Veiling glare (Lens flare)

Veiling glare is stray light in lenses and optical systems caused by reflections between surfaces of lens elements and the inside barrel of the lens. It is a strong predictor of lens flare— image fogging (loss of shadow detail and color) as well as "ghost" images— that can degrade image quality in the presence of bright light sources in or near the field of view.  It occurs in every optical system, including the human eye.

Lenses with low flare have been traditionally known for their excellent color performance. Color saturation is higher, especially in shadows. Low flare may be as responsible as sharpness for the exalted reputations of some classic Leica and Zeiss lenses. Even though lost color saturation and contrast can be recovered with digital processing, lenses with low flare will always have an edge in quality. There are several ways of dealing with lens flare.

• In lens design, veiling glare is controlled by using high quality lens coatings (multiple coatings are best), baffling in the lens barrel, and careful design.
• In the field it is controlled by lens hoods and anything that can shield the lens surface from the sun. Ansel Adams used his hat, as well as a bellows lens shade, made famous in a Datsun (Nissan) commercial.
• In the studio it is controlled by "barn doors" on light sources.

Obligatory bad photo showing severe lens flare caused by the sun (far left), and moderate flare with the sun blocked (near left). Canon EOS-20D, 24-70mm f/2.8L, 24mm, f/8, no hood. Taken during Colorado's notorious "Winter of 2007" (five feet of snow in three weeks in December and January)   The veiling glare measurement in Imatest Stepchart is designed to give a good prediction of a lens's susceptibility to lens flare in challenging situations: the larger the number, the worse it's likely to be. But it's not perfect in every detail. For example, it can't predict ghosting (visible behind the Stop sign in the image on the left). The measurement is fairly simple to perform, but several pitfalls can cause inaccurate or misleading results unless measurements are made with care. Some degree of exposure control— either a manual setting or exposure compensation— can enhance the accuracy of the results. Exposure may be a problem with cameraphones and other simple devices, where little, if any, control is available.

Veiling glare is measured by photographing a perfectly black object against a uniform white field that extends well beyond the image frame. It is defined as the ratio of the light reaching the sensor from the black object (which emits no light) to the light reaching the sensor from the surrounding white field.

Veiling glare = V = L(black object) / L(white surface), where L is the illuminance at the sensor.

Since digital image sensor output is linear, this measurement is straightforward

• if you have access to the linear (RAW) output of the sensor,
• if the white area is unsaturated,
• if the signal in the black area is large enough to be well above the noise, and
• if the black area is perfectly black.

We deal with the last of these issues first: how to create a perfectly black object— a black hole. Don't worry: it's more environmentally friendly than the type that sucks entire planets into its core.

Preparing the target

The black hole The darkest surfaces— materials or pigments— reflect about 0.5 to 1% of the incident light, i.e., they are are far from perfectly black. To obtain a surface suitable for measuring veiling glare you will need to construct a surface that little light reaches— a black hole— the darkest possible object that can be photographed in a bright environment. The black hole should be constructed inside a box or tube that is approximately 3x4 inches on its top (or 4 inches in diameter) and 4-8 inches deep. The bottom (inside back surface) should be lined with black velvet— the darkest material you can buy. The sides can be lined with any matte black material. We used black art paper because it was easy to work with. The top is a piece of black foam board with a 1x2 inch opening cut in the center. The structure should be kept as lightweight as possible. If necessary it can be constructed entirely out of black foam board, with the black velvet in the inside back surface.

Construction of the "black hole"

The black hole is mounted next to a standard step chart such as the Kodak Q-13 or Q-14, which performs two functions. It allows you to measure the camera's tonal response (pixel level as a function of scene luminance) and it provides a reference for determining the white level from a deep gray patch, so you don't have to measure it directly. This is important because many, if not most cameras, have nonlinear response in the bright regions, i.e., the response curve has a (film-like) "shoulder"— a good thing pictorially because without it digital cameras have a strong tendency to burn out highlights.

An alternative: Bart van der Wolf sent this link, which suggests that a cone painted glossy black can be used to make a superior black hole.

Gamma (image contrast) for measuring veiling glare is calculated differently from the average gamma shown in the figures. It is determined from the region where the step chart pixel level is between 0.1 and 0.6 of the minimum-to-maximum pixel range, i.e., the brightest areas are excluded. The white reference is determined from the patch in the middle of this region. For example, suppose patch 12 is selected. Since the Q-13/Q-14 has density steps of 0.1, patch 12 is 1.1 density units darker than (0.0794 as bright as) patch 1, which is the reference white, i.e., we infer the white level from the measurement of patch 12. If there is any nonlinearity in the response curve, this results in a much more accurate veiling glare measurement than you'd get by measuring the white region directly. Tonal response for an in-camera JPEG and a linear (simple gamma curve) RAW conversion are shown here.

Assembling the target A Kodak Q-13 step chart and the black hole box are mounted on a piece of mat board, roughly twice the width of the Q-13 chart— its dimensions are not critical. A 3/4x1 inch hole is cut in the mat board, just to the right of where of where the Q-13 chart will be mounted. The black hole box is mounted behind it. The Q-13 is mounted to slightly overlap the opening. The mat board is attached to the 40x60 inch white foam board with Velcro so it can be easily removed for safe storage. Black hole location detail

Typical framing for measurements

The entire target

Measuring veiling glare

• Illuminate the target evenly, with no more than about ±20% variation in illumination across the target, as described in Imatest test lab. Keep the ambient light near the camera as low as possible ti minimize the light entering the "black hole."
• Note whether you are using a lens hood or a filter (like the ubiquitous UV filter most photographers leave on their lenses for protection). Both affect veiling glare.
• Set the camera to the lowest available ISO speed to minimize the noise in dark patches.
• Adjust the exposure (or exposure compensation) so the white level is close to saturation, i.e., bias it in the direction of overexposure. It doesn't hurt if some highlights are blown out: the Imatest veiling glare algorithm is insensitive to highlight saturation.
• RAW output is preferred, though JPEG may be perfectly fine if the response is reasonably linear.
• Photograph the central portion of the target, leaving some white area around the Q-13 chart and "black hole." Typical framing is shown above, to the right of Assembling the target. The length of the white target to the left and right of the frame should be about equal the frame itself. Similarly, the length of the white target above and below the frame should be about equal to the frame itself. (Light entering the lens from outside the image frame is an important contributor to lens flare.)
• Run Imatest and select Stepchart. Open the image file. If Automatic Zone detection is unchecked and the image is the same size as the previous run you'll be asked if you want to repeat the same ROIs (Regions of Interest). If you enter No, you'll be asked to make an ROI selection. It's OK to make a rough selection: another window will enable you to refine it. After you've made the rough selection, the Stepchart input dialog box, shown below, appears. To measure veiling glare, Automatic Zone detection must be unchecked, Veiling glare must be checked, and the number of patches must be set to one greater than the number of patches in the step chart: 21 in this case.

Stepchart input dialog box, set for veiling glare

• Click OK. The fine adjustment box appears. Press Lighten, just to the right of Zoom out to view the dark patches clearly. The Lighten button changes to Normal when the image is in lightened mode. It also helps to maximize the window and click the Coarse radio button (middle, left).

Portion of Fine ROI adjustment window; Lightened view ( Normal box is displayed)

• Click Yes, Continue to run Stepchart and calculate veiling glare.

Results

The first illustration shows the results for the Canon EOS-20D with the 24-70mm f/2.8L lens set at 70mm, f/8. Veiling glare is shown on the middle-right of the first figure.

with UV filter (not multicoated)

No filter: 23% lower veiling glare

Overexposed about 2 f-stops, with UV filter: Results are insensitive to overexposure.

These plots show the difference in linearity between JPEG and RAW (to TIFF) density response. The EOS-20D only deviates slightly from linearity (a straight gamma curve).

JPEG density response, showing nonlinearity in highlights

RAW/TIFF density response, showing good linearity in highlights

Most of the results are not surprizing. The Canon 90mm has the lowest flare, while the cheap Canon 28-80 is the worst. The only surprise is the excellent performance of the Sigma 18-125: a lens know for its fine performance and value, though its autofocus performance is mediocre. I use mine on manual.

Limitations

The veiling glare may be underestimated for telephoto lenses— especially if they are measured without lens hoods— because telephoto lenses form an image of only a small fraction of the light reaching the front element (the portion is much larger with normal and wide angle lenses). The target, as shown above, may not be large enough to simulate all the light that reaches the lens. If a hood is used, this error is considerably reduced.

If there are nonlinearities in the camera response at low light levels, the results may be incorrect (although relative results, i.e., comparisons, will still be valid). It's always safest to work with RAW images and convert them with a "linear" (i.e., simple gamma curve) setting. You can read many RAW formats into Imatest, using the dcraw converter.