For this assignment I had to take A series of colour pictures with 9 black subjects and 9 white subjects (At least 90% has to either white or black ). From each setting I had to take two pictures one measuring the light with the camera on the subject the other by measuring the light on a gray card. The first picture is the way the camera sets the exposure the second when measurement is done on the gray card.White Balance had to be set to Daylight, which explains the yellow colouring.
(From Wikipedia, the free encyclopedia)
Gray cards are used, together with reflective light meters, as a way to produce consistent images in film and photography.
A gray card is a flat object of a neutral gray color that derives from a flat reflectance spectrum. A typical examples is the Kodak R-27 set, which contains two 8x10" cards and one 4x5" card which have 18% reflectance across the visible spectrum, and a white reverse side which has 90% reflectance. Note that flat spectral reflectance is a stronger condition than simply appearing neutral; this flatness ensures that the card appears neutral under any illuminant (see metamerism).
A major use of gray cards is to provide a standard reference object for exposure determination in photography. A gray card is an (approximate) realisation of a Lambertian scatterer; its apparent brightness (and exposure determination) therefore depend on its orientation relative to the light source. By placing a gray card in the scene to be photographed, oriented at a defined angle relative to the direction of the incident light, and taking a reading from it with a reflected light meter, the photographer can be assured of consistent exposures across their photographs. This technique is similar to using an incident meter, as it depends on the illuminance but not the reflectivity of the subject.
In addition to providing a means for measuring exposure, a gray card provides a convenient reference for white balance. White balance, or color balance, allowing the camera to compensate for the illuminant color in a scene.
Gray cards can be used for in-camera white balance or post-processing white balance. Many digital cameras have a custom white balance feature. A photo of the gray card is taken and used to set white balace for a sequence of photos. For post-processing white balance, a photo of the gray card in the scene is taken, and the image processing software uses the data from the pixels in the gray card area of the photo to set the white balance point for the whole image.
Most digital cameras do a reasonable job of controlling color. For the casual user, a gray card is unnecessary. Many serious photographers or hobbyists consider gray cards an essential part of the digital photography process.
Gray cards are made of a variety of materials including plastic, paper, and foam. Some photographers hold that any neutral white or grey surface, such as a white piece of paper, a concrete or stone wall, or a white shirt are suitable substitutes for a gray card; however, since bright white papers and clothing washed in typical detergents contain fluorescent whitening agents, they tend to not be very spectrally neutral. Gray cards specially made to be spectrally flat are therefore more suitable to the purpose than surfaces that happen to be available.
A gray card is only useful for setting or correcting the balance of neutral colors. Other charts, such as various color charts, provide standard reference patterns with calibrated reflectance spectrum and color coordinates, for use in adjusting color rendering in a larger range of situations
Interessante experimenten Erik! In beide gevallen lijkt me de gulden middenweg tussen de meting op het onderwerp en de kaart het mooiste.
Het verschil tussen de belichting van donkere en lichtere onderwerpen is wel mooi te zien zo trouwens.
goed experiment en ik hoop dat andere er wat van kunnen leren. Was dit een opdracht voor een opleiding?
Ja Niek ben een aantal weken geleden gestart me de Fotoacademie in Groningen.
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