On Sharpness, ISO and Shutter Speed

Started 8 months ago | Discussions thread
The_Suede
Contributing MemberPosts: 553
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Quite a lot of problem parameters...
In reply to Jack Hogan, 7 months ago

A mechanical resonance in a complex system is quite complex.

The problems with most sensor-stabilized systems are well known, and there's even been some formal attempts at quantifying them. Falk Lumo, also present here on DPR, among others have some material in the public domain concerning mirror and shutter vibration effect on image sharpness.

Recent cameras (modern constructions) have very low mirror-slap impact on sharpness, but stronger shutter-induced vibrations.

Since it's a damped resonance, you have a fast motion peak buildup and a slightly damped motion fade-out.

Some things that has an effect on this vibration displacement maximum in time is:

  • Lens>body system resonance (when hand-held)
  • Body flexibility between tripod plate and sensor-cage and lens mount (on tripod)
  • Sensor mount compliance/damping (for sensor-stabilized cameras)

Then different combinations of lens weight, mount-point (lens collar or camera), lens mount stability and so on further modulate the effect.

In general you have a vibration movement maxima coinciding with a point about 20-50ms after the first curtain hits the end stop. This means that the maximum vertical vibration effect is visible in exposures between 1/160s and up to 1/80s. As you get longer exposure times, the very short peak in vibration has less effect on the overall light throughput.

When the exposure is 2x longer than the time that fully encloses the full vibration peak (1/80-1/100s?) you have half of the exposure in almost stationary mode, also halving the shake PSF effect. At 1/20-1/25s you have pushed the shutter vibration two full stops down in effect, leaving just a slight lowering in MTF, and almost certainly no visible effect.

At 1/200s you're left with an effect that's equally strong, but only visible in the top half of the image..

On DSLR-type cameras with no sensor-stabilization, the highest amount of shutter-induced shake you get is typically when you shoot with longer lenses mounted to a tripod via the lens-collar foot.

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