Some easy ways to judge whether your monitor performs well enough to be used for image editing…
Using a monitor you can trust not only makes printing your images easier and more enjoyable; it will also save you time and money.
When the monitor you use can display the full range of hues and tones your camera has captured and keep them in the correct balance, you reduce the number of test prints required before committing to an enlargement, and at the same time decrease the time you take to produce a great-looking print.
Here at Photo Review we are firm believers in the adage that an image isn’t a photograph until it’s printed. We also believe there’s nothing quite as satisfying as producing an A2 print that faithfully captures the picture you shot. That satisfaction can be compromised if you can’t trust your monitor to:
1. Display hues and tones accurately enough to allow the image to be printed as closely as possible to the picture you see on the screen.
2. Have a wide enough gamut (range) to reproduce all the hues and tones in the image file.
3. Provide even illumination and colour reproduction across all of the screen with no ‘hot spots’.
4. Maintain the same quality of reproduction with consistency between calibrations.
5. Support hardware calibration (which is more reliable than software tweaking).
Your monitor must be able to ensure accurate reproduction of image hues and tones when you edit your images.
One basic way to see whether a monitor is suitable for imaging is to check the range of adjustments it provides. The ability to calibrate and profile a monitor depends on being able to adjust both brightness and contrast as well as independently adjusting the red, green and blue colour channels. Some monitors are sold with one or more of these controls locked. This prevents accurate profiling.
Resolution should be listed in the monitor’s specifications. Assess it in the light of the largest frame size you want to display, particularly if you’ll be editing video clips. If you plan to work with 4K footage, theoretically you need a screen that can display 3840 x 2160 pixels. From a practical viewpoint, however, when editing still images, a screen resolution of 2560 x 1440 pixels should be adequate, regardless of the resolution of the camera used to capture the shot, providing other key parameters are up to scratch.
You can’t evaluate some criteria by reading the published specifications because many parameters aren’t standardised, which means it’s impossible to compare like with like. Viewing angles are often measured differently by different manufacturers and measurements tend to be largely subjective. What’s satisfactory in one manufacturer’s view can be substandard in another’s.
If you will be working alone, and sitting in front of the screen, viewing angles can usually be ignored. If you want others to view the screen while you’re working, it’s important to have a wide viewing angle horizontally and vertically combined with a high level of screen uniformity.
Understanding test results
When we test monitors we look at all the parameters listed below and provide a rating based upon the screen’s performance when measured with a Spyder calibrator. Readers who own one of these devices can use it to check their own monitors.
The software bundled with this device includes an advanced section that makes objective measurements of seven key parameters: colour gamut, tonal response, white point, contrast, luminance uniformity, colour uniformity and colour accuracy. The software then provides an overall rating based upon these measurements.
The best-performing monitors should have ratings of at least four (out of five) for all parameters. The following criteria are important when judging a monitor’s capabilities and performance:
Colour gamut. Most low-priced screens can only display the sRGB colour space. Laptop screens frequently fall into this category, which is why they aren’t recommended for serious editing.
Displays that will be used for image editing and/or printing should be able to reproduce as much as possible of the Adobe RGB colour gamut, partly because it shows blues and greens with a natural-looking balance but also because it can reproduce all the hues and tones used in printing, regardless of the printer and/or media.
Unfortunately, simply covering the full gamut won’t guarantee accurate colour reproduction. Many recently-released monitors claim to cover (or nearly cover) the entire Adobe RGB colour gamut but, while they can meet that claim, the distribution of colours across the display is often seriously out-of-whack.
Two colour gamut plots obtained from our monitor tests. The red triangle shows the colour gamut covered by each monitor, with the green and purple triangles showing the sRGB and Adobe RGB gamuts, respectively. Nothing in these plots indicates the monitor on the left provides better colour fidelity; in fact, the monitor on the right showed much better colour accuracy in subsequent tests.
Colour accuracy should be viewed in conjunction with colour gamut. This analysis shows how specific hues are reproduced on your screen, giving you a good idea of the overall colour balance and indicating which colours are more accurately reproduced. Displays used for image editing require all colours to be relatively evenly balanced.
The table on the left shows a monitor with good overall colour accuracy, while the one on the right shows serious deviations in two hues. The figures at the bottom of the Result Lab column indicate the minimum and maximum deviations from the desired colour plus the degree of overall colour variation. The closer the ‘Average’ figure is to zero, the higher the overall colour accuracy of the screen.
Uniformity from corner to corner is another criterion that is often overlooked. Yet a screen that is darker in one or more areas ““ or one with a central ‘hot spot’ ““ will make it difficult to assess exposure levels.
The upper plot represents a screen with excellent brightness uniformity, while the lower plot shows a screen with a central ‘hot spot’, which would produce unwanted edge darkening in displayed images.
The same applies with colour uniformity, where uneven screens can make it hard to determine whether colours are reproduced correctly. Both brightness and colour can vary with ambient temperatures so the ideal monitor should include some ways to counterbalance these fluctuations and provide a stable image display.
The upper plot represents a screen with excellent brightness uniformity, while the lower plot shows a screen with large differences in the lower third of the screen. This screen would be much less suitable for image editing.
Tonal response provides a guide to how well hues and tones are displayed, yielding a parameter known as the ‘gamma curve’. Bit depth plays an essential role in performance. Professional monitors can be distinguished by 10-bit (or higher) gamma processing, whereas other displays offer 6-bit or 8-bit support.
A monitor with 6-bit colour reproduction can only display 64 colours per channel (red, green and blue) or a total of 262,144 discrete hues and tones. Monitors with 8-bit processing can calibrate the gamma curve in 256 steps and display 256 colours per channel, a total of 16,777,216, which is significantly more.
However, a 10-bit processor can calculate the gamma curve in 1021 steps and display more than one billion colors simultaneously. The result will be a smoother gamma curve and greater hue and tonal accuracy. Expect to pay a premium price for a monitor with high-bit gamma processing.
In our monitor tests, the software provides an Overall rating that shows how the monitor scored in the various parameters. This provides an easy visual guide to whether it is a good choice for image editing. The best monitors rate highly for all parameters (although we have yet to find a monitor with a perfect score).
Overall ratings for two of the monitors we have tested. The top table shows a monitor that rates well across all parameters, while the bottom one shows a monitor that scores poorly in Contrast and Luminance Uniformity, despite having a perfect score for colour Gamut.
Several manufacturers of imaging-related products provide online resources that can be helpful for evaluating monitors, particularly those that will be used for printing photographs.
Eizo in Belgium has a very comprehensive test that will show up defective pixels, uniformity, colour gamma and separations, sharpness, viewing angle and response time. You can select any parameter individually or use the default setting, which will run through all the tests. The test is available at https://www.eizo.be/monitor-test/.
The Eizo online monitor test provides a simple and effective way to see whether your monitor’s performance meets the expected standards for image and video editing.
On-screen testing can be useful but to be really sure your monitor is displaying the hues and tones you need for printing, you should compare a printed photograph with the image displayed on the screen. If your image editor supports soft proofing it will simulate the image hues and tones to show how the image will look when output to your printer.
An accurate soft proof requires a computer display that can reproduce all these hues and tones, along with a calibrated monitor, printer profile and colour-managed workflow. Make allowances for the huge difference in dynamic range between an on-screen image and one that is printed on paper. A decent monitor will have a dynamic range of at least 1000:1, whereas papers typically range between about 200:1 for glossy media and 100:1 for matte papers.
You can find links to a variety of printable test images at www.bit.ly/ColorWiki-test. We particularly recommend the PDI test image from Photodisc, which includes the Gretag-Macbeth colour patches plus a range of human skin tones. It can be downloaded from www.photodisc.com.
Article by Margaret Brown
Excerpt from Photo Review Issue 74