All keyboard shortcuts are for PC. I don't have a Mac, but as far as I know the main difference is use of the COMMAND key and OPTION key on a Mac instead of the Ctrl key and the Alt key on a PC.
This is the photo to be restored. It is slightly damaged; the contrast is very poor. In fact, there is a sunny day - but you can hardly guess it looking at the unretouched photo.
I tried to keep old photo look and therefore decided not to remove all the scratches and imperfections. Besides, I added a little color to increase the effect.
Move you mouse over the image to see restoration result.
Step 1. Scanning the photo.
Never ever scan grayscale photos in grayscale scanner mode!
Though the image may have no color, ALWAYS scan in RGB. That may depend on the particular scanner, but usually grayscale-scanned images are much more noisy.
Just move your mouse over the image to compare amount of digital noise when scanning in grayscale vs scanning in RGB.
Scanning resolution is not so critical. Usually 300 dpi is enough.
Step 3. Remove scratches and correct damaged areas.
First of all use Filter - Noise - Dust and Scratches to remove most little scratches. I used radius of 4 pixels and 20 levels for threshold.
Dust and Scratches filter settings are very individual and vary greatly depending on the photo. For your own old photo retouching first choose the smallest radius that removes most scratches and then adjust threshold to remove artifacts.
Then use Patch tool and Healing Brush tool to work out more damaged areas. Press J and Shift-J to switch between the tools.
Step 4. Sharpen the image.
Great tool for sharpening (and most used one) is Unsharp Mask, or USM.
I used it twice. First with large radius and small amount for increasing overall image contrast (10%, 60 pixels). Then with small radius and considerable amount to increase local contrast (40%, 1.6 pixels).
Move your mouse over the image to compare "before" and "after" sharpening.
Step 5. Adjust Shadows/Highlights.
Shadow/Highlight is very powerful tool for quick correction of darker image areas. I used it to emphasize "sunny" feeling. The most important Shadow/Highlight parameter is Shadows amount (I used 15% for this image). You may also experiment with Tonal Width amount.
Step 6. Add little color.
Press Ctrl-U for Hue/Saturation adjustment. I checked "Colorize" and set Hue to 40 and Saturation to 10.
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