised amount in MB after Pixel Dimensions if the document size is important to you.

• Click OK

To alter the image size for screen images

• From the Image menu, select Image Size...

The Image Size dialogue box is displayed.

• Check Constrain Proportions if you wish to maintain the image's current proportions. Check Resample Image to allow the file size to be altered
• Enter new values in the fields under Pixel Dimensions (if Constrain Proportions is checked you only need to enter one value).

Note the revised amount in MB after Pixel Dimensions, if the document size is important to you.

• Click OK

If Resample Image is checked when the image size is altered, resampling usually takes place, resulting in pixels being added or deleted.

Increasing the pixel dimensions (file size) results in the creation of brand new pixels based on existing tonal/colour values. Large increases in size, four times the current size and upwards, leads to dithering on the edges of images, obvious pixelation, lack of sharpness and loss of detail.

Reducing the pixel dimensions (file size) results in the removal of pixels. Extreme reductions have no detrimental visual effects, provided the resolution is appropriate for a given use.

Any resampling involves interpolation, a process of assigning colours to pixels. The method of interpolation can be altered to suit a particular image. Bicubic gives the best quality; Nearest Neighbour retains sharp edges within images and is appropriate for certain graphic subjects.

In Photoshop 4.0, the method is selected within the pop-up menu at the bottom of the Image Size dialogue box.

Recropping an Image

To recrop an image freely

• Select the Cropping tool
• In the Cropping Tool Options palette, uncheck Fixed Target Size
  
  
  The Cropping Tool Options palette

• Click-drag diagonally across an image to define the crop area
• Select the Move tool to implement the cropping

An alert box saying 'Crop the Image?' is displayed.

• Click Crop or Cancel

Note: Cropping freely reduces the dimensions of an image whilst maintaining its resolution. No resampling takes place when the tool is used in this way.

To constrain a crop and/or alter the resolution

• Select the Crop tool
• In the Cropping Tool Options palette, check Fixed Target Size
• Enter values in the Width and/or Height fields. Select the unit of measurement you wish to use in the adjacent pop-up menus. If you do not wish to fix one of the dimensions, leave its field blank
• Optionally, enter a value in the Resolution field. Select the unit of measurement you wish to use in the adjacent pop-up menu. If you do not wish to change the resolution, leave this field blank.
• Click-drag diagonally across an image to define the crop area

A cropping border is displayed.

• Move the pointer to one of the handles of the crop border and click-drag to re-adjust a crop
• Move the pointer to the edge of the crop border (but away from any one handle) and click-drag to rotate a crop
• Select the Move tool to implement the crop

An alert box saying 'Crop the Image?' is displayed.

• Click Crop or Cancel

Note: Entering a resolution results in the resampling of the cropped area, resulting in the creation or removal of pixels. See Resizing Images.

To create extra working area

You can create extra space around an image to give you more working area. This function is especially useful when composing montages.

• Select a current Background colour. This colour is used for the extended space
• From the Image menu, select Canvas Size....

The Canvas Size dialogue box is displayed.

The Canvas Size dialogue box

• Enter values in the Width and Height fields
• In the grid at the bottom of the dialogue box, click where you wish the existing image to be in relation to the extra space
• Click OK