Photoshop Levels

After opening a photograph in Photoshop, click on Image, then Adjustments, then Levels

Tone Scale A box will appear - this is the levels Histogram. The black ‘mountain-like’ areas show how much of the image there is at each tone, going from pure black to pure white of the Tone Scale . The taller the ‘mountain peak’, the greater the area of image at that particular tone.

This image has lots of dark tone areas Tone Scale The image does not have many grey mid-tone areas The image has lots of light grey areas

The lightest areas in the image are here on the tone scale The lightest areas in the image are here on the tone scale. This is not white but a light grey tone.

This histogram is for an image of the grey square and shows a peak around the middle of the tone scale because the square consists of just this one tone.

In this histogram for a black square, where is all the information on the histogram?

In this histogram for a white square, where is all the image information on the tone scale?

This is the histogram for the striped square This is the histogram for the striped square. There are peaks for pure black, grey and pure white

The histogram has three small triangular markers – one to show where pure black tone is, one in the middle for midtone (grey) and one on the right to show the position of pure white.

You can use your mouse to click and ‘grab’ any of the triangular markers to slide it left or right. This will change the appearance of the image.

Imported Image. Note position of darkest tone in the image – this is not pure black.

Moving the slider has made the darkest tone in the image pure black, so that the tonal range of the image is from black to white rather than dark grey to white. The black tone level has been dragged right until it is in line with the darkest tone in the image.

Imported Image Image after Levels Adjusted

Imported Image Image after highlights adjusted Although adjusting the levels has slightly improved by making the highlights lighter, the image is still not good & shows how important it is to get the exposure correct!

A correctly exposed image of a scene containing a wide range of tones. Levels histogram for the image as it was imported from the camera. No adjustment to levels is required.

Adjusting the levels can dramatically change the look of an image. Imported image Note how the histogram indicates that most of the image is white or near white! Levels adjusted

Levels Histogram and Exposure Viewing the histogram on your camera screen can be a useful way of checking exposure, especially when it is sunny and you cannot see the screen clearly. This photograph was over-exposed. The histogram shows a large section extending beyond the white end of the tone scale.

Correct exposure will record detail in both highlights and shadow areas. When an image is correctly exposed the histogram will Not have large areas going outside the end points on the tone scale, as in the example above.

A word of caution ! This photograph was deliberately under-exposed to give a semi-silhouette look in which shadows lose detail. Although the levels histogram looks wrong, in this case it was what the photographer wanted in order to achieve this effect.

Summary Using the levels adjustment can improve the image quality. Using levels adjustment as a creative tool can radically alter the look of an image. All images should be checked for levels immediately after importing. Any ‘blank space’ at either end of the tone scale should be removed by adjusting the highlight and shadow sliders.

For Maths Whizzes Only … (Normal boring humans please skip) Jpeg digital images are recorded in ‘8 bit’ code. Computers use Binary Arithmetic (0’s and 1’s) An 8 bit binary number means numbers that are 8 digits long, using 0’s and 1’s, e.g. ‘10101001’. Since binary arithmetic uses a base of 2, the maximum number of an 8 bit binary number is 11111111 This equals 1+2+4+8+16+32+64+128 = 255 (in normal base 10) You may have noticed that the photoshop levels panel includes this number (255). This is because an 8 bit image can only have a range of 255 different tones, from 0 to 255. [For large top quality prints professionals use TIFF files which can record data with 16 bit numbers and therefore a finer tonal range].