Image Quality Factors


Sharpness

Noise

Dynamic-Range

Color-Accuracy

Distortion

Uniformity

Chromatic Aberration

Flare

Color Moiré

Artifacts

Compression

 

Summary table— Image quality factors and corresponding test charts and modules
Sharpness   Noise   Dynamic range, tone, & contrast   Color accuracy
Lens Distortion   Uniformity   Blemishes   ISO Sensitivity
Lateral Chromatic Aberration   Lens Flare (Veiling Glare)   Color Moiré   Artifacts
Data compression losses   Dmax   Color gamut    Texture Detail

 

Overview of image quality and Imatest measurements

 Image quality is one of those concepts that’s greater than the sum of its parts. But you can’t ignore the parts if your goal is to produce images of the highest quality. Every image quality factor counts.

This page introduces the key image quality factors and briefly describes how Imatest™ measures them— with links to detailed pages. It is a guide to Imatest organized by image quality factors. Other guides include the Tour (organized by module) and Imatest documentation (the Table of Contents).

To illustrate the quality factors, we use this early morning image of Monument Valley from Hunt’s Mesa, near the Arizona-Utah border. 

Image quality measurements are affected by the

  • LensImatest cannot measure lenses by themselves, but lenses can be effectively compared to one another using a single camera body with consistent image processing settings.
  • SensorImatest can measure the performance of the Lens+sensor from minimally-processed RAW images if they are available. Sharpness, distortion, vignetting, Lateral Chromatic Aberration, noise, and dynamic range are the principal factors that can be measured at this stage. Most of these measurements can be clearly classified as good/bad.
  • Image processing pipeline— typically includes demosaicing, color correction, white balance, application of gamma and tonal response curves, sharpening, and noise reduction. Measured from the image delivered to the user (such as in-camera JPEG images). Additional image quality factors include tonal response (contrast, etc.), color response, and many others. The output of the pipeline may be compared to the minimally-processed lens+sensor measurements.The effect of the pipeline on subjective image quality can be highly scene- and application-dependent, making it difficult to assign “good” or “bad” rankings. Imatest results for these factors need to be interpreted carefully. Examples:
    • Higher contrast images often score higher in visual quality assessment tests, but high contrast can cause clipping (visible as burnt-out highlights) in contrasty (often sunlit) scenes. Tonal response curves with “shoulders” can help. See Stepchart and Multicharts.
    • Most consumers find accurate color to be boring, so camera manufacturers “enhance” colors in a number of ways, such as boosting chroma. See Colorcheck and Multicharts.
    • Most consumers dislike noisy images, but software noise reduction (typically lowpass filtering in areas that lack contrasty features) can remove fine texture, resulting in a cartoon-like “plastic” skin appearance. See Log F-Contrast and Random/Dead Leaves.

 

Summary table

This table summarizes the image quality factors and links to pages where they are described in detail. Most of the charts are available from Imatest Store.

Quality factor Chart Module Comments
Camera, lens
Blemishes, Sensor defects Plain, uniformly-illuminated surface
(flat field)
Blemish Detect Can be displayed on flat screen monitor with Screen Patterns and opal diffusing glass. Note [1]
Color accuracy X-Rite ColorChecker (24-patch) Colorcheck, Multicharts, Multitest  
IT8.7 Multicharts, Multitest  
ColorChecker SG, general mxn grids, and many other charts Multicharts, Multitest Note [1]
Dynamic range (DR),
Tonal response,
Contrast
Grayscale step charts Stepchart, Multicharts, Multitest Transmission charts such as the Imatest 36-Patch Dynamic Range chart or the Stouffer T4110 recommended for DR. Algorithm
Reflective step charts (also works with transmissive charts) Dynamic Range Postprocessor for Stepchart, Multitest. Calculates DR by combining several analyses at different exposures. Doesn’t require a transmission chart.
Special charts: ISO-16067-1, QA-62, EIA Grayscale, ISO-14524
OECF
, ISO-15739
Noise
Stepchart, Multicharts, Multitest Note [1]. Many are available from the Imatest Store.
ColorChecker, ColorChecker SG, IT8.7, Step Charts Multicharts, Multitest  
SFRplus, ISO 12233:2014 E-SFR SFRplus, eSFR ISO SFRplus does not measure DR. eSFR ISO measures ISO 15739 DR. Highly automated. Measures several factors. Available from the Imatest Store.
Exposure accuracy Step charts (reflective) Stepchart, Multicharts, Multitest  
X-Rite ColorChecker Colorcheck, Multicharts, Multitest  
ISO Sensitivity
(closely related to
Exposure Index)
Step charts Stepchart Two ISO sensitivity measurements are displayed when incident light (lux) is entered. Details in ISO Sensitivity and Exposure Index
Various color and step charts Multicharts, Multitest
SFRplus SFRplus
X-Rite ColorChecker Colorcheck, Multicharts, Multitest
Lateral chromatic aberration Slanted edge, ISO 12233 charts SFR Available from the Imatest Store. Note [2]
SFRplus, ISO 12233:2014 E-SFR, Checkerboard SFRplus, eSFR ISO, SFRreg, Checkerboard Available from the Imatest Store. Note [2]
Dot pattern Dot Pattern Available from the Imatest Store. Note [2]
Lens distortion Square or rectangular grid or checkerboard, Distortion Printable by Test Charts or displayed on LCD flat screen monitor with Screen Patterns.
SFRplus, ISO 12233:2014 E-SFR SFRplus, eSFR ISO, Checkerboard Highly automated. Measures several factors. Results in the Image & Geometry display.
Dot pattern Dot Pattern Available from Imatest Store.
Light falloff, vignetting Plain, uniformly-illuminated surface Uniformity Can be displayed on flat screen monitor with Screen Patterns. Opal diffusing glass recommended.
Noise Step charts Stepchart  
X-Rite ColorChecker Colorcheck  
SFRplus SFRplus Measures flat areas near slanted-edges. Best with low (4:1) contrast charts.
eSFR ISO eSFR ISO Measures noise from grayscale surrounding center of chart.
Wide variety of grayscale stepcharts and color charts Multicharts Works with a large variety of grayscale and color charts if patches are large enough. Can measure sensor (raw) noise.
Sharpness (MTF) Slanted-edge, ISO 12233 SFR, Rescharts
ISO 12233:2000 and 2014 charts available from Imatest Store. Edges printable by Test Charts. Algorithm
SFRplus, eSFR ISO, SFRreg, Checkerboard SFRplus, eSFR ISO, SFRreg, Checkerboard Highly automated. Measures several factors. Available from Imatest Store.
Other charts: Star, Spilled Coins, Wedge, etc. Rescharts, Star, Log F-C, Random, Wedge Both interactive and fixed modules. Each responds differently to signal processing.
Texture detail Log F-Contrast, Spilled Coins (Dead Leaves) Log F-Contrast
Random/Dead Leaves
 
Lens flare (Veiling glare) Reflective Q-13 or Q-14 step chart with “black hole” Stepchart See Veiling Glare. Note [1]
Color moiré Log Frequency Log Frequency, Wedge  
Artifacts Log F-Contrast
Any arbitrary image
Log F-Contrast
SSIM
Notes [1,2] SSIM measures degradation due to image processing (primarily compression) by comparing two images: reference and processed.
Prints
Dmax (deepest black tone) Custom test chart printed from file, scanned on profiled flatbed scanner Print Test Gamutvision extracts these properties from ICC profiles.
Color gamut

Notes: [1] Not available in Imatest Studio. Available in Master, Image Sensor, etc. [2] can be printed from Test charts, but we recommend purchasing it from the Imatest Store.