The focus remains the same in this third part of our introduction to digital photography - don't be put off; the Levels tool is seriously useful and surprisingly easy too
In part 1 and part 2 of our Digital Photography 101 series, we discovered how the look of an image can vary considerably, and how to interpret these differences by using histograms. Today, I'm going to show how a image histogram plays another very useful role; to help adjust an image and make it look better. To do this, we're going to explore the use of a very useful and fairly easy to use tool called a Levels adjuster, or plain 'Levels', for short. further down this page we have four short video clips that show you how these tools work in a practical sense.
Levels can be very simple, but they can be used in a more powerful and complex fashion too. As this series of articles aims at expanding your digital imaging knowledge in simple bit-size chunks, we'll start with the basics and the three tone adjusters (1, 2, and 3 in the illustration below:
Levels tool (from Photoshop CS4, but despite minor differences in layout, this tool is present in many other imaged editing software packages).
Item 1 (above) is the black point adjuster. By moving this you are changing the point at which the darkest pixels in your image will (or should) be. If the image starts off with no true blacks, only various shades of grey, then you could move the black point to a new position that still doesn't connect with the darkest tones in your image. However, the mid tone point (2) will also be moved if you adjust either the black point (1) or the white point (3). This is because the mid point stays in the same point relative to the black point and white point.
Item 2 (above) is the mid point adjuster. More technically, it is the gamma adjuster. Gamma is a concept that we will cover in the near future, but for now just think of it as the mid point or mid tone adjuster. Move this left or right and it pulls tones from the side you are moving into across towards the centre. So move it left in to the darker tones and you are actually pulling darker areas of the image into the brighter centre of the tonal range. Move to the right and you will darken lighter tones by pulling them left towards the darker end of the tonal range. The effect is most pronounced at the centre of the range, gradually decreasing towards the end points (1 and 2), and the actual end points remain unchanged.
Item 3 (above) is the white point adjuster. It's just like the black point adjuster, but in reverse. It marks the end of the image tonal range where pixels become totally white. Move it left and you will tend to darken brighter tones. Just like the black point adjuster, it also shifts the mid tone point towards the darker side of the tonal range because the mid tone point remains in the same position relative to the two end points.
If you are not totally comfortable with the above, never mind. With luck, the following video clips will make the theory come alive and it will all start to make sense.
Using levels to brighten an under-exposed image
In this clip I briefly review the three adjustments we're concentrating on from the Levels tool. I then work to brighten up the image by reconnecting the white point with the lightest tones in the image, then moving the mid-tone to stretch the darker areas across to the right so that they find a new home in the brighter mid-tones. The end result is a rescued image that has near normal brightness.
Reducing brightness
In the second video, we're starting off with a picture that is too bright. The pattern of the stones in the tower is almost lost because the image is too light. Luckily, by examining the histogram we can see that the image isn't 'burned' out - not much of the highlight, or brightest tones, have been lost. Instead, what has happened is that they, and much of the mid tones, have been concentrated to the far right of the histogram. All we need to do is use the Levels mid-tone adjuster to bring the lighter tones back to the darker mid tone area. In fact, there is a school of thought that says setting your camera to expose so that most of the tones are recorded 'to the right' of the histogram is a good idea as cameras are more sensitive in this area, so more good detail is recorded and less noise (graininess) results.The main precaution is to ensure no highlights are burned out as a result; the right hand foot of the histogram has to stop short of the very right hand wall of the tonal range.
Uncompressing dark and light tones
In our third video clip, I'm going to use Levels to cure an image of a lifeless, flat, and grey malaise. Here, the histogram stops well short of both the black point and the white point. Without any good deep dark tones, nor any dynamic bright tones, the image consists only of mid tones. This means there is a general lack of contrast and bite. But in the video I reconnect the black and white points with their respective sides of the image histogram, and this stretches the dark and light tones across the full range, making the darker and lighter, respectively. The mid tone stays where it is, but the tones in the image either side are either darkened or lightened. The shadows now have depth, the highlights are cleaner and brighter, and there is mode contrast in the mid tones.
Finally, there are some limits to what Levels can achieve
We round off this article with a look at an image that is not going to benefit much from Levels adjustment. Although the histogram has a neat peak in the centre of the tonal range, which would normally indicate perfect exposure, the problem is that both ends of the histogram indicate 'clipping' - or where there is a sudden end to the slope of the histogram as it reaches the extreme side of the range. In the example below the worst damage is done on the right hand side, the white point. A crucial level the image's pixels are completely white and if you look at the image you can see these burned out pixels make up significant areas of the flower petals. Once a pixel has reach total whiteness, it basically represents no useful detail any more and so can't be adjusted, or darkened. It's the same if an area of the image is clipped at the dark end of the scale.
If you have a good image editing package, like Photoshop, or Paint Shop Pro, etc., why not try to use the techniques covered above with some of your own images? If you get some really good before and after results, let us know, or even better, post them on the DPNow forum.
Reader feedback:
Discuss this story:
Digital Photography 101 - the Levels tool is easier than you might think
