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Digital Camera Testing - Measuring Characteristics, Camera settings, OECF measurements, Resolution

image values quality iso

DIETMARLLER
Dietmar Wüller Image Engineering

Measuring Characteristics

While most people will never test a digital camera themselves, they may nevertheless be required to evaluate the results of someone else’s camera test when shopping for their next camera or reading technical information about a specific camera. This essay will introduce the issues that are important when testing a camera and describe a few image quality aspects in detail.

The measurement procedures behind the described camera characterization are based on ISO standards. 1,2,3,4,5,6 Work on these standards started in conjunction with the development of the cameras themselves. It is important to keep in mind that most of the characteristics that are measured with these tests are also being measured with standardized methods in conventional photography as well. The theories that these tests are based on may use the same tools, which are new for the digital age. 7 However, certain characteristics are important for digital cameras that did not exist for conventional ones.

Measuring specific image quality features can help in characterizing a camera, but these measurements always have to be related to the real-world applications. There are numerous aspects of the real world that we will never be able to measure. It is therefore impossible to completely test a camera without taking pictures of common scenes.

To acquire sufficient characterization information for a digital camera, a couple of characteristic values seem to be mandatory while others may be recommended or optional. The two characteristics that seem to be the most important are the measurement the OECF 2 (opto electronic conversion function) and the resolution. 5 A few others belong to this group as well.

The following aspects are mandatory:

  • OECF
  • White balancing
  • Dynamic range (related scene contrast)
  • Used digital values
  • Noise, signal to noise ratio
  • Resolution (limiting resolution center, corner)
  • Sharpness
  • Used digital values

Recommended values are:

  • Distortion
  • Shading/vignetting
  • Chromatic aberration
  • Color reproduction quality
  • Unsharp masking
  • Shutter lag
  • Aliasing artifacts
  • Compression rates
  • Exposure and exposure time accuracy and constancy
  • ISO speed

Optional values may be:

  • View angle, zoom range (at infinity and shorter distances)
  • Hot pixels
  • Detailed macro mode testing (shortest shooting distance, maximum scale, distortion)
  • Flash capabilities (uniformity, guiding number, light source, etc.)
  • Startup time
  • Image frequency
  • Video capabilities (pixel count, resolution, frame rate, low-light behavior)
  • MMS capabilities for mobile phone cameras (resolution, frame rate, compression, etc.)
  • Display (refresh rates, geometric accuracy, color accuracy, gamut, contrast, brightness, visibility in sunlight)
  • Optical stabilization
  • Auto focus accuracy and constancy
  • Metadata (Exif, IPTC)
  • Watermarking
  • Spectral sensitivities
  • Bit depth of raw data
  • Power consumption
  • Battery life
  • Detailed noise analysis
  • Color resolution

Camera settings

The measured values are influenced by the settings of the camera. To ensure the correct interpretation of the data, it is necessary to mention the proposed settings. Two different methods are used among testers to ensure the correct settings:

  1. Use the predefined factory settings; most users do not change these settings. They should have been carefully selected by the manufacturer.
  2. Select settings that provide an optimum image quality.

Both methods can lead to the same settings. For example, the manufacturer may choose a high-compression JPEG file format as the standard format to ensure high-speed image storage and minimum file sizes. The compression leads to an image quality that may not be the optimum image quality of the camera and therefore leads to different results. For the proposed test procedure we chose the settings mentioned in the table below to maximize the image quality provided by the camera. For all aspects that are not explicitly mentioned, the default factory settings should be used.

If it is possible to adjust the parameters, use the following settings:

File from camera to computer transfer

The image transfer from the camera/camera phone to the PC depends on the existing interfaces. Interfaces should be checked in the following order:

  1. Memory-Card/Card Reader
  2. USB
  3. Bluetooth
  4. Irda
  5. MMS

The first interface that is available should be used for the camera being tested.

OECF measurements

The OECF (opto-electronic conversion function) describes how the camera transfers the illumination on the sensor into digital values in the image. This information or at least the images of the test chart are necessary to answer the following questions:

  1. What is the maximum contrast in a scene that can be captured by the camera in all its tonal details (dynamic range)?
  2. Is the white balance operating correctly?
  3. Does the camera use all possible digital values in the image?
  4. Is there a gamma or tonal correction applied to the captured linear image?
  5. What is the signal to noise value for different gray levels?
  6. What is the ISO speed of the camera?

The camera OECF, as specified in ISO 14524, 2 is measured using a test chart with patches of different gray levels aligned in a circle around the center.

Resolution

Unfortunately, people think that resolution is the same as pixel count and, in fact, think of the pixel count as the sample rate is a limiting factor for resolution. When increasing pixel count, the other parts of the imaging system are increasingly the bottleneck for the resolution, such as the ability of a camera to capture fine detail, especially because manufacturers are trying to keep the sensor size as small as possible.

There are several ways to characterize the resolution of a digital camera. Methods developed so far have various drawbacks. Currently there is a new method under discussion in the respective ISO committee.

Visual analysis of the resolution leads to the problems described in Figure 15: moiré effects make it impossible to see which line pairs are still resolved. The slanted-edge analysis with the SFR algorithm as defined in ISO 12233, 5,8,9 was found not to represent the resolution of a camera if sharpening or other image-processing algorithms are applied to the edges in the image.

What is desirable is to have an idea of the modulation transfer function (MTF), as shown in Figure 16.

These are just a few of the tests that have been developed to characterize digital cameras. A full description of more tests can be found on the following website: digitalkamera.image-engineering.de/index.php/Downloads.

The requirements for an ideal digital SLR camera are:

  • Live image
  • Autofocus accuracy and speed
  • Manual focus
  • Image quality

    • Exposure control, level correction
    • Color appearance
  • Metadata
  • Standardized raw format
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over 4 years ago

Dear sir/mam

I Prashant.Hiremath working as Engineer in ADA
Bangalore. We need test procedure of colour camera(CCD) .We are using
colour camera on a "Head Up Display" (HUD) with auto iris in both day and
night modes. You are requested to suggest the parameters which should be
checked for above camera. If There are any equipments or software for
testing of camera let me know ,we are ready to buy.

I shall be very thankful to you

Thanks & Regards
Prashant

Vote down Vote up

over 4 years ago

Dear sir/mam

I Prashant.Hiremath working as Engineer in ADA
Bangalore. We need test procedure of colour camera(CCD) .We are using
colour camera on a "Head Up Display" (HUD) with auto iris in both day and
night modes. You are requested to suggest the parameters which should be
checked for above camera. If There are any equipments or software for
testing of camera let me know ,we are ready to buy.

I shall be very thankful to you

Thanks & Regards
Prashant