RPP/ACR 6.1; round whatever

Started Jul 14, 2010 | Discussions thread
gollywop
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Camera profiling in ACR
In reply to malch, Jul 17, 2010

In what follows I have been strongly influenced by Iliah Borg and by my own experiences.

Lighting:

Ideally, one would employ an appropriate lighting arrangement in a studio, where the lighting temperature can be controlled. For very exacting work, this may be beneficial. If you don't have a studio (I don't), good ol' sunlight works to excellent effect. That's what I use.

Studio Lighting: place two appropriate bulbs in good reflector shields each angled at 45° to the target. Make sure the lighting is completely even on the target with no stray reflective light. The camera is placed square before the target, bisecting the right angle formed by the two lights. The Solux 4700 °K, black-backed, bulbs are good for the 5000 °K illuminant.

Sunlight: Shoot the target on a nice clear sunny day. A very slight, even haze doesn't seem to hurt, but, unless you're making a cloudy-day target, stronger overcasts are to be avoided. Defined clouds are definitely to be avoided since they alter the lighting minute-by-minute. Place the target at a 45° angle to the sun with its face roughly perpendicular to the ground, and set the camera directly orthogonal to the target face. The target should be placed in direct sunlight, not shadowed or filtered through trees, and nothing nearby should produce reflections or color casts. Eyeball from the camera to make sure the target elements show no glare (reflections) -- this is particularly a problem with the ColorChecker SG (semi-gloss) target. I set my target on a music stand (firmly attached) to allow easy adjustments of angles and height.

The best time is either in mid morning or mid afternoon. The 9:30-10:30 a.m. and 2:00-3:00 p.m. ranges are nice. But it's better to avoid the harsh, high-angle light of high noon (Standard Time). Wind that moves the target is also to be avoided.

You can make different targets for different sunlights (bright daylight, cloudy, overcast, shade). I've done this but find that a well-made target for typical daylight (5000-5300 °K) makes an excellent overall profile.

Target:

The Gretag MacBeth 24-patch color checker (CC24) has been a standard for this work, but the newer X-rite ColorChecker Passport (CCPP), with similar 24-color array, appears to be more accurate. It is also much easier to make flat. The GM CC24 has got to be glued or tacked down to a rigid flat surface.

• Place the target over a larger gray background so a gray border of at least a couple of inches shows. This isolates the target from its support and, more importantly, helps to check for evenness in illumination (see below). Neutral gray matte boards from frame shops can serve here.

• Be sure the target is perfectly flat. The natural curvature of the CC24 can really mess things up.

• Set the target at 45° to the sun with its face perpendicular to the ground.

• Place the center of target directly perpendicular to the lens axis in a place free from reflections and/or color casts.

Exposure:

• Place the camera on a tripod. Have it set (or zoomed) so that the target (color grid) occupies the center third of the view (horizontally and vertically). Iliah suggests using a simple lens, but a sensible alternative is your favorite lens if it is frequently used.

• Use Manual Mode with a medium aperture, say 5.6.

• Be sure lens and target are clean.

• Set WB. Don't use auto so it won't change on you between different exposure shots. Iliah likes to set the WB exactly here, but I don't find this makes any real difference. Both profile makers are going to set the WB based on the second-lightest gray patch anyway.

• The target should appear centered and rectangular. Make the bottom or top of the target edge parallel to the horizontal helper line in the view finder.

• Use manual focus (you don't want AF shifting between shots) and place the target very slightly out of focus.

• Take a series of 5 or 6 exposures, varying shutter speed one increment at a time, that range from under to overexposed.

Processing:
• Convert the target images to DNGs -- both profile makers require this.

• Bring the images into ACR with completely flat settings (everything zero), no contrast behind the scenes, no sharpening, no nuttin'.

• Set the WB with the second-lightest gray patch, and check the level of the white patch.

• Pick the image whose white-patch level in the prior step is roughly 250. I have varied this from 230 to 250, but never above 252. The color results are virtually identical, but it does cause the default brightness and saturation to differ slightly in processing new images. Since you're going to be making adjustments to these anyway, this is an inconsequential difference.

• Read the levels of the gray background border near each of the four corners of the color checker to insure they are essentially the same (within a 6-point range). This is important. If they are not, you're back to square one because the illumination is inadequately even and your results are problematic. Iliah recommends checking this fact with a test shot before doing the exposure sequence, but I'm a gambler. Doing this check first guarantees a trip to the computer and back, but if everything works right from the start, you save this trouble.

• Process the chosen DNG target image in either the Adobe DNG Profile Editor or the ColorChecker Passport profile maker. Either one should automatically install the profile in the appropriate folder to be recognized by ACR for the specific camera.
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gollywop

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