In the pre-digital days adjustments to an image were the responsibility of the laboratory: they decided if improvements could be made and how. The trend now is for the photographer to retain control and make the adjustments before ordering the print so that the lab cannot misinterpret the style of the required print. That sounds an ideal solution but at The Darkroom we often find ourselves offering advice on how to make those image adjustments. The first recommendation we make is to save the original version, work on a copy file and to ensure that any changes made do not adversely affect the print quality.

One of the basic adjustments that can be made is to 'Levels' or alternatively Brightness / Contrast / Curves / Exposure, which are all tools which can perform much the same task. Expert practitioners of Photoshop will be familiar with these techniques, but you do not need a degree in computer science to make simple improvements. This article illustrates how to identify the key components of a histogram and how to avoid the heffalump traps!

The first set of pictures are made from normal print film, in this case Fuji Superia 200 iso on a rather damp November day in a Cheltenham park. It may seem odd to start with a non-digital source but put into historical context, this is where image adjustment started, so it makes sense to begin here. The first sequence shows different film exposures: under exposed, over exposed and correctly exposed ( not enough light, too much and just right ). The fourth image and its associated histogram, shows the final adjusted result.


under exposed image
3 stops underexposed film image with histogram

Under exposed film The image is too dark with little shadow detail and the inset graphic plots how the different tones have been recorded by the film. The histogram can be read from left to right, from dark to light and the high peak at the left hand margin tells us there are large areas of dark tone in the image. That makes sense: it is a dark picture. There is also a spike at the right hand side that corresponds to the patch of bright sky.

ver exposed imagealt text
3 stops overexposed film image with histogram

Over exposed film The histogram shows that the distribution of the tones has shifted to the right, the light end of the scale. This time the spike on the right hand side is taller suggesting there are areas of the print that are pure white, with no detail. ( The underexposed image recorded some blue sky ) Conversely there are no tones recorded on the left hand side i.e. there are no black tones in the image.

correcty exposed image
Correctly exposed film image with histogram

Correctly exposed film It might look a little washed out at this stage but the tones are all grouped comfortably within the left and right margins of the histogram with excessive levels of any particular tone. The triangular sliders under the baseline can then be moved to adjust the tonal range and so improve the image.

correcty exposed image optimised
Corrected film image with histogram

The histogram after corrections Here the values in the histogram have been adjusted using the sliders and the resulting image looks much improved: instead of ghostly dark greys there are true blacks in the image and the intensity of colours is boosted up at the same time. The centre slider can also be moved to change the balance of mid tones.

The tone values of the histogram are evenly distributed, tailing off towards the left and right margins. This tells us there will be good detail retained in shadow and highlight areas of the print.

The next three examples were taken on our pocket sized Canon digital camera and we've reproduced the same effects of under, over and correct exposure and inset the histogram diagram.

under exposed digital
2.5 stops underexposed digital

The digital camera has struggled a little with underexposure: as expected the tone values in the histogram are bunched up on the left hand side but the slope down to the left is cut off by the margin. This means there will be less detail in the shadows and some areas in the picture will be pure black and this cannot be retrieved by adjusting the sliders.

over exposed digital
2.5 stops over exposed digital

The tones of overexposure are too light and instead of a gentle slope down the right the values are sliced off abruptly. This means parts of the print are pure white with no detail - no matter what is done with the levels adjustment those white tones can never be made to reveal detail. Cameras that record in RAW format can allow for some extra detail to be teased out of highlights or shadows this cannot always be relied on. It will always be better to take care with the initial exposure and then take a sensitive hand to adjustments in Levels. Overly enthusiastic adjustments in Levels will always lead to unnatural results.

over exposed digital
Correct exposure digital

A near perfect exposure and histogram! The slopes of the tonal pattern tail off perfectly to the left and right margins meaning that there are no unnatural blocks of pure black or pure white in the photograph. The overcast sky produces a naturally soft light without the extremes of tone so the little digital camera has been able to record the full range.

If you'd like to download these pictures and have a play with levels then click on the links to the individual files below: you'll then need to drag or copy them from your web browser to your desktop.

Film Underexposed | Overexposed | Correct

Digital Underexposed | Overexposed | Correct

What to Avoid

At The Darkroom UK Ltd we sometimes see digital images that have been treated a little roughly by adjustments to Levels. Typically, and most noticeably, it is the highlights that are affected. Below is an example of a good initial exposure followed by the result of inappropriate adjustment. Comparing the histogram shows how the blown-out highlights are represented: the graph does not tail off gently but is sliced off, leaving pure white where there was once detail.

Sample A

Sample B
wedding photo normal wedding photo overexposed

Section of original 17Mb file with text

Reduced to 9Mb file and enlarged to match

That's been a lengthy article but hopefully it has given some insight into Levels. A concluding piece of guidance from our side of the printing process is to save a copy of the original file before making adjustments: once the highlight details of a brides dress have been overly brightened there is no going back! Generally it is better to ask us to print with extra contrast, if that is what you want, than force the image to be contrasty at the expense of detail.

Alistair Baird
The Darkroom UK Ltd
01242 239031