X Rite Color Passport and DPMx?

Started Apr 5, 2013 | Discussions thread
yvind Strm
Veteran MemberPosts: 4,129
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Re: Clarification and misconceptions.
In reply to cquarksnow, Apr 9, 2013

cquarksnow wrote:

yvind Strm wrote:

I know Gunther Borgermeister calibrated his SD14s, but found he needed to use a 144 patch gretag card for getting correct colors. I also think it was him that said that you could not calibrate the camera once and for all, but that each different light setting required re-calibration.

Also think Gary Mercer did calibrations (also with SD14), using 16 patches.

A few misconceptions in this thread:

1. A gray card has nothing to do with calibrating a camera. Grey card is for WB adjustment (and/or exposure measuring), and can never replace calibration. (Only if the camera is perfectly calibrated, of course). A WB adjustment influence ALL colors in the scene. This also applies if you use a grey card and Custom WB.

2. Calibration is used to correct individual colors (not all, of course, but hopefully the adjustments also influence the rest of the colors correctly). It is vital for correct color reproduction with Foveon. (and for most other cameras) WB will just shift all colors.

2. Grey cards are 12% (at least they should be), not 18%. Light meters, handheld and in-camera (film and digital) tries to match 12%.

3. A grey card is not a grey card. Get a good one, because lighter or darker grey might cause color shifts.  I can´t seem to dig up a Imatest for the DPxm series, but the Imatest for the SD14 shows that white to black points varies - meaning that colors (wb) will change if you click on different shades of grey (and white).

Kind regards

Øyvind

Hi, Yvind -

Indeed it's 12% and one can check this if any doubt http://www.bythom.com/graycards.htm
By the way I would like to know your opinion on neutral rather than standard color setting with the DP*M when it comes to taking photographs of color patch cards.

Sorry, I have no knowledge of the differences between the various color modes. I only have the SD14.

If I had one of the cameras with color mode, I would certainly photograph a color patch card in all modes under equal conditions. And probably not only one scene, but daylight, flash, indoor.

This is to gain knowledge of how the various modes influence different colors in various conditions. As you may have read, people here have some recommendations about color modes for different types of photography. Absolutely vital information to know the behavior of your camera.

I supose you would pick the color mode being closest to correct colors for a particular setting.

Also is there anything wrong in your view with doing color balance, e.g. using CBL lens for the DP*M ? http://www.cbllens.biz/gallery.htm

Absolutely not. But there is a cave cat. Even some pros gets this wrong.

You can use an Expodisc or (similar) in three ways.

  1. At camera position, pointed at the subject.
  2. In subject position, pointed at the main light source.
  3. In subject position, pointed at the camera position.

To kill the suspense, number 3 is the correct one. (The other two might have some limited use, if one knows why)

To understand why:

With 1, you do not measure the Color balance of the light, but what is reflected from the colors of the subject. In principle, with that method, you would be just as well off by using AWB on your camera, and save the money.

With 2, if there is a really dominant light source, it might work just fine. BUT, very often the reflections of other objects also plays a role - like green grass in front of the subject.

With 3, CWB algorithms of the camera will see all light that is reflected on the camera with the WB device (aka the subject).

An example: You want to take portrait of someone in front of a blue wall. The subject is standing on a large green lawn. The person is lit by direct sun, and so is the wall and the grass.

With method one, you point the camera at the subject, hold the CBL device in front, and do a CWB. The algorithm will pick up the color of the reflection of sunlight and the green from the face.  So skin color, and how that reflects matters. This is the same as the camera do in AWB (without the CBL). This will probably reflect as some Yellow-green, so the WB algorithm will compensate with magenta. To make matters worse, the CBL will see to that the background color also is considered. The camera will try to balance the strong blue color with some red-orange compensation. All in all you will probably get a pretty red skin out of this. Pretty useless CWB in any case.

With method 2, you will measure the color balance of the sun, and not from the reflected green from the grass. The background is not considered. In this case, camera is not aware of the green, and green it will be. In cases where secondary and tertiary reflections are weak, method 2 will work. Typically, you will use method 2 if you need to force the camera to consider only one of several sources.

With methode 3, the reflected light (with its color) is also considered.

This is exactly the same way when using the CBL device for metering. The exact same considerations applies, only for exposure. This is the way handheld meters is use, to measure incident light. Again, pointed at the camera. A studio photographer will also use 2, to see differences between light sources.

On a side note, for a professional, critical shot, a photographer might want to eliminate secondary unwanted reflections with gobs, reflectors, extra (possibly mired) light.

Hope this was clear enough.

Thanks,

Chris

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Kind regards
Øyvind

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