Some cameras will allow you to see a histogram of the shot you have taken, this can be very useful as it will indicate whether or not you have blown highlights or have an over-emphasis in the dark areas, both leading to no information being recorded on the sensor. If the information/detail is in the information then it can be brought out.

This picture you can see is too dark. Travelling on a boat in Venice there is no time to adjust the settings of the camera as you are moving.

To adjust the picture (post-processing) the histogram is invaluable.

Here you can see the histogram for this image. The black 'mountains' are representing pixels and you can see that there are more pixels at one end (left), and none at all at the other end (right). At the bottom of the graph there are three small arrows, on the left a black arrow, in the middle a grey arrow and on the right a white arrow. These control the amount of light and dark in the picture. Moving any of the arrows - white to the left, black to the right, and grey either way makes the picture lighter or darker. So to make this image lighter I need to move the white arrow to the left.

You can see that I have moved the white arrow to the point where the histogram started to register pixels and immediately the picture becomes lighter. (Please note that there are two layers and I am making changes on the 'copy' layer. This enables me to keep the original so that any errors I make I can start again with no problems.)

Although the picture is lighter it is somewhat 'flat'. In Photoshop that flatness can be adjusted too, clicking on the box at the top which says 'Custom' reveals a menu which can increase contrast etc.

I think you can just about read the choices that are there - these will help to bring out the colours more. There are also 3 little 'eyedroppers' at the right hand side. Again Left is black, centre is grey, and right is white. Click on the white eyedropper and then click the eyedropper on a point of pure white in the image (Grey goes on grey and black on black) - I have chosen white and am using the vest on the washing line.

This as you can see has brought the colour and detail back into the picture. Still many things wrong with it. I will continue more adjustments in another tutorial. You will notice that the histogram has become spread out so that the pixels now go from black to white but with gaps inbetween. Working on the Histogram in camera prevents this happening and will result in a better photo as there will be more information recorded on the sensor. Ideally the pixel cover should be from black to white, but there are occasions when it may not be possible or even impossible.

Below is a photo I took in Moonlight with its colour histogram (the 'Light' Histogram would be the same shape)

Adjusting the Histogram of this would lose the 'moonlit' effect.

Most software has a histogram adjustment section. You will probably find it under 'Levels' - which refers to the light levels in the photo. In photoshop it is 'Image' > 'Adjustments' > 'Levels'.

To adjust the Histogram in camera is usually a case of adjusting the amount of light entering the camera. This can be done by adjusting the shutter - quicker or slower, and by using the Aperture - making that bigger or smaller. Experiment with your own camera and software and I hope you will be delighted with the transformations. ISO also has an effect on light but I inderstand this is more like an amplifier and as a general rule ISO is better kept as low as possible. Exact amounts depend on the camera and the setting.

I apologise if there are errors in this information - but hope it helps some folk anyway.