Win a Pentax Limited Lens: Raffle Now Live!

Falk Lumo blog: LumoLabs: Shutter-induced blur with an SLR camera

1. The mechanical focal plane shutter indirectly can increase the blur in an image. The exact amount of additional blur depends on the direction in the image. It is zero at a vertical contrast edge (aka yaw blur, blur due to yaw movement). And it is up to 11 µm (on average) at a horizontal contrast edge (aka nick blur, blur due to nick movement).
   
   The exact amount of average blur is shown in the opening figure of this article. It has its maximum for shutter speeds of about 1/100s to 1/80s. It is less than 5 µm for 1/25s and slower. Or 1/250s and faster.
   
   Note that any single image can be affected more or less. Add or subtract +/-50% to get an idea of variation from image to image.
   
   Note that one pixel is 5 µm large and the blur effect is only visible if all other sources of blur are very well under control (sharpening, defocus, shake, subject blur, lens abberation, noise etc.). Normally, these other sources mask the effect. Nevertheless, if you want tack sharp images then you need to understand the shutter blur effect.


2. The effect for the Pentax K-7 is larger than for the Pentax K20D. About 2 - 3x larger.


3. Mirror slap or shake reduction have no negative or positive impact on the effect. Shake reduction works as advertized but cannot counteract the perturbation from the focal plane shutter as it is too fast really. Mirror slap is very well dampened in the K-7 camera and has no negative impact on image resolution except on a weak tripod.
   
   There is a delay of about 10 ms between end of mirror slap and begin of shutter operation which suffices to keep the mirror slap perturbation out of the image.


4. The blur effect is an indirect one:
   
   First, the moving masses of the shutter (curtain etc.) make the body move (with surprising speed and acceleration of its stiff body!).
   
   Second, the body movements cause a classical blur effect lasting as long as the shutter works. The K-7 shutter is faster and stronger than that of the K20D probably increasing the effect by some 60% or so.
   
   Third, the body accelerations cause additional vibrations in the imaging sensor which last a bit longer than the first shutter curtain operates and which magnify the effect by another 60% or so.
   
   Preventing the first from happening (which requires a heavy and sturdy tripod) will kill the effect. There is no "loose" magnetically held imaging sensor and no negative direct impact from shutter curtain or mirror slap causing air flow in the mirror box or whatever.


5. In practice, you'll only see any effect with wide angle lenses.
   
   At about 1/100s you would normally have blur due to free-hand shake (we can ignore the case of a tripod as only weak tripods would cause any trouble with the shutter). At 50 mm and longer, the shutter blur will be masked and at 30 mm it will have comparable magnitude. It is at 10-20 mm that the effect will be noticeable most.
   
   In these cases, we highly recommend to shoot at 1/25s (or slower) and to enable shake reduction as it is highly efficient at such exposure speeds. The images will be sharper than at 1/100s!


6. Early efficiency tests of the K-7 shake reduction suggested that it may be ineffective at fast shutter speeds as required for long focal lengths. This was a preliminary conclusion we proved to be wrong.
   
   The Pentax shake reduction is effective even at 1/500s! It just cannot prevent the shutter blur at about 1/100s. We may soon publish an update to our SR guide reflecting this.