Calibration of Photographic Monitors
Why don't my prints match what I see on my computer monitor
Part 1: Background and Basics
This is a multi-part series that will appear in the newsletter over the coming months.
It is a integral part of modern photography to view and edit our photographs on a computer monitor. The result of this process may be a photographic print, a web display, the projection of your image on a club night or a slide show on our iPad or the many other media that display our photographs. But are we always happy with the end result, are the colours the same as we saw when we edited the photo? Is it too dark or light? Is the contrast correct? If something is not right then we may have a problem with the calibration of our monitor. But what does it mean to calibrate our monitor?
It may be best to start with an example. Say that our desired output is a print on high gloss paper. Our first step is to make sure that the colours on our screen are the same as the colours on our print, but that is not the whole story, what about the brightness, is it the same as on the print? Are the whites in the print the same as on the screen? Does the screen contrast match that of gloss paper? It may not be possible to get a perfect match between our print and the monitor screen but we can get close and how close we get depends on how many of these parameters we can adjust accurately.
So if we want the best possible match between our monitor and our desired output then we need to be able to accurately adjust the following parameters:
Colour, Brightness, Gamma, White point or colour temperature, Black point, and Contrast ratio.
Firstly we may be limited because only high end monitors specifically designed for photo editing will be capable of adjusting all these parameters and secondly we may not need to adjust all the above parameters to get a match that we will be happy with. When we first get into monitor calibration out usual reason is so that the colours we are viewing on the monitor are correct and that is most important. However it is just as important to make sure the brightness is correct especially if our end product is a print. The next in the list of importance is gamma but then white point or colour temperature, again especially for prints. In general it is much easier to calibrate our monitor for viewing on the web and computer slide shows than it is for prints and projected images.
Computer monitors can usually be calibrated, that is adjusted so that the image accurately portrays what the camera captured, that is to give a display that more closely matches the colours and brightness that should be displayed. Many devices are available on the market to calibrate monitors but all require some knowledge of what you are doing, so even with this equipment it is not necessarily a plug and play solution. Besides this not all monitors are created equal, some monitors supplied with computers can be calibrated reasonably well but there are are not so flexible. In my experience I have found it difficult to calibrate laptop monitors. There are still other monitors that are specifically designed to be calibrated to a high degree of accuracy and it is these that are used by many professional and amateur photographers who require a high degree of colour, brightness and contrast accuracy.
When we first come into the world of monitor calibration we are met by a number of technical terms that we find confusing, but without some understanding of these terms good monitor calibration may be difficult. We need to be able to set up the monitor the way that is best for us and our photography and so we need to be able to control how the calibration is done. These terms include colour temperature or white point, gamma, black point, brightness and contrast ratio. I will give an explanation of each of these terms in turn.
Colour temperature - Colour temperature or white point describes how white is white. As a black body radiator (BBR) gets hotter the light from it changes in colour. At a relatively low temperature of about 2000 degrees kelvin the BBR glows orange/red and is the colour of light at early sunrise. As the temperature is increased to about 5500 degrees kelvin the light will be white, and is the colour of light in the middle of a clear day. If it is increased further to 6500 degrees kelvin the light will get a bluish tinge and is
the colour of light from the blue sky. It is also the colour of light that penetrates into the shadows on a clear day. So colour temperature can be used to describe the colour of the light. The light directly from the sun in the middle of the day is white and is described by a colour temperature of 5500 degrees kelvin. The other term that I have used to describe the colour of the light is "white point". When we are printing our images the white point not only depends on the colour of the light but also on the colour of our printing paper which may not be pure white. If we are to be exact we should measure the colour of our printing paper under white light and use this to determine our colour temperature/white point for the purpose of printing.
Many monitors that are supplied with computers are set in the factory to a colour temperature of 6500 degrees kelvin. This is becoming the de-facto web standard and has a slight blue tinge similar to the light from many television sets. If your final outcome is to display your images on a computer screen or television then your editing monitor should be set to 6500 degrees kelvin. But this colour temperature does not allow a good match between the monitor and the printed photograph. A good match is better achieved with a colour temperature of about 5000 degrees kelvin. This is a bit lower than the colour temperature of daylight (5500 degrees kelvin) to take into account the slightly warm tone of the paper.
Gamma - The gamma describes how the electronics in the computer changes the RGB values in your image file to a brightness value on the computer screen. The two major computer software companies Apple and Microsoft have traditionally used a different value for gamma. The gamma value of 1.8 used by Apple was based on the need to match the onscreen and printed values in the digital file. The value of 2.2 used by Microsoft was more about what looked better onscreen for business applications. Now this is not the case and the value of gamma is determined by the application rather than the computer platform. The value of gamma used in imaging applications is 2.2. A lower value of gamma will result in lower contrast on the screen and as a result lighter shadows.
Brightness, Black Point and Contrast Ratio - I have put these three terms together as they are related. Brightness is a critical factor in any monitor calibration. Many people are able to get the colours correct but are not happy with the match between their monitor screen and their prints because the brightness is set incorrectly. Most monitors supplied with computers are set very bright at about 100 to 140 cd/m^2and certainly much too bright for print matching. The monitor brightness should be set to about the same as the viewing conditions for the final print. A value of the brightness of about 80 cd/m^2 may be more appropriate. Black point is how black will be black on our monitor screen. The value of the black point should be set by the value of the contrast ratio that is required. The contrast ratio required is that of the final print which is about 200:1. The contrast ratio is the brightness divided by the black point. If we choose a brightness of 80 cd/m^2 and a black point of 0.4 cd/m^2, we get 80/0.4 = 200 which gives a contrast ratio of 200:1.
John Doody Photography Training