Levels Adjustment
This feature allows you to adjust the brightness levels of an image. It is best performed as an Adjustment Layer as this allows retrospective changes, provides a mask for selective alterations and doesn’t make permanent changes to the image pixels. The Levels dialogue box shows a histogram of the average brightness throughout an image. A colour image doesn’t actually contain coloured pixels. It has a range of brightness values for each of red, green and blue from zero (no colour) to 255 (full colour).
The histogram that appears in the dialogue box has a range across the horizontal axis from zero to 255 and the vertical axis shows the Input Levels ie the average number of pixels (red, green and blue) which fall into each brightness value. You can select the individual channels from the drop-down box to see the specific contribution of each of the three colours.
The image above has a well balanced exposure showing a full range from black to white.
On the other hand, this image is under-exposed, probably because the camera was fooled by the window in the centre. You can just see the window piled up at the right hand side of the histogram. To recover this image, drag the small triangle at the right hand end across to the left until it just touches the main body of the histogram (see next page) and drag the centre triangle of the three to left or right to adjust the overall balance. (Other images may want adjustment to the shadows end and others perhaps to both.)
This has improved the range, but has unfortunately, largely burnt out the window. However, this can be substantially improved by painting out the adjustment to the window using a black brush on the Adjustment Layer Mask. See Note on Layer Masks.
This is still not a very satisfactory image, but it’s a big improvement on the original. I should really have used a tripod, taken separate shots exposed for both areas and blended them!