Using Levels

The Levels tool

Most image editors have a levels tool. It's probably one of the quickest, easiest and simplest tools to use to make general image corrections. It combines a Histogram view with adjusters that allow the image parameters to be altered. Here is the Photoshop version.

At the very top is a drop down box that allows the user to choose which colour channel to alter. The default is RGB which is all three, Red, Green and Blue. When this is used the relationship between the colour channels remains fixed, they are linked and stay in step with each other. But if you wish you can choose to alter them individually, by picking them from the drop down menu. When this is done then the image colours change as it is the amount each colour channel produces which determines the overall colours that the tonal values in the image have. 

Below this are the input level boxes that indicate where their respective sliders have been set. These are called the input levels because the sliders concerned control how much of the image's tonal information is used as input.

Three triangular sliders are provided with which to make these alterations. The dark slider at the left controls the limit of blackness, whilst the slider at the right the limit of whiteness. In 8bit images where the number of tonal values that can be described is 256 - 2^8 -  the black point has a value of 0 and the white point a value of 255. Moving these sliders controls where the image dynamic range input stops, in other words how much highlight and shadow detail in the image is used.

The middle slider controls the brightness of the image, the luminosity, and this value is on a scale that uses 1 as the mid point and rises when moved to the left, and falls to the right. The value of 9.99 is the maximum and 0.10 the minimum. Moving this lightens or darkens the image without altering the overall levels. As a general rule movement of the sliders is not that great as just a small one can often result in a big change.

At the bottom is another set of boxes and sliders. These control the output. How much of the image information gathered is used. There are just two sliders here. Which respectively control the limit of black and white tonal output. Altering these changes the overall tonal range produced, the limit of blackness and whiteness.

There are also some dialog boxes down the side along with some tonal pickers. The dialog boxes are really self explanatory, the 'auto' box invokes the auto levels procedure. This works okay with some images under certain conditions, but is a bit like giving control to someone else. The result is often not what you want. The tonal pickers can be used to automatically set either the mid-grey, black or white point limits. Again the results can often be not what you want. 

For this particular exercise we will stick with using the levels manually, in RGB channel mode, and with maximum output levels. This is how the levels dialog usually pops up when invoked - Ctrl+ L is the keyboard shortcut in Photoshop.

Making Image alterations 

If it is the case that histograms may look totally different but the image can be quite correct, see  Histograms , what is the point and usefulness of a histogram? Well, that very much depends on your experience and abilities, but basically it's main use is indicating where changes can be made when it is already clear that an image is wrong. Either where highlights have burnt out, shadow detail has been lost, or it's just under or over exposed.

To be perfectly honest, any experienced photographer will already know when an image or scene is wrong or is going to be difficult to shoot correctly, and will know what to do, or what options they have. Quite often it's a case of having to make a choice when it's not possible to cover and expose every tone in an image correctly, by for example spot metering on a light coloured flower to expose and preserve the detail in it, and allow it's darker surroundings to under-expose. 

This is often the case with scene's where some sunlight is present, or there is strong directional lighting of some kind involved. One of the problems with digital camera sensors at present is that they don't have as wide a dynamic range as film, and more importantly electronic sensors respond differently to that of film, so there are greater problems in high contrast situations, where either highlights burn out, or shadow detail blocks in. In these cases, as we are dealing with files that are positive images, rather than negative, it's always preferable to underexpose to preserve highlights, as generally shadow detail can be 'pulled out' later in a photo editing application, yet burnt out or 'blown' highlights cannot be restored. 

Please refer to  Image Exposure  and  Dynamic Range Assessment  for more details.

An Example

Here is a shot of a snowdrop. It was taken in program mode using multi-zone metering. The background has been correctly exposed but the petals of the snowdrop are over-exposed and the detail has been lost. The highlights are 'blown' as you can see in the close up crop. This is because the metering 'averages' the scene, and the flower petals take up a relatively small proportion of the whole image. If the image shot had been as the crop on the right, then in all probability the exposure would have altered and not been as great, and less detail lost.

                                             full size                                                       cropped

Exposure: 1/1000 sec @ F3.5 ISO200. Multi-zone metering

Here is the histogram for the image. If you look closely at the left side you can see that all the dark tones have been captured as they finish before the edge is reached.

Making Exposure Choices

So a choice has to be made. Which is more important, the flower petals or the background? If the answer is the flower, what is the answer to the problem? Experience tells us, and is backed up by the histogram, that less exposure is needed. There are several ways of achieving this, depending on what options the camera you are using has. Exposure compensation is one way. Most cameras have this ability. Reducing the exposure in 0.5 or 1.0 steps until the correct exposure is found is not difficult with a digital camera, and costs nothing except your time and effort. A better and quicker way, is to change the metering so the flower is the main focus. Depending on how much of the image is occupied by the flower, using either centre-weighted or spot-metering should give a better exposure. 

Because the flower was small the next shot used spot metering, the reading being taken off the petals.

                                             full size                                                     cropped

Exposure: 1/4000 sec @ F3.5. ISO200. Spot metering.

Here's the histogram for the image. As you can see by looking at the image and the histogram, all the light tones have been captured and preserved, but the result is that the dark tones have now been squashed and detail in the shadows lost. The histogram also indicates that as the light tones finished well before the edge, the exposure could have been longer and therefore made the general scene and the background lighter. 

However, this can be corrected in Photoshop or any similar image editing application using the 'levels' command as we will now show.

Making Alterations in Photoshop Levels.

To start with we moved the white slider left from 255 until it reached where the light tones started at 206. This increased contrast to the limit that could be used before highlights became affected, i.e. blown. Then we moved the mid-tone point to 1.58 which lightened the overall image background to a point we felt happy with. 

Here is the result. If you compare it with the overexposed shot you will see that the background exposure is roughly the same, but the flower petals are no longer blown and detail is present.

And here's the histogram that results.

The tonal range has been adjusted so it runs from 0-255. There are no blown highlights and no dark areas blocked in.

Taking the Correct Exposure

Now here is a correctly exposed shot of the snowdrop. This was taken using centre-weighted metering and in this instance has produced the best image of the three metering patterns tried.

                                             full size                                                    cropped

Exposure: 1/2000 sec @ F2.8 ISO200. Centre-weighted metering.

And here is it's histogram. You will see that it is virtually identical to the corrected shot's histogram.

Altering Output levels

When it is not possible to take alternative exposures, or you find out later after taking a shot that highlight detail has burnt out or a portion of sky is glary there is another alteration using levels that can be made. Whilst it doesn't recover lost detail it can help in some cases to reduce the impact over-bright and glary tones can have on an image. And this is by making changes to the image output levels, the limit of blackness, or more usually the limit of whiteness. Careful use is needed not to over-do the effect obtained because image contrast is reduced as a consequence. But it is a useful alternative option when other alterations don't have quite the required effect.

Here are three more shots of the snowdrop. At the top left is the original over-exposed image, at the right the correctly exposed one. And below these is the over-exposed shot changed by reducing the image whiteness output limit.

and here is the levels histogram dialog showing where the output was set at to obtain this result.

The tonal range has been reduced so it's limited to 0-220. The contrast has reduced as a result, more subdued or 'flat' looking, less richness between the tones, but still preferable to the original blown highlights version.

Being able to correct or 'save' shots that are not right is one of the benefits of digital imaging, although getting the shot correctly exposed in the first place is the best option if it's possible. If not then using a histogram in conjunction with the image, as in levels, to understand what's wrong and where to alter it to put things right, is of great benefit.