Friday, October 4, 2013

Mirror-Slap Versus Shutter Shock

via Wikimedia Commons

Seasoned DSLR enthusiasts will be familiar with the phenomenon of mirror-slap, which is the vibration caused by the mirror swinging out of the way of the light path during an exposure. The problem with this is that it occurs while the shutter is open and the sensor is recording an image. If the exposure time is very long, then the vibration generally has little impact as the light is too faint to record during the time that the vibration's are subsiding. If the shutter speed is very fast, the exposure finishes before a cycle of the vibration frequency completes; again, the camera body movement is not recorded. However, within a certain shutter speed range, mirror-slap can be recorded, and will be exacerbated if the camera is mounted on a rigid surface like a tripod or a table top. Depending on the camera and the mounting, mirror slap's effects are most pronounced between exposures of 1s-1/60s. However, when performing long exposures, it's important to watch for bright light sources, which 'expose quickly':

Mirror slap. Note the 'dancing' street lights.


Given the improving quality of mirrorless cameras, many DSLR owners have been tempted into the possibility of shedding the weight of their systems for something lighter. One might be lead into thinking that the grass is greener with a Micro Four Thirds or Fujifilm X-System camera because there isn't a mirror to (literally) slap around. Lower safe shutter speeds for hand-holding, right? Nope, it's another case of "the grass is always greener on the other side of the fence."

Shutter Shock


Before you read further, one important caveat: Shutter shock is not the end of the world; it's one of those things that are a normal  part of operating a camera. Indeed, camera operation is all about managing compromises. Less discriminating people may not notice it as much, but this is by no means a strike against the many excellent mirrorless cameras on the market today. 

Mirrorless cameras (m4/3, NEX, X-System) have a problem to deal with that DSLR's don't, and that is shutter shock. To understand what it is, first consider how a mirrorless camera is different. Doing away with the reflex mirror and optical viewfinder, these cameras rely on the main image sensor to produce a preview image for either the rear LCD display or for an electronic viewfinder. That means that while you are composing an image, the shutter is open on a mirrorless camera, where as the shutter would be closed for the same on a DSLR. When you press the shutter button on an Olympus PEN or Sony NEX, the rhythm is:
  1. Close shutter - The imaging sensor gets emptied
  2. Open Shutter - The exposure begins
  3. Close Shutter - The exposure ends
  4. Open Shutter - Live view is re-engaged
Note that this is twice the movement of a DSLR shutter, which simply opens, then closes. The total motion is  not as violent as a DSLR mirror, but it's a considerable amount of movement in a device that is supposedly simpler than a DSLR. Note that the problem is in the extra movement of the first shutter closing; the final re-opening has no bearing on the exposure. The range of shutter speeds over which the blurring occurs ranges from 1/20s to 1/200s; some have reported that the peak effect is around 1/100s. Note that this is a wider range of shutter speeds than seen with mirror slap, and that the peak blurring occurs at a commonly used shutter speed.

Vibration damping over time.

The degree to which this is occurring seems to vary, with some cameras more afflicted than others: Panasonic G6 vs Nikon D5200 vs Nikon V1. In this example, the m4/3 camera is clearly demonstrating it, whereas it does not show up on the DSLR in live view mode to the same degree. The V1, also a mirrorless camera, appears to have the vibration damped out better than in the Panasonic.

A dissenting opinion is that not all instances of vibration loss are due to shutter chock, and that in some cases it's a case of the image stabilization system not working optimally. As described by Thom Hogan, this is the case when the vibration blur occurs at higher shutter speeds (when there isn't enough time to record the vibration of the camera body), or if it occurs in a burst sequence of otherwise well-defined images.

One way to completely avoid shutter shock in a mirrorless camera is to do away with the first shutter closing by using a completely electronic shutter operation. What this means is that the imaging sensor is "flushed" in the same manner in which it operates during video mode between frames. Certain Sony NEX cameras can do this: the NEX-5N, NEX-5R, NEX-6 and NEX-7.

Shutter Shock on a DSLR


So why don't Nikon and Canon DSLR's experience this when they are in live view mode? This is where the larger size of these cameras is beneficial. Because of the proportionally greater mass of the camera relative to the shutter mechanism, the shutter shock that would normally be visible in live view for these cameras is dampened out. That's the quick and dirty explanation anyway. In reality, the shutter on a mirrorless camera like an Olympus OM-D E-M1 or a Nikon V1 has less mass because it is covering a smaller sensor area, and the blades have less distance to travel for the same reason. The upshot is that while the camera is lighter and easily more jostled, the shutter blades are imparting less force on it. However, with a larger camera, there is more opportunity to dampen the vibration, and the shutter motors don't scale down with the sensor size. The upshot is that the total amount of motion that a mirrorless camera experiences is in greater proportion than in a DSLR.

A practical analogy would be to think of the ride-quality of a small car versus a large one. There are some very comfortably-tuned compact cars on the market, but in general, the mass of a larger car soaks up bumps better than a small one. In the case of a camera, say you have a Sony NEX and a Nikon D7100. Both are APS-C sensors, so both would have similar-sized shutter mechanisms, and roughly equivalent mass. Since Force = Mass*Acceleration, the force induced on each respective camera body is roughly the same. As the cameras have different mass, the acceleration (Acceleration = Force/Mass) that they experience from each is different. In the case of the heavier Nikon body, there is less acceleration from the same amount of force. Hence, less camera shake.

However, not all is perfect in DSLR land. Shutter Shock can also degrade image quality in a DSLR. As per the work done by Falk Lumo, the effect is noticeable around 1/100s. The long version of his work can be found here. I highly recommend reading to the end if you are technically inclined, as a lot of work went into the article.

What Doesn't Help


That would be the image-stabilization system. On every camera, whether it be lens-based or sensor-shift, it's designed to pickup movement of the optical system. Since you can't have one part that is good at all things, nearly all of the current image stabilization systems are incapable of counteracting it.

Theoretically, better hand-holding technique would not matter, but in practical usage it does. No matter how steady you think your hands are, you are always loosing some resolution to camera movement. By improving your holding technique, you are mitigating some of this loss... it's a case of gaining resolution by loosing less of it.

There's a heated discussion around whether or not the Olympus PEN E-P5 suffers from this; the OM-D E-M1 doesn't seem to be as affected. What's telling is that the DPR staff found that image shake was reduced when they used the touchscreen LCD to operate the camera, indicating the the camera is somewhat prone to unintentional vertical movement with the shutter button operation. (This is a common problem with many small camera)
 

How to Avoid It


Here are a few suggestions. Not coincidentally, they are the same suggestions that apply to DSLR's when avoiding mirror-slap.

  • Know the shutter speeds that are affected for your camera. 
  • Avoid these shutter speeds if outright image quality is your goal
  • Practice damage mitigation by improving your hand-held technique. Steady hands won't stop shutter shock, but there's no sense in loosing more image quality because of bad technique
  • If your camera has it, use the electronic shutter setting
  • Use a shutter delay mode. On Olympus cameras, this is the "Anti-shock" feature
  • Use a mirror-up delay mode if you are using a DSLR, as opposed to live view

1 comment:

  1. Actually, mirrorless shutters have the same total amount of movement as DSLR shutters. The shutter doesn't operate as one contiguous unit, there are two shutter curtains that move independently of one another. On some cameras, sometimes the first curtain is electronic rather than a physical curtain, but it's still a fundamental principle of how focal plane shutters work.
    http://www.dpreview.com/forums/post/39043130

    Mirrorless cameras exhibit more shutter shock due to their smaller size making them more susceptible to resonance, and because of one of the curtains closing and ramming into the opposite end of the frame just before the exposure begins, starting a vibration in the camera--whereas on a DSLR, both curtains are pretty much moving simultaneously.

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