Exposure Time and Unsharpness
Short exposure times are no problem, the longer the exposure time gets the higher is the risk of unsharp pictures for two reasons:
Subject motion means that your subject moves during the exposure time. This is more visible for subjects close to your camera than when they are further away.
A listing of specific exposure times in this context will always be inaccurate and just be a rough guidance as it depends on the situation and your expectations what will still be „sharp enough“.
My rules of thumb are: Above 1/60s I get cautious, pictures of people can already get blurred while they are speaking or making gestures. 1/250 or shorter is welcome for moving subjects like kids or sports. For really sharp pictures of fast movements you get quickly in the range of 1/1000 s or even less.
There is no way around trying yourself and gathering your own experiences if you want to judge about the risk of such motion blurs for you.
Camera motion means that the camera moves within the exposure time.
With millions of pixels on a tiny little sensor a movement by fractions of a millimeter are enough to create visible blurs.
Along lines with sharp contrast you can see such motion blurs like a shadow.
The risk of blurs due to camera motion is depending on the focal length – long tele lenses show a smaller picture frame and will get blurred more easily than wide angle lenses as you can already notice while looking through the viewfinder.
Again listing exposure times as thresholds when you will get motion blurs is difficult.
A well liked rule of thumb says that the exposure time should be less than the reciprocal of the focal length – so e.g. 1/100 s or less for a focal length of 100 mm.
But it is not that easy, this rule existed already decades ago, before the era of digital photography. All digital cameras except „full frame“ DSLRs are more sensitive to camera movements because of their smaller senser and for the same picture frame they have a shorter focal length.
But this rule is better than nothing and seems to be well liked because it is simple and gives the illusion of accurate figures.
On the other side: Almost all cameras or lenses have some kind of mechanism to reduce camera vibrations. If you consider this you have a chance again that this rule works.
And additional the blurs because of camera motion also depend on the following points:
- If you take a stable, still position of your body for hand held shooting and concentrate on holding the camera as motionless as possible
- If you are blessed with less shaking hands than others – there are photographers who take sharp pictures hand held where others get only blured pictures
- If you have good luck – give it a try, not all pictures get the equally blurred. I like to take three or four pictures in a row when I use longer exposure tims with a risk of blur and keep only the best.
So it applies here too: Gather your own experience, watch and learn what works for you. I use the rule above as a guidance for DSLR cameras that are not full frame and have a kind of vibration reduction; without I would be more careful.