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Using Your Camera To Check Monitor Luminance And Print Viewing Light

One of the most common complaints that we hear about matching the print to the monitor is that the prints are too dark compared with the image on the monitor. Often, the fault is not with the print but with the monitor luminance (brightness). If the monitor is too bright, you will invariably reduce the image brightness to make it look "right". Result - genuinely dark prints.

Another common cause is that even if the monitor luminance is correct, the prints are viewed with low illumination. To get the most from a print you will need a fairly high level of illumination; in poor light the shadow detail will be lost, giving the impression that the print is too dark overall.

To realistically compare the printed image with the monitor image it is necessary to make the perceived brightness of the print match that of the monitor. This means viewing the print with a light level of at least 500 lux - the kind of level normally found in a brightly lit office or supermarket. The level in a typical living room may be as low as 50 lux, so you cannot expect to see your prints at their best while sitting in your armchair.

Few photographers have the special equipment needed to measure both monitor luminance and illuminance of the print viewing area. However, every photographer has a camera, and a good camera can do this job with an accuracy much better than you can do with your unaided eyes. Try the simple procedure given below.

Checking monitor luminance

The factory settings for modern LCD monitors are always far too bright for photo editing; so, unless you use a hardware calibrator to set luminance, it is worthwhile spending a few minutes to check that you are at least in the right ball-park. A good quality camera will allow you to do this.

If you are reasonably confident that your camera gives correct exposure, follow this procedure to check or set your monitor luminance.

1 In your image editing application, open a new, blank document with a white background. Open this document in full-screen mode so that most of the screen is filled with white.

2 Set your camera to manual focus and set the focus to infinity. Do not use a close focus mode because this will change the effective aperture of the lens and your readings will be inaccurate.

3 Set your camera as follows: ISO 400; Aperture Priority; f/5.6

4 Point your camera at the centre of the screen at close range (50cm or less) so that the viewfinder is completely filled with white. Don't worry that the image is completely out of focus - it will not affect the exposure reading. In fact, you should find that moving the camera back and forth between 20cm and 50cm does not affect the exposure reading.

5 Note the exposure reading. This will normally be displayed as a fraction of a second - in other words, 1/60th will be displayed simply as 60. This displayed number will be (approximately) the luminance of your monitor in candelas per square metre (cd/m2)
6 The recommended luminance for photo editing is 120 cd/m2 so you should adjust your monitor brightness to get an exposure reading of between 100 and 125. (1/100th to 1/125th)

Naturally, this procedure will not guarantee you a precise measurement of luminance, but it is at least objective. We have no innate reference for luminance so any measurement technique is probably better than doing it by eye.

Note: The light from LCD monitors is polarised, and this will have an effect on SLR metering systems. If you use an SLR to check an LCD monitor, take one reading with the the camera in the landscape (horizontal) orientation and one in portrait (vertical) orientation. The difference will probably be 1/3 to 1/2 a stop. The higher reading will be the more accurate.

For some monitors the lowest achievable luminance will be higher than 120cd/m2; the iMac is one well known example of this. If your monitor cannot be set to 120cd/m2 don't be tempted to use one of the software methods to reduce the luminance. These methods usually reduce the number of colours that you can display, and may well produce colour banding where smooth colour gradients have visible steps. Better to live with the higher luminance and adjust to it mentally!

Checking illuminance of print viewing area

Checking the illuminance of the print viewing area can be done in a very similar way to that used for checking monitor luminance. Follow all the steps above, but in step 4, instead of pointing your camera at the monitor, point it at a sheet of matt white paper in your print viewing area. Ordinary copier paper is perfectly adequate for this job.

Fortunately, you are not too concerned with the actual lux level. All you need to do is to make sure that the exposure reading is the same, or higher, than the reading you obtained from the monitor. You can then be sure that the print luminance is similar to that of the monitor.

As a very rough guide, a desk lamp with a 60W bulb (or low-energy equivalent) will give the right illuminance at about 50cm (18 inches) from the print.

Exhibition prints or armchair prints?

A custom printer profile will generally give the best possible prints from your printer. That means the most accurate colour and the widest range of tones. Unfortunately, "perfect" exhibition prints may not look right in poor lighting. You may therefore decide that you prefer lighter prints for day-to-day "family" viewing.

If you decide to do this, then a simple "Levels" or "Curves" adjustment in your image editing application can be applied just before you print the image. Be aware though, that although the colours will still look natural, your lightened prints will probably look a bit washed-out in exhibition level lighting.

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