Imatest™ Uniformity (Light Falloff)  measures lens vignetting (dropoff in illumination at the edges of the image) as well as other types of image, illumination, and sensor nonuniformities. Examples include

  • the uniformity of flash units (using light bounced off a white wall),
  • the uniformity of flatbed scanners,
  • sensor noise and spots of the type caused by dust on the sensor (Imatest Master only).

Display options include contours on top of images and pseudocolor with color bar. In Imatest Master, Light Falloff can also display a histogram of pixel levels, analyses of stuck (hot and dead) pixels and color shading, and a detailed image of fine nonuniformities.

Simplified instructions

To prepare an image for Uniformity, set your camera lens for manual focus and focus it at infinity (the worst case for light falloff). Uniformity works without a lens if the sensor is evenly illuminated. Photograph an evenly-lit uniform subject— a smooth sheet of matte paper (white or gray) or a light box— with opal diffusing glass (available from Edmund Optics) in front of the lens— works well. There are many options.

The subject does not need to be in focus; the goal is to measure lens light falloff and sensor uniformity, not features of the subject. For typical lens vignetting measurements, set exposure compensation to overexpose by about one f-stop. (You may, however, use any exposure you choose.) More details can be found in the Uniformity instructions.

Channel contour plot

This plot shows contours of the level (normalized or not) of the selected channel (Red, Green, Blue, or Y (luminance = Y = 0.30*R + 0.59*G + 0.11*B)) of the image file. The contours are superposed on top of the image. In this unnormalized display, the maximum value of 1 corresponds to pixel level = 255 in image files with bit depth = 8. Some lightning nonuniformity is evident in the plot: the top is brighter than the bottom. The contours are derived from a smoothed (lowpass-filtered) version of the image.

Picture Window Pro Light Falloff transformation dialog boxThe text displays the maximum luminance value, the worst and mean corner values (in normalized pixel levels and percentage of maximum), and the side values. Two hot and two dead pixels (which were simulated) were detected with thresholds of 246 and 10 (pixels), respectively. More details of the hot and dead pixels are in the CSV and XML output files. Selected EXIF data is shown on the right.

The setting for correcting light falloff in the Picture Window Pro Light falloff transformation is also given. The PW pro Light Falloff dialog box is shown on the right. Film Size (mm) remains at 36 (the PW Pro default value: the width of a 35mm frame). Picture Window Pro is the powerful and affordable photographic image editor that I use for my own work. The Lens Focal Length is rarely the exact focal length of the lens. Light falloff depends on the lens aperture (f-stop) as well as a number of lens design parameters. Lenses designed designed for digital cameras, where the rays emerging from the rear of the lens remain nearly normal (perpendicular) to the sensor surface, tend to have reduced light falloff.

For aesthetic purposes I generally recommend undercorrecting the image, i.e., using a larger Lens Focal Length. This makes the edges somewhat darker, which is usually pleasing. Ansel Adams routinely burned (darkened) the edges of his prints. Part of the reason was that he had to compensate for light falloff from his enlarger (when he wasn’t contact printing).

“My experience indicates that practically every print requires some burning of the edges, especially prints that are to be mounted on a white card, as the flare from the card tends to weaken visually the tonality of the adjacent areas. Edge burning must never be overdone…”

Ansel Adams, “The Print,” p. 66. 1966 edition.

F-stop contour plot

shows contours of the normalized channel levels (R, G, B, or Y) of the image file, measured in f-stops. A pseudocolor display with color bar has been selected. The colors in the color bar are fixed: colors always vary from white at 0 f-stops to black at -4 f-stops and darker. For this plot to be accurate, a good estimate of gamma (the camera’s intrinsic contrast) is required. Gamma is measured by Stepchart, using any one of several widely-available step charts. (Reflection charts are easiest to use but transmission charts can also be used to measure dynamic range.)

Gamma can be tricky to measure for several reasons. (1) Many cameras have complex response curves, for example, “S”-curves superposed atop gamma curves. This means that gamma can vary with brightness. (2) Some cameras employ adaptive signal processing in their RAW conversion algorithms. This increases contrast (i.e., gamma) for low contrast subjects and decreases it for contrasty subjects. This improves pictorial quality for a wide range of scenes, but makes measurements difficult, especially since Light falloff targets have the lowest possible contrast.

Color shading (nonuniformity)  (Imatest Master only)

Light Falloff displays color nonuniformity. This image shows shading measured as the pixel ratio between the red and blue channels, normalized to 1.0 to minimize the effects of white balance errors. The background shows exaggerated colors (HSV saturation S has been increased by 10x for low saturation values; less for high values.) Several display options are available. Details here.

R/B color shading plot

Uniformity profiles   (Imatest Master only)

Uniformity profiles, introduced in Imatest Master 2.3.16 (August 2007), displays profiles of image levels along several lines: Diagonal Upper Left-Lower Right, Diagonal Lower Left-Upper Right, Vertical Top-Bottom (center), and Horizontal Left-Right (center). Several display options are available, including unnormalized levels (max 1), pixel levels (max 255), normalized levels (max 2), and R/G and B/G ratios (G constant). Details here.

Uniformity profiles display

Histograms  (Imatest Master only)

The histogram plot, introduced in Imatest Master 2.3.11 (July 2007), facilitates the detection and the setting of thresholds for stuck (hot, dead, etc.) pixels. Histograms of log10(occurrences+1) are displayed for the red, green, and blue channels. Details here.

Light Falloff histogram, showing hot & dead pixels

In this example, single stuck pixels are plainly visible near levels 0 and 252. You can quickly see if the thresholds are set correctly— if they are outside the valid density region and if the dead and hot pixels are above and below their respective thresholds. You can change threshold settings and rerun Light Falloff if needed.

Noise detail  (Imatest Master only)

shows an exaggerated image of the noise detail, a pseudocolor image of image variation, or a pseudocolor image emphasizing spots.. The algorithm is in the Light Falloff for Imatest Master instructions. The spot emphasis is very elementary: Blemish Detect has a more sophisticated approach to detecting visible blemishes.

The first image (below) shows noise detail for the Canon EOS-20D at ISO 1600 with the 10-22mm lens set to f/4.5. No surprises here; electronic noise dominates.

The second image (below) shows noise detail for the Canon EOS-20D at ISO 100 with the 10-22mm lens set to f/8. Thanks to the small aperture, dust spots are visible. The dust is on the anti-aliasing/infrared filter/microlens assembly in front of the sensor. This assembly can be well over 1 millimeter thick. Stopping the lens down (increasing the f-stop setting) reduces the size of dust spots but makes them darker.

The image below is an enlargement (a zoom) of the above image that includes the dust spot to the left of the center, shown with superposed contours.You can zoom into an image by using the mouse to draw an rectangle, or by simply clicking on a feature you want to enlarge. You can restore the original image by double-clicking anywhere on the image.

 Blemish Detect has a more sophisticated (and tunable) approach to detecting visible blemishes.

 Full instructions for using Uniformity are on Using Uniformity