Dealing with digital camera pictures

Some measurement tools make easy to normalize the way pictures are taken. Scanners, microscopes and other scientific measurement tools always take pictures from samples placed at the same distance or in the same environment. If you know  the DPI (Dots Per Inch in the scanned image) value and the microscope amplification factor, you can easily calculate the Conversion Factor, no matter if there is no known length object in the image.

Other tools, like digital cameras, produce images taken under far more variable conditions. The distance to the object may change from image to image, as well as the zoom or the camera lens. That makes more difficult calculating the Conversion Factor unless there is a known length object in the image that can be used to get the Conversion Factor.

Measurements taken from pictures produced by digital cameras hardly can be 100% exact. Unless you can normalize your shooting environment with great precision it should be best taking a measurement tape and performing such measurements in the traditional way.

To increase the measurement accuracy, place the camera perpendicular to the target and as far away from the object as possible, so perspective and lens distortion are negligible.

Place a known length object (like a ruler) near the middle of the object to measure (to reduce perspective inaccuracy) and keep that object perpendicular to the camera lens.

After the picture is taken, by measuring that known length object, using the Conversion window, you will be able to get the proper conversion factor for that image and measure real word units in the image.

If you must use a camera, please consider these guidelines: