Photoshop Workshop - Lesson 4

This lesson covers basic image manipulation: changing an image's overall dimension or resolution; sharpening the image afterwards; cropping an image; and changing the canvas size.

First, watch this Quicktime video lecture - navigating images (third item under "basics")

Please download (right-click and save-as) any or all PSD images to your hard drive; reserve healing.psd for the last activity.

Topics in this lesson follow. Remember that C* stands for the Control key on PCs and the Command (Apple) key on Macs.

• Changing an image's resolution and dimensions
• Applying the Unsharp Mask Filter
• Cropping an image using Marquee
• Cropping an image using Crop command
• Applying auto-color correction

Changing an image's resolution and dimensions

1. Open the image (C*-O). Choose Image > Image Size
2. Ensure Resample Image is checked. I leave the setting at Bicubic because it degrades the image the least.
3. To preserve image dimensions, ensure Constrain Proportions is checked.
4. Set the Resolution to 72 pixels/inch (ppi).
5. Enter a new pixel dimension in either the width or height box.
6. Click OK or press Enter/Return.

Applying the Unsharp Mask Filter

1. Open the image (C*-O). Choose Filter > Sharpen > Unsharp Mask
   
2. Choose an Amount for the percentage increase in contrast betweeen pixels.
   Tips: use low settings (< 50) for figures or natural objects; use a higher setting if the image contains sharp-edged objects. For a high-resolution image, use an amount between 150 and 200 percent. Experiment!
   
3. To choose an appropriate Radius value, factor in final size, resolution, and the subject matter of the image. The Radius value controls the number of pixels surrounding high contrast edges. Try between 1 and 2 pixels. A larger value may produce too much contrast in areas that are already high-contrast.
   
   There is an inverse relationship between Amount and Radius.
4. Choose a Threshold value (0-255) for the minimum amount of contrast an area must have before it will be modified. At a Threshold of 0 will apply the filter to the entire image. To prevent noise from distorting skin tones, specify a Threshold between 8 and 20.
5. Click OK.

Cropping an image using Marquee

1. Open the image (C*-O). Choose the Marquee (crop) tool (C).
2. Drag the Marquee over the portion of the image that you want to keep.
   
3. Modify properties
   Look at the properties located just under the menu bar. Check Shield if you want the area outside the crop marquee to be darkened. Click the Color swatch if you want to change the color of the darkened area, and choose an Opacity for the shield color.
   
4. If you need to resize the marquee, drag any handle (double-arrow pointer). Shift-drag will preserve proportions. A*-drag will resize the marquee from its center.
5. If you need to reposition the marquee, position the pointer inside the marquee and drag, or use the arrow keys to nudge.
6. Press Enter/Return or Double-click inside the marquee.

Cropping an image using the Crop tool
I often use the crop tool when I don't want to change the original image.

1. Open the image (C*-O). Choose the Rectangular Marquee tool (M).
2. Drag the Marquee over the portion of the image that you want to keep.
3. Choose Image > Crop, then deselect (C*-D).
4. You can go straight to Image Ready (C*-A*-shift-S) or save as (C*-shift-S) a new image (you probably want to avoid C*-S unless you don't think you will ever need the original image).

Applying auto-color correction
Photoshop 7 has built-in auto color correction options: Auto Levels, Auto Contrast, Auto Color. I use these for quick-and-dirty correction for Web images. Next week, Dan is going to show us how to do custom correction.

1. Download and open (C*-O) healing.psd.
2. Make a copy layer - work with the copy, not the original. Be sure to "toggle" the eye "off" so that what you can see the changes on the copy. This protects the original in case you forget and save the image without changing its name.
3. Choose Image > Adjustments > Auto Levels. See if the image changes.
4. Do the same for Auto Contrast and Auto Color. Remember that C*-Z is Undo but sometimes what you need is Step Backwards (C*-shift-Z).