Hi!

Thanks for advice and information. I examined suggested shutter speed calculators and formulae. I thin that the formula recommended by the site

Shutter speeds to freeze subject movement might be the most appropriate, at least for preliminary calculations. I a rearranged the formula a little to get shutter speed in seconds:

Shutter speed <= [[Circle of confusion (in mm)] / 1000] x Distance to object (in m)] / [Object’s speed x [Lens focal length (in mm)/1000]]

I input this formula in Excel and got sensible results. It interesting, that this formula yields the traditional “1/focal length rule” if the circle of confusion is chosen 0.02 mm, the distance to object is chosen 25 m and the object’s speed is chosen 0.5 m per second. So this rule has quite sensible foundation.

It also interesting that this website suggests that the smaller circle of confusion should be used than in standard DOF calculators, for example 20 microns (0.02 mm) instead 30 microns for full frame sensors and 13.3 microns instead of 20 microns for 1.5 crop factor sensors. So I presume that these reduced values for circle of confusion allow getting sharper pictures.

However, I noticed that the very online calculator of minimum shutter speed works correctly only if the distance is measured in meters and speed in m/sec. If other units of measurement are chosen, especially in miles/hour, then the calculator yields incorrect results.

I tried to carry out calculations for my situation with a bus. I assumed that the bus speed is about 90 km/h our 25 m/s and the distance to object is about 50 meters. As I have a Pentax K-x, I used circ of confusion 13.3 microns or 0.013 mm. I chose the focal length 18 mm (the wide “end” of my standard kit lens). According to my rearranged formula, I got that the shutter should be 1/675 or faster. So I can set the camera to TV mode and choose shutter speed 1/675 or something faster and try. Of course, to get a correctly exposed picture I also should set correct aperture and ISO as well as take into account DOF aspects. But it is a different issue. However, I think that it daylight time shutter speed 1/675 allows achieving normal exposure without extremely wide aperture or extremely high ISO.

If I want to shoot objects that are located 25 m from bus, then the minimum shutter speed is 1/1350. This shutter speed is harder to handle if it not very sunny day. If want to shoot objects with focal length 35 mm, then even for objects 50 m away the minimum shutter speed will be 1/1313. So it seems to be very complicate to shoot quite near objects from moving bus with larger focal length.

I hope that I have rearranged the formula correctly and that there are not errors.