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Basic image adjustments tutorial-

Basic image adjustments tutorial-
Levels and histograms



The histogram on the back of your camera is one of the most useful but under-used features on a modern digital slr camera. I am surprised that many seasoned photographers don't seem to understand it - or it's benefit. It all looks a bit technical, so many shy away from using it. In actual fact it is your best ally in getting correct exposures in your pictures.By the end of this tutorial you will love it - trust me!

You will also come across the histogram again in Photoshop - both in the RAW converter and in the Levels dialogue box - so there is no escaping it - you are going to have to come to grips with these mysterious looking graphs! But don't worry - they are not half as bad as they look

If you have shot your images in RAW, then you will have already made your basic image adjustments in the raw converter software as described in the "art of raw conversion" tutorial , you will already have come across the histogram and used it to adjust your image's tonal values. What follows is useful for subsequently tweaking your brightness/contrast and colour balance to make your pictures really "pop".

If you shot the images in jpeg format, then the following is an essential basic procedure as your images will look dull, washed out and lacking contrast and impact without it. Additionally, your images may have a colour cast if you did not get the white balance spot-on at the time of shooting.

Adobe Photoshop's Levels command lets you correct the tonal range and colour balance of an image by adjusting intensity levels of the image's shadows, midtones, and highlights. In this tutorial, you'll learn how easy it is to do in a simple three step process.



The levels dialogue box, first look at the histogram and how to use it to adjust an image to look much punchier.
1. Open your image in Photoshop.
Choose File > Open or double-click a thumbnail in the File Browser in Photoshop CS or Bridge in CS2.. Before you start making adjustments, evaluate the image tonality. Is it too light or dark? Does it have too much or too little contrast? Is there a color cast?

2. Create a new Levels adjustment layer.
Choose Layer > New Adjustment Layer > Levels, and then click OK in the New Layer dialog box. You can also open the Levels command by choosing Image > Adjustments > Levels. However, with an adjustment layer, you are applying the tonal correction on a separate layer. The original image is untouched. If you decide you don't like your changes, you can change them at any time or simply discard the adjustment layer and return to your original image.


Enter the histogram....

A very useful tool for evaluating an image's tonality is the histogram displayed in the Levels dialog box. A histogram illustrates graphically how the pixels which go to make up an image are distributed, by mapping the number of pixels at each color intensity level. In other words it shows you the distribution of pixels in the image in graphic form. The shadows are shown in the left part of the histogram, the midtones are shown in the middle, and the highlights are shown in the right part of the graph.

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In this image of a mute swan you can see the distribution of pixels in the histogram - those to the extreme left represent the very small number of pixels that are very dark (the black on knob of the swan's beak for example). The bulk of the pixels are in the midtones (which represent the water and reeds). The area to the extreme right represents the highlights - and notice that they are bunched right up to the right of the graph and beyond. You may have noticed that there is also a tall spike at the extreme right of the box.. This is indicative of over-exposure, the highlights are "blown or clipped " If you look at the swan itself, you can see that this area corresponds to the wing and tail - which has burnt out to pure white and it is not possible to distinguish the feathers - not good.

Now look at the shadow part of the graph. It is not clipped at this exposure level, so in this example, it would have been better to have underexposed the shot by around half to one f-stop at the time of shooting it in order to retain the missing highlight detail. This would have driven the graph to the left - at the expense of the shadow detail. If we overdid it, the shadows would then start to clip !

This scene of the swan is very "contrasty" as it is in full sunshine - i.e. has a very large dynamic range. That is to say, the blacks are very black and the whites are very white and shadows are strong and harsh under such conditions.

A bright-overcast day offers less contrast than full sunshine and is often preferable for photographing white subjects - where your aim is probably to retain detail in the shadow and highlight extremes. On a really grey day, the contrast is lower still - blacks look greyer and highlights also look greyer, so it becomes easy to capture the full dynamic range of the scene, but as grey days offer very "flat" lighting with few shadows, pictures tend to look bland and unexciting.

Another way of lowering contrast is to use a reflector or fill-flash to lighten the shadows - but this would not have been very practical with our swan example.

You will quite often find that you will come across scenes which have a greater tonal range (dynamic range) than the camera can handle - both shadows and highlights are clipping. Under these circumstances you will have to decide which are more important to your image and expose accordingly. It is possible to combine over and under-exposed images together in Photoshop to produce a high-dynamic range (HDR image) but this is beyond the scope of this tutorial.

If your camera is a digital slr, you should be able to check the histogram on the back of the camera immediately after taking the shot. Look for "clipped" highlights or shadows and adjust the exposure accordingly and retake the shot if necessary. Some cameras have a setting where you can set the highlights to blink in the areas of over-exposure.


The correctly exposed shot

The next image of a collared dove is perfectly exposed - no clipped highlights or shadows, it was easy for the camera to get this right as the scene is predominantly mid-tones with no extreme highlights or shadows to cause problems of clipping. The full "dynamic range" of the scene has been captured. Even so, we can improve the look of this image and give it far more "punch" by adjusting the levels a bit. So let's pep it up:

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Adjusting the levels by setting the shadow and highlight sliders.


1.0 Move the Black Point Input slider (located directly beneath the histogram) inward from the edges of the histogram. Moving the Black Point Input slider maps all image values at its position or below to the Output Levels black point (set by default to 0, or pure black). This will blacken up the shadows and add more of a contrasty feel to your image, but don't overdo it - as you will losing shadow detail in the process.

2.0 Similarly, moving the White Point Input slider maps image values at its position or above to the Output Levels white point (set by default to 255, or pure white). For example, if your image is too dark, move the Input White Point slider to the left. This maps more values in the image to 255 (the Output Levels white point), making them lighter. If there are important highlight details in this part of the histogram - such as subtle detail in the feathers of a swan, you will loose them - as you are making these values pure white, so be careful. In this example of the dove, the pale areas around the bird's eye and chin will be sacrificed just a little to give the image a little lift.

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A good starting point for many images, is to adjust the sliders up to where the histogram begins to rise steeply at each end as I have done above example. Compare the two images of the dove and you will see how washed-out the first image looks compared to the adjusted one.
As you move the highlight slider you will notice that the middle (midtone) slider adjusts itself automatically - which is useful.

3.0. Adjust the midtones in the same way by moving the middle input slider to the left to lighten the image or to the right to darken the image in the mid-tones. The middle input slider actually adjusts the image gamma. It moves the midtone (level 128) and changes the intensity values of the middle range of grey tones without dramatically altering the highlights and shadows.

That's all there is to it. When finished click OK to apply the levels adjustments - job done.


4.0. Removing color casts within levels.

Select the Set Grey Point Eyedropper tool in the Levels dialog box. Click an area in the image that contains only a grey tone, or an area containing as few colors as possible. It's easier to color balance an image by first identifying an area that should be neutral and then removing the color cast from that area. With such a correction, all other colors in the image should be color balanced, too. The eyedropper tools work best on an image with easily identified neutrals.

If there are no easily identified neutrals in your image, the Levels command may also be used to adjust individual colour channels (red, green or blue) to remove a color cast. Choose a channel from the Channel menu of the Levels dialogue box and then adjust the Input sliders for either red, green or blue.Many images with a blue sky in the picture benefit from moving the blue channel slider a little to the left in the highlights - it really perks up the sky - try it for yourself.


The under-exposed shot

This image of a nuthatch was purposely under-exposed to illustrate a point. (Well that's my excuse and I am sticking to it) :

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See how the shadows are clipped off at the left hand side of the graph ? The mid-tones are bunched up to the left, and the highlights are most definitely not clipped - they seem to drag on forever along the baseline.

Can the levels adjusters deal with this situation and retrieve the image ? The answer is essentially yes, but there is a price to pay in terms of " noise". Digital noise is the unsightly equivalent of film grain and is usually concealed within the dark areas of the image. If we open up the shadows and midtones using the levels sliders, this will expose the noise to the detriment of the image. If the image was really precious to us, there are several good noise-reduction filters available (such as Neat Image, Noise Ninja and Photoshop's own noise filter) which will help a lot in reducing noise without loosing too much detail and sharpness in the image.

To demonstrate this noise, here is the adjusted image of the Nuthatch at 200% so you can see the noise in the red-circled area .I have purposely over-sharpened the image a bit to show it more clearly here.

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Below is the equivalent from the correctly exposed image. Notice how the noise in the circled area is much lower. It is difficult for me to illustrate this very clearly on this website as the images are such low resolution here, but hopefully you can get the general idea. The examples are using images shot at low ISO (200). If they had been shot at high ISO in order to gain shutter-speed on a dull day the noise would have been much worse.

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More histogram examples

Let's look at a few more histograms and endeavor to interpret what they are telling us:

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Here is an easy image for the camera to get right - it is predominantly midtones (hence the huge mid-tone peak) - all the highlights are captured without clipping. There is a bit of clipping in the extreme shadow area to the left of the histogram - but this only represents the few black shadows between the planks which I am more than happy to allow to go to 100% black as they are non-critical and it even gives the image a bit of punch.


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This is an image of a Puffin. The histogram looks very different to the last example. The three peaks to the right of the histogram represent the white on the bird, the sky, and I am guessing here - the pale coloured rocks. Notice that the three peaks are taller than the box ? This simply means that they go outside the scale of the box - they are not actually missing or clipped - and this is nothing to worry about.

The image looks a little bright at first, but the histogram tells us that we have (just) captured the full dynamic range of the image. I would simply adjust the midtone slider a little to darken the image - which will minimise noise and make a great picture. This is a good example of "exposing to the right " in action - click here to see the tutorial on exposing to the right.

This example also illustrates that histograms can look very different according to the scene - sometimes smooth and rounded - sometimes spiky like this one. There is not a right and a wrong as such - histograms just are !


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This is a night image of Tower Bridge in London. At a first glance at the histogram, it looks under-exposed.

The image has a lot of dark night sky which has clipped to black, but I like the contrast that gives to this image - so that is fine by me. The midtones are mainly to the left of the image - which would normally suggest under-exposure, but look at the highlights - which are important in this picture - they too are starting to clip at the extreme end (the lights on the top of the building). I don't mind them clipping - as I am happy with them being pure white on this occasion. What I don't want is the light on tower bridge itself to clip - which it isn't . So despite the initial appearance of this image being under-exposed, it is not, at least it is not within the concept of what I am trying to achieve with the image.

Now let's say that I wanted to show a bit more light in the sky rather than the pure black that it is. Then I would say that the image is under-exposed! I would have needed to have shot the image with a little positive exposure compensation (say plus 2/3 of an f-stop). As I didn't I could now try opening up the shadows with the midtone slider, but I would soon start to expose unacceptable levels of noise in the image. So which is it - underexposed or not underexposed ? The answer depends entirely on what you are trying to achieve. Histograms are not always showing you right and wrong - they just showing you how it is - and you have to decide if it's right for you - this is supposed to be art after all !


Hopefully this tutorial has given you an understanding of the histogram and how you can harness it's power to make better pictures

Good luck !


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