Whitehead Institute BaRC

Updated 7/12/05




Basic Tools
Magic Wand


Inside WI > Biocomputing > Graphics > Photoshop > Introduction to Photoshop


Introduction to Photoshop


Photoshop is unlike other common software interfaces which emulate virtual typewriters or graphing paper. Photoshop creates an artist's virtual studio/darkroom. When you open the program you see a toolbox on the left with tools you will use to manipulate your images, and on the right, a white square which is your "canvas" or work area. The gray area surrounding the canvas is not part of your image, but only defines its edges.

To change the canvas dimensions, go to Image > Canvas size. You can type in any dimensions you like, but remember that the area of the image is directly proportional to the file size.

Basic Tools .............Back to top

(You can use the letter next to each tool to switch between tools quickly)

Marquee Tool : The images in Photoshop are stored pixel by pixel, with a code indicating the color of each. The image is just a big mosaic of dots. Therefore, before you can do anything in Photoshop, you first need to indicate which pixels you want to change. The selection tool is one way of doing this. Click on this tool to select it, then click and drag on your image to make a dotted selection box. Hold shift while you drag if you want a perfect square or circle. Any pixels within the box will be affected when you make your next move. If you click and hold on this tool with your mouse button down, you will see that there is also an oval selection shape, and a crop tool .

Crop Tool: . To crop your image, draw a box with the crop tool. Adjust the selection with the selection points, and then hit return to crop.

Lasso Tool : The lasso tool lets you select freeform shapes, rather than just rectangles and ovals.

Magic Wand: Yet another way to select pixels is with the magic wand. When you click on an area of the image with this tool, all pixels that are the same color as the pixel you clicked will be selected. Double click on the tool to set the level of tolerance you would like (i.e. how similar in color the pixels must be to your original pixel color. A higher tolerance means a broader color range).

The Move Tool: This is a very important tool, because up until now all you have been able to do is select pixels, and not actually move them. The move tool not only allows you to move areas you have selected, but also to move entire layers without first making a selection. If you hold the option (or alt) key while clicking and dragging with the move tool, you can copy the selection.

Airbrush Paintbrush and Pencil tools can be used to draw with the foreground color on whichever layer is selected. To change the foreground color, double-click on it in the toolbox. You will then see a palette of colors from which to choose. Select one and click OK. To change the brush size, go to Window > Show Brushes.

Eraser Tool: Erases anything on the selected layer. You can change the eraser size by going to Window > Show Brushes.

Line Tool: Can be used to draw straight lines. Click on the tool to select it, then click with the tool on the canvas area and drag to draw a line. When you release the mouse button, the line will end. You can change the thickness of the line or add arrowheads to it by double clicking on the tool to see this dialog box:


Text tool: Click on this tool to select it, then click in the Canvas area. You will be given a dialog box in which to type your text, and choose its attributes. Each new block of text goes on its own layer, so you can move it around with the Move Tool. Once you have placed the text, however, it is no longer editable. To correct mistakes, you must delete the old version (by deleting its layer) and replace it.

Eyedropper: Click with this tool on any color in the canvas to make that color the foreground color. (You can then paint or type with it).

Magnifier: Click with this tool on a part of your image you want to see closer, or drag with it to define the area you want to expand to the size of the window. Hold down the Option or Alt key to make it a "reducer" instead and zoom back out.

Grabber: Click with this and drag to move the entire page for better viewing.


Options Bar

The Options bar appears at the top of the screen and is context sensitive, changing as you change tools. The tool in use is shown in the left corner, and options relating to the tool appear to the right of that.

For example, in the snapshot above, the move tool is selected.

Auto Select Layer when checked means that the move tool will switch layers automatically depending where you click on the canvas. This makes it feel almost like a vector program when you are selecting objects. The layer palette still shows you what is on each layer and you can use it to select objects as well.

Show Bounding Box makes it easy to rotate or transform an object when you click on it. When the cursor is held over the corners of the bounding box, it turns into a curved arrow. Click and drag to rotate.


Layers ............Back to top

Photoshop works on a system of layers, which are like sheets of transparency. When you place objects on a layer, they become part of the layer, just as if you had drawn on a transparency with a marker. If two objects are on the same layer and you want to move them closer to each other, you need to cut them out with the lasso or marquee tool and physically move them together. If they are on different transparent layers, however, all you have to do is move the layers with the move tool until the objects are closer to each other.

In a file with overlapping elements, putting them on different layers allows me to move them around without having to select them first. To see all the layers in your document, go to Window > Show Layers. Here are the layers for the image on the left. The eye icon on the left indicates that the layer is visible. Click on the eye to make a layer invisible. Select a layer and then click the trash icon at the bottom to delete a layer. Click and drag on layers to change their stacking order.

Resolution / Dimensions of Image / Size of File .............Back to top

Pixels per Inch
Photoshop files are made up of tiny squares of color called pixels. Using large pixels will make a grainy image, and using tiny pixels to make the same image will be much smoother. However, the smaller the pixels, the more of them there are in the file, and the larger the file size will be. If the file size is large, it opens slower, takes longer to save, and takes up more room on a disk. The the key in choosing a resolution is finding the balance between image quality and file size.

For example, if I want to re-tile my bathroom floor to make a smiley face, I can either use large tiles or small tiles. On the left I use 10 tiles on each side for a total of 100 tiles. On the right I use 25 tiles on each side, for a total of 625 tiles.


If each of these images measures an inch on each side and each tile is a pixel, the image on the left would have a resolution of 10 pixels per inch, or 10 ppi. The image on the right would be 25 ppi. Notice how a small increase in resolution corresponds to a much larger increase in file size, because the increase is exponential.

You can begin to see why images of different resolutions sometimes change size when combined in one file, or when brought into other programs. For instance, what if I wanted to draw the image on the right in tiles the size of the image on the left? I would have a much larger image in the end. This is what happens when you bring an image with a high resolution into a file which has a lower resolution. There can only be one resolution per file, so the pixels of the image coming in are resized to the size of the existing pixels in the file.

Image Size Dialog Box

JPEG images from the Internet have a low resolution of 72 dpi. However there is no reason that your JPEG images have to have such a low resolution. Image files generated by digital cameras often begin with a low resolution but very large dimensions. You can use that to your advantage by trading area for resolution. To change the resolution or the size of your image, go to File > Image Size. It will look something like this.

At the bottom of the window there is a box marked "Resample Image". This box determines whether or not area can be exchanged for resolution. When it is checked, resolution remains independent of the image dimensions. If you change the dimensions or the resolution, you will see the file size change accordingly.

To exchange area for resolution, uncheck the Resample Image box:

You can see that the linking symbol on the right now encompasses width, height and resolution. Perhaps you don't need a 10 inch wide image. If you make the width smaller, look what happens to the resolution:

What is happening is simply that the pixels in your image have been decreased in size. The total number of pixels and therefore the file size remains the same. If you had used the resample option, pixels would have been added or removed from the image and the file size would have increased or decreased.

What resolution is best?: 300 is optimal, between 200 and 300 is okay, but below 200 the image will begin to show pixelation.

When to resample: When you have two images of different resolutions that you want to combine in Photoshop and you don't want either of them to change in dimension, you will have to resample one of them to bring them to the same resolution before combining them. This is because a single file can have only one resolution. When you bring in an image from a file with pixels of a different size, Photoshop will enlarge or shrink the pixels to match those in the file into which the image is going. Bringing an image with a resolution of 300 ppi into a file with 72 ppi will make the first image appear much larger when it arrives in the new file. Why? It is the same as if I decided to make my detailed smiley face in the larger tiles instead of the small ones. I would need a much bigger bathroom!

Tiling my detailed face in larger tiles This is why I really need to change the tile size first!

Also, if you have an extremely high resolution that you don't need, you can resample down to 300 to save disk space.


Tonal Adjustment .............Back to top

Sometimes a scan or even a digital photo won't look like what you saw under the microscope. There are some easy ways to adjust the color in an image. As long as the adjustment is made to the entire image, and not to selected parts, it should be OK ethically, although you should use your own judgment.


Sometimes an image will seem faded or washed-out. Using the Levels adjustment can increase the contrast in the image. Go to Image > Adjust > Levels. You will see this dialog box with a histogram showing the tonal values in the image. Here the curve is shifted to the right, which corresponds to what we can see--the image is too light.


By moving the circled sliders toward the bell-shaped part of the curve, we can reset the value of the darkest and lightest pixels in the image. Moving the black arrow to the right will darken the darkest values in the image, and moving the white slider to the left will lighten the lightest values in the image.


Watching how the changes affect the image, let's gradually move the sliders inward to the edges of the curve.

When we click OK, Photoshop will reset the curve, with our placed arrows as endpoints:

(Going back into Levels, we can see our new tonal values curve)

As you can see, what the levels adjustment did was take a segment of the tonal range and reassign the values from black to white. In the readjusted levels dialog box, you can see spaces indicating values that no longer exist. This is the difficulty in making image adjustments. When you over-adjust, you begin to leave gaps in the tonal range, and this will eventually cause a posterized look (too much black & white and not enough middle gray tones). The only way to avoid this is to capture a large range of tonal values when scanning or taking a picture, and to use only subtle adjustments to the images.

Using Levels on color images:

In this image the background wasn't quite black. Moving the dark slider to the right deepened the darks.


Values are lacking at both ends of the curve. Move sliders inward Click with highlight dropper in the lightest part of the image to correct color imbalance.



Back to top