In keeping with the idea of this site, to note here things that I either refer to, or crops up during forum discussions or the like on a regular occurrence, this is some information that I keep in my notebook and look at often.
A matter that is raised daily in photography discussions is that of movement blur - often stemming from comments that a lens is soft or questioning why a particular shot isn't sharp etc. Movement blur, for the purposes of this discussion is the movement of the subject within the photograph - the other type of movement that is often captured by a camera is that of the photographer themselves, moving the camera within the duration of the shutter actuation - that tends to be referred to as camera shake, but confusion between the two often happens and it's perfectly possible to have both within the same image.
Whilst camera shake tends to occur in slower shutter speeds - and the rule of thumb is that the longer the focal length of the lens, the faster you need to keep the shutter speed in order to eliminate photographer movement being recorded - usually accepted as the reciprocal of the 35mm equivalent focal length - movement blur is determined by the speed your subject is moving at - a posing family group will offer less of a challenge to this than a fast moving athlete or racing car.
These are some rough speeds I've noted over time for potential subjects, from which you could estimate other activities - I've included some less common ones to give some relative comparisons. Below, I'll outline the way I work out the distance travelled during the short time periods involved in shutter actuations.
- Strolling along ~ 2mph
- Average walking pace ~ 3mph
- Speed walking ~ 5mph
- Olympic Walking ~ 9mph
- 100m sprinters ~ 27mph
- A fast racing dog ~ 35mph
- A dog running and playing ~ 15mph
- Car in urban traffic ~ 30mph
- Racing car ~ up to 225mph
- Road racing cyclist ~ average 37mph
- Horse trotting ~ 10mph
- Horse at a brisk gallop ~ 25mph
- Horse racing ~ 35mph
So how far travelled is that while the shutter is open?
This is my own calculation, there are other ways of working it out, but this just happens to be the way I remember to do it. There are 63360 inches in a mile, so I use this calculation:
Inches travelled while shutter open =
63360 x speed (in mph) ÷ 3600 to covert to seconds ÷ shutter speed.
So a 30mph subject in a 1/100 second shutter speed would be:
63360 x 30 =1900800 inches in an hour ÷ 3600 = 528" per second ÷ 100 = 5.28 inches in a 1/100 second shutter actuation!
This is a rather simplified view, distance from the subject and other factors will play a part in how successful the shot appears too, but this gives some guidance as to why that 1/30 second shutter speed shot rendered your active toddler as a blur!
Let's add some shutter speed times to that list:
The right hand column is how far that subject would move during a 1/100 second shutter actuation:
|Average walking pace
|A fast racing dog
|A dog running and playing
|Car in urban traffic
||Up to 225mph
|Road racing cyclist
||Average 37 mph
|Horse at a brisk gallop