Grey card, matt surface, perfect neutrality, 18% reflectancy, 20x25cm (GC18)

The gray card is a tool for determining the lighting conditions on the stage. It is neutral gray, size 20x25cm, on a solid surface. 

Gray calibration cards are suitable for:

• determining the correct exposure
• measuring the contrast of the scene lighting
• color balancing in duplication procedures
• Protect the card from moisture, dust, light and especially mechanical damage.

Instruction Paper for Photo and Video Application of Gray 18% Calibration Card 20x25cm GC18

Grey card is designed for photographers to aid in determining lighting conditions on the scene and in balancing image in computer before print. The card has matt neutral grey surface with 18% reflectance (grey area reflects 18% of the light hitting it). Exposure meters (incl. digital camera meters) are calibrated to the reflection of an average scene so that the camera will record it in the optimum of the sensitometric/OECF curve. A grey card represents the standard reference value which all photo light meters are calibrated against. The hypothetic average scene and the grey card reflect the same - approx. 18%. Real scene does not have aver-age reflection very often, so it is necessary to use 18% card.

A) EXPOSURE DETERMINATION.
Place the 18% card so that it received the same light as your subject, without shadows and glares on card itself. Measure the exposure either by camera with built-in exposure meter, or by hand meter from the distance approx. 15 cm. When measuring, avoid casting a shadow on the card. • For artificial light, place the card close to the subject, aimed between the main light and camera. • For daylight place the card in front of the subject, pointing at the camera. You can measure the exposure with the card near your camera; be sure, that the card is in the same relationship to the light source as the subject. • If the subject is very dark, increase measured exposure by 1/2 or 1 stop; for very light subject decrease the exposure to the same extent.

The Grey Card is designed

• to determine EXPOSURE LEVEL
• to measure LIGHTING RATIO
• to evaluate COLOUR BALANCE
• to set up WHITE BALANCE

B) LIGHTING RATIO

is relationship between entire illumination (main plus fill-in) and fill-in illumination alone. This ratio should not exceed 3:1 (or max. 5:1 - accord-ing to the type of the final positive: either paper print or dia, black&white or colour, for exhibition or for print). Using 18% card you can determine your lighting arrangement to achieve the appropriate lighting ratio. For measurement, place the 18% card in close proximity of the subject. • Let all lights on to measure entire illumination of the subject. Turn the card to obtain the highest reading on your meter (mostly it is when the card is aimed towards the main light). Be sure, that no of your lights shine directly to the meter (you must switch off such light). Record the reading.

• Turn off the main light to measure fill-in illumination. Aim the card toward the camera. Record the reading.
  
• Compute difference of both readings in stops. The table will give you the lighting ratio:

C ) COLOUR BALANCING

• FILM CAMERA
  Place 1 8% card in your scene as a reference that records the colour of illuminator. Colour variations of lighting are the major reason for balance variance. Be sure, that the card is illuminated in the same way as the subject. By carefully placing the card it can be masked out of the finished print, or simply make two exposures one with and one without the grey card.
• DIGITAL CAMERA
  Take a shot using the same method as above. Open Photoshop; open your grey card image. Choose IMAGE > ADJUSTMENT > LEVELS, click Option, choose Middle Grey patch, set levels L=50, a=0, b=0. Position the eye dropper over the grey card and click. You should see an instant improvement in colour. Leave the LEVELS dialogue box open, then click on SAVE. Use this saved "Custom Colour Balance" for all shots taken under the same lighting conditions.

D) SETTING THE WHITE BALANCE (DIGITAL CAMERA)

Before taking a shot (preset):

• Set the White Balance switch on your camera to appropriate position (Custom/PRE). Point the camera to WHITE side of card (in the same way as for exposure determination (A)). Measure white readings and save them to "white memory" of the camera.
• Use this "Custom White Balance" for all shots taken under the same lighting conditions.