As said in the m/s photobook, The speeds of the objetcs represented in pictures must be learnt outside the pictures themselves; there’s no way of knowing those speeds by the study of pictures’ metadata. What is feasible, albeit not practical, is determining the critical shutter speed to freeze the movement of the objects captured. This has no real usefulness but can be interesting.
Factors influencing the critical shutter speed for freezing movement of the object on a picture are:
- Visual acuity (VA = 0.2 mm). It is considered normal vision (20/20) a resolving power of 0.2 mm at a distance of 25 cm. In such a case, 2 points at a distance less than 0.2 mm would be seen as one point.
- Eye to print distance (E2P = 25 cm; from a 21x14 cm print).
- Object to camera distance (O2C).
- Object trajectory to focal axis angle (O2F). From 90 (perpendicular) to 0 degrees. The quotient of the angle of trajectory divided by 90 is to be multiplied by the real speed of the object to obtain its apparent speed, which is what will influence the result of the capture.
- Appearent object speed (AOS). As said, it equals to Abs(O2F/90) * real speed.
- Crop Factor of sensor (CF). APS-C = 1,5; Micro 4/3 = 2. FF = 1.
- Lens Focal Length (F) (mm).
These variables can be grouped so that partial calculations can lead sweetly to the final result.
Distances must be treated together. The first that must be determined is Df (Diagonal of the frame “seen” by the diagonal of the sensor at D distance with a lens of F focal distance): Df = (Ds * D)/F.
From here, Proportion 2 (Df/Dc) determines what distance in the frame represents the 0.2 mm of the 25 cm copy seen at 25 cm. Those 0.2 mm will distinguish frozen from blurred object rendition. Let’s call the corresponding distance in the space framed by the picture (CDf).
The inverse of the time needed by the object to span the CDf distance is the critical shutter speed. In the first column of calculations of the next excel figure, critical shutter speed has been calculated to be 1/965 s. That exact speed will probably be unavailable with any camera; with a Fujifilm X-T1, the nearest speeds available are 1/1000 s and 1/800 s. Capturing with the former, the object will look frozen; capturing with the latter, blurred.
And that’s all. There is blurring from movement when some point of the image has moved 0,2 mm or more on the 25 cm diagonal print due to movement of the object.
NOTE: We have made all calculations in meters per second (m/s).