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Paper: DSDN144 Digital Photographics

Exercise: Experimenting with Levels

Note that this is simply an experimentation exercise to get you to understand how Levels work.

1. Select the File and go to New (or press Control + N). Create a new file

600 pixels wide by 450 pixels high, resolution at 72 pixels/inch, RGB with

a white background. Click OK.

2. Press D on the keyboard to set the foreground and background colours

in toolbox to black and white. This is the default setting.

3. Select the Gradient tool (or press G on the keyboard).

4. In the options (below menu) click on the drop down arrow on the Gradient Picker to open it. Make sure the Foreground to Background gradient is

selected (if you are not sure which is which, position your pointer over

each gradient to see the tooltip description).

5. To make your gradient linear, select the first of the group of five icons to

the right of the Gradient Picker.

6. In the Gradient options palette, set Mode to Normal and Opacity to

100%, make sure Dither and Transparency are ticked but Reverse is

not.

7. Hold down the Shift key (in order to your gradient perpendicular) and

starting at the left of the image, drag your pointer right over to the right

side. This will create a gradient from black to white (0 to 255).

8. If no rulers are showing on your image press Control + R.

9. Select the Move tool from the toolbox (top right tool), position your pointer

on top of the horizontal ruler, click and drag a guide half way down your

image.

10. Click on the Rectangular Marquee tool (top left on toolbox – if the oval

marquee tool is showing, click and hold down on it and then select the

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rectangular version) and use it to select the top half of your image. The

bottom of your marquee should snap to the halfway mark guide.

11. From the Image menu select Adjustments then Posterise (second from

the bottom of the list). Make sure that Preview is ticked and type some

different numbers in the Levels box to observe their effect. Lastly type in

9 and click OK.

The tonal range of the top section of the gradient is reduced from 256 levels to

just nine. You will now adjust the tonal characteristics using Levels and Curves

and it will be easier to identify and measure the changes than with the portion of

the image that has retained 256 levels.

12. With your Marquee selection still active, go to the Layers Palette and

click on the Create new fill or adjustment layer icon (fourth to the right

on the bottom of the palette) and from the pop up menu select Levels.

The levels graph shows nine very high peaks to indicate the large number

of pixels present in the nine tones created for the step wedge. Drag the

Imput Highlight and Shadow Sliders (black and white triangles on

bottom of graph) towards the centre to see the effect on the highlight and

shadow tones of the step wedge.

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13. Slide the middle slider to various positions. Observe the effects on the

midtones, shadows and highlights.

14. Drag the Output Sliders (at the bottom of the dialogue box) and observe

the effects on the highlight and shadow tones. Output Sliders reduce the

contrast of the image whilst Input Sliders increase the contrast.

15. From the Window menu select Show Info. Move your cursor (it appears

like a pointer tool) to the centre midtone and note the levels in the RGB

info (127). This is the digital midtone if all three channels are 127.

16. Press the Cancel button to close the Levels dialogue box.

17. Press Control + D to deselect the Marquee selection.

18. Once again use the Create new fill or adjustment layer icon on the

Layers Palette to bring up the Levels dialogue box, then experiment with

moving the different sliders. Notice how including the 256 tones of the

bottom half of the image has effected the graph. Click the Cancel button.

19. From the Image menu select Mode then Grayscale. A dialogue box will

appear asking Discard Colour Information?, click OK. Move your cursor to

the centre midtone again. Note how a tone sampled in a greyscale file will

give only a percentage black reading (shown as K) in the information

palette and no specific information regarding its precise level.

20. Press Control + Z to return to undo the Mode change and return to RGB

mode.

21. You can keep this image open and use it for the next exercise

Experimenting with Curves, if you do this you can start the curves

exercise at step 10.

The levels graph is similar to a bar graph and shows the relative number pixels

for each of the 256 levels. The sliders beneath the levels can be used to modify

the shadow, midtone and highlight tones. The white and black eyedropper tools

can be used to set white and black points within the image, e.g. click on the

darkest tone within the image with the black eyedropper to move it to level 0. The

grey eye dropper tool in the centre is used to remove casts from selected tones in

colour images.