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 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:
Try to center the object .
If possible do not use wide angle lenses, since they have more distortion than other lenses. When using wide angle lenses it may be useful to calibrate the lens to correct its distortion. See Lens Calibration.
Since the relation between pixels/real size will change along the line from the center of the lens to the borders, also calibrate the measurement taking a reference object, located near the same place where you will place the object to be measured.
If you plan to take more than one shot, put the camera on a tripod to keep the distance and angle between the lens and the object constant.
After setting a conversion factor, do not zoom in/out. If you do you will need to recalculate the conversion factor.
Place the camera perfectly perpendicular to the object and as far as possible from it. If the camera is not perpendicular and there area perspective distortions correct then using the Perspective window.
Using a flatbed scan instead taking
a picture with a digital camera, will improve the measurements accuracy
In some cases it may useful using food wrap plastic to flatten the objects against the sheet and keep them in place as the following pictures shows: