lens focal length greatly influence critical image sharpness.
Why? Very simple.
The more you enlarge the picture using a zoom or telephoto lens, the more you magnify camera shake. Simple as that.
One golden rule that you can apply to all photos is to use a shutter speed that matches the focal length set on the lens. If you don’t match the focal length, chances are your result will be blurred or shaky.
Here are some indicators of an ideal shutter speed to match a range of popular lens focal lengths.
The Crop Factor
One feature of all consumer cameras (i.e. non-full frame cameras) is a phenomenon called the crop factor. It could also be termed the magnification factor.
Lenses make circular pictures. Cameras take a rectangular chunk out of that circular image. If you use the same lens on both a full frame and a smaller APS-C sensor camera you’ll note that the resulting APS camera photo looks approximately 1.5 times bigger, as if you have zoomed in on the subject (this is because the frame is smaller than the 35mm sensor).
So, the smaller sensor picture looks like it has been cropped into, hence the name. FourThirds cameras have smaller sensors. Their crop factor is two times (2x). In effect you get more bangs for your dollar. If you have a 100-400mm lens, on a full frame camera this is 100-400mm but on an APS-C consumer camera, this is equivalent to a huge magnification 150-600mm lens. On a FourThirds camera its double to focal length: 200-800mm.
Sharpness TIP#1: Always shoot in Hi-Speed Drive Mode. This not only gives you a range of shots to choose from but critically, the first shot might be blurred by your pressing the shutter. The following shots will not suffer that same initial movement error...
Sharpness TIP#2: Shoot in Aperture mode. For the fastest possible shutter speed in the light available, open the aperture to the max (i.e. to a small f-number). If this is not enough, increase the ISO rating.
Increase ISO = Instantly Sharper
If you keep an eye on the shutter speeds and simply can’t open the aperture or raise the shutter speed further because the light is defective, increase the ISO. This is the easiest, fastest and most effective way to boost a shutter speed.
Double the ISO and you double the shutter speed...
Monopod = Instantly Sharper
Stick the camera on a monopod and you can get away with a shutter speed of approximately half the recommended speed (providing the subject is reasonably static). Monopods are good because they are compact, lightweight, use the same head that you might already use on your tripod, yet are manoeuvrable and incredibly supportive, especially when using heavy telephoto lenses.
Flash = Sharper pictures
You’ll note that many wildlife photos appear dark, or shadowy, especially in the eyes. Use flash and at the barest minimum you’ll punch a tiny keylight into the animal’s face. Better still, a small burst of flash will also lighten the shady parts of the subject without over-brightening the critical highlights...
There are always factors that can make your results look considerably worse:
Extreme subject movement
Incorrect ISO settings
But there are techniques to make the image sharper:
Tripod or Monopod
Multiple shots in Drive Mode