Capturing a Calibration Image
- Nathan Dunne
To create an Exposure Preset you will need to photograph a grey card under the same lighting conditions of your typical scan. An accurate measurement from the camera sensor to the centre of the grey card is also required.
Grey Card
Place a grey card close to where the subject is usually situated. M-XR recommends the Datacolor SpyderCheckr24.
Try to ensure the grey card positioned as close to perpendicular to the camera as possible.
You will need to know the grey value of at least one of the tiles, ideally a mid-grey, as a percentage for calibration.
See: Grey Cards | Known Values
Optimal Camera Settings
For best results, it is recommended to shoot -2.5 stops underexposed when spot metering on a middle grey card (50%). An under-exposed image will preserve the specular highlights, whilst maintaining a good amount of the shadows.
Be sure to take note of the exposure settings for future scanning sessions. The same settings should be used when capturing an actual subject, otherwise a new calibration should be created
Take Measurements
Measure the distance from the Light to the Grey-Card. This can be done by holding a piece of string from the lens to the subject, and marking the ends, then measuring it with a ruler.
When measuring the distance, include the distance from the front of the lens to the sensor by looking for the image plane symbol (ɸ). This distance can be measure separately, and added to the distance from the lens to the grey card.
Capture Image
Once an exposure value has been identified, take a RAW photograph. Save it with a descriptive name. This will be used in the Marso calibration process.