Correct exposure problems and red eye

To add contrast to a photo
If the image needs overall contrast because it doesn’t use the full tonal range,
choose Image > Adjustments > Levels. Then drag the Shadow and Highlight
input sliders inward until they touch the ends of the histogram.
Adjustment layers
An adjustment layer applies color and tonal adjustments to your image without
permanently changing pixel values. For example, rather than making a Levels or
Curves adjustment directly on your image, you can create a Levels or Curves adjustment
layer. The color and tone adjustments are stored in the adjustment layer and apply to all the layers below it.
Adjustment layers provide the following advantages:
1. Nondestructive edits. You can try different settings and re-edit the adjustment
layer at any time. You can also reduce the effect of the adjustment by lowering
the opacity of the adjustment layer.
2. Reduced loss of image data through combined multiple adjustments. Each
time you adjust pixel values directly, you lose some image data. You can use
multiple adjustment layers and make small adjustments. Photoshop combines all
the adjustments before it applies them to the image.
3. Selective editing. Paint on the adjustment layer’s image mask to apply an adjustment to part of an image. Later you can control which parts of the image are
Photoshop Photo Manipulation adjusted by re-editing the layer mask. You can vary the adjustment by painting on the mask with different tones of gray.
4. Ability to apply adjustments to multiple images. Copy and paste adjustment layers between images to apply the same color and tone adjustments.
5. Adjustment layers increase the image’s file size, though no more than other
layers. If you are working with many layers, you may want to reduce file size by
merging the adjustment layers into the pixel content layers. Adjustment layers
have many of the same characteristics as other layers. You can adjust their opacity
and blending mode, and you can group them to apply the adjustment to specific
layers. You can turn their visibility on and off to apply their effect or to preview
the effect.
To create an adjustment layer
Do one of the following:
• Click the New Adjustment Layer button at the bottom of the Layers
palette, and choose a layer type.
• Choose Layer > New Adjustment Layer, and choose an option from the
submenu. Then name the layer, set other layer options, and click OK.