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Measuring monitor contrast ratios with a camera

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It is nearly impossible to accurately estimate the contrast ratio of a monitor by eye, and the manufacturers are often inflating the numbers to unrealistic values. However, with a digital camera and this web page, it is quite easy to check it yourself. It is most accurate with a camera that gives you some manual control over exposure time and the like, but it will work even with a point-and-shoot camera.

The idea is simple: take a picture of a white area and of a black area on the screen. The brightness of the picture, combined with camera settings such as ISO, F-number, and shutter time tells us how much light comes off the screen in either case. For the most accurate results, all camera settings should be identical, except for the shutter time, which is chosen such that both pictures end up as a middle grey.

How to use your camera Measuring the contrast ratio with a camera. Note how the camera is held close to the screen.

These are the minimum settings:

Cameras vary in how easy it is to adjust the exposure. A good starting point seems to be ISO 400, F 2.8, and shutter times of 1 and 1/400 second. (For laptop screens, use 1/4 and 1/400 second). Always keep the same ISO and F number for the two images, and vary the shutter times if necessary to get two images with approximately the same mid-grey level (2.5 stops below saturation). Also try to keep the shutter times between 2 and 1/500 second. Set the camera to a low-resolution mode; it will reduce image noise. Also don't try to focus on the screen, because we are not interested in seeing individual pixels.

If the above is cryptic, you can try the following procedure:

Then proceed with the test as follows.

  1. Make the environment as dark as possible.
  2. Set the shutter time to about 1 s (or 1/4 s for most laptop screens). Put the camera very close to the black area below, at a straight angle, and take a picture of the black area. Don't worry about warnings about camera shake and autofocus. (If the camera has manual focus control, set it to infinity, ∞) If the image is very dark, increase the shutter time (or change the EV adjustment) and take a new picture to get a mid-grey image.
  3. If the black level of your monitor is worse close to the edges ("backlight bleed"), then you may repeat the procedure for the borders of the screen. Make sure that you don't photograph the light from the menu or task bar and such.
  4. Decrease the shutter time to about 1/400 s. Make a picture of the white area. Change the shutter time or EV adjustment if the image is (almost) saturated white or very dark rather than mid-grey.
  5. Upload two representative JPG files in the form below. The result page will warn you if you need to change your camera settings. Optionally, you can add a third JPG file taken at the edge of the black screen.
Black screen photo
White screen photo
Black screen edge photo (optional)
Monitor brand/model (optional)
Color space (optional)

By uploading, you give permission for your files to be stored on the server and to be used for any purpose, including public display without personally identifiable information.

(max. file size 1 MB)

Alternative method

If you don't use the upload form, you can proceed as follows.

  1. Review the pictures. If either of the two is vastly underexposed or overexposed, try changing the exposure time or EV adjustment. If that doesn't work, you can change the aperture F number and the ISO number, but these two should be identical for the black and white photographs. That means: don't shoot the white area at ISO 100 and the black area at ISO 800. Also, avoid extremely short shutter times (1/1000 or less).
  2. Look at the pictures and if your camera has one, the histogram. For both the dark and the light pictures, locate the peak of the histogram. In the example image, the peak is at 2.5 stops. If the peak (measured at the base) is wider than about 1 stop, I would suggest to read the value 0.5 stops from the right edge. Alternatively, enter the RGB values that are in the resulting image file (see below for hints).
  3. Enter the numbers in the form below. (This uses javascript. It should work with Firefox 1.5/2.0, Opera 9, and even Internet Explorer 6.)

Quantity Black image White image Explanation
Exposure time Best between 0.5 and 1/500 s.
F-number Preferably equal
ISO value Preferably equal
Histogram peak
RGB values

Need 0.1 or better accuracy.
RGB values in the range 1–255.
sRGB   Adobe RGB   (see below)
Luminance ?? ?? Read the explanation below!
Your contrast ratio
Contrast ratio ??

Black area
White area
You can also try the white and black squares below instead, for example if your monitor has some kind of dynamic contrast. Make sure that you point the camera lens accurately.
You can also check the procedure with the images below, which should have a contrast ratio of about 4:1, if your display gamma is correct (see gamma test).
25% luminance100% luminance


Using RGB values

Picking RGB values in GIMP.
Picking RGB values in MS Paint.

If your camera does not have a histogram, or a histogram without a scale, and you don't want to upload your photos for some reason, you can open the JPG image file in an image editing program such as the GIMP or MS Paint. Of course, there are plenty of other programs, too, but these are easy to get. This method is more accurate than looking at the histogram, but also more time-consuming. The RGB values are converted into luminances (relative to image saturation) according to the sRGB standard [Wikipedia] or Adobe RGB standard [Wikipedia]. Most consumer-grade cameras use sRGB.

Especially with the black image, there might be a considerable difference between the center and the edges. In that case take the RGB values at the lighter parts of the image.

a. GIMP procedure

GIMP is available for all common platforms (Linux, Mac, Windows) and is free. [download]. Select the color picker tool (1; it looks like an eye dropper). Set the "sample average radius" to 50 or so and click at a representative part of the image (2). Depending on the GIMP version, you will get a popup with the RGB values (3), or you can click on the currently selected color (4) to get a window with the RGB values (5).

b. MS Paint procedure

MS Paint is included in Windows (it's under the Accessories menu). Select the color picker tool (1; it looks like an eye dropper). Click at a representative part of the image. Then go to Colors (2) → Edit Colors → Define Custom Colors (3) and read the RGB values (4). Repeat this a few times and calculate the average RGB values.

Contrast ratio accuracy

I guess that it's fairly evident how this test works. The result should be fairly reliable if your camera can show a histogram. There are a number of error sources:

Luminance accuracy

The form also shows the absolute luminance as calculated from the data entered. All numbers should be known and not guessed or this number will not be meaningful. And even then, the real ISO numbers of cameras may vary considerably from one camera model to the next, so don't take this number as an absolute truth.

The calculation assumes that the sensor or RGB image saturates at a luminance

Ls = 85.5 × (f#)2 / (ISO × t),

where f# is the aperture number, and t is the exposure time. This equation uses the "standard output sensitivity (SOS)" definition of the digital ISO value. [see Wikipedia]. From some experiments with a calibrated luminance meter and various cameras, it appears that the actual sensitivies can differ considerably from this equation, and that it is not always consistent on the same camera at different "ISO" settings. Here are the numbers for a couple of cameras:

Camera modelISO setting F-number Ratio real/nominal
Canon Powershot A540 80 5.5 1.4
100 5.5 1.5
200 5.5 1.5
400 5.5 1.4–1.5
800 5.5 1.4
Canon Powershot A710IS 80 4.8 3.0
200 4.8 1.2
800 4.8 1.3
Nikon D40 400 8.0 1.8
1600 5.6 2.0
Nikon Coolpix 990 100 4.0 2.0
200 4.4 1.60
400 5.6 0.80
Nikon D80 1007.1 2.1
400 7.1 2.1
1600 9.0 2.8
Olympus Camedia C-350 zoom 128 5.2 2.30
Sony Cybershot DSC-P120 100 5.2 2.3
100 10.0 2.2
400 5.2 1.7
Sony Cybershot DSC-W1100 5.2 1.8
200 10.0 2.0
200 5.2 1.9
400 5.2 1.7
The column "Ratio real/nominal" is defined as the measured ISO/SOS sensitivity divided by the number provided by the camera. For these cameras, the luminances (in cd/m2) provided by this pages should be divided by this number. For example with a Canon Powershot A540, this page may report a black level of 0.5 cd/m2, but actually, the black level is 0.5/1.4 = 0.36 cd/m2


I'd like to hear your feedback. Mail me at lcdtest /at/ lagom /dot/ nl.

Laatste verandering: 04 Mar 2008   Copyright