A Study of the Effectiveness of Shake Reduction in the Pentax K7February 2010

P Smith

Introduction

Shake reduction, image stabilisation and optical stabilisation are all terms used to describe techniques to reduce image blur caused by camera shake. Pentax use the term 'Shake Reduction' to describe their technique. In the Pentax K7 this is done by mounting the image sensor on a movable plate which is moved at high speed to compensate for camera shake. The image sensor movement is controlled by circuitry which senses camera shake and sends corrective signals to an electro-magnetic controller to provide compensating movement to the sensor plate. When the shake reduction is inactive one can hear the plate move as the camera is rotated.

While the consensus is that shake reduction is effective there have also been many reports of perceived problems, mainly in the Pentax K-x but also in the Pentax K7. This study sets out to investigate how effective is shake reduction and in the process obtain a better understanding of the perceived problems. The author has a K7 so this study is limited to the K7.

Perceived problems

1. The sensor is loose and this can cause unsharp photos. This is a persistent belief and seems to have its origin in the fact that the sensor plate can be heard to move while the control circuitry is inactive.2. Mirror slap causes the sensor to vibrate at shutter speeds of between 1/125 sec to 1/80 sec. There have been many anecdotal reports of reduced sharpness at these shutter speeds. The same reports suggest that motion blur is well controlled at speeds between 1/20 sec and 1/60 sec. This has led to the belief that there is a harmonic resonance effect related to mirror slap leading to sensor vibration to peak around 1/100 sec.3. Shake reduction is ineffective. Here the problem may be that photographers expect shake reduction to be more effective than it really is, leading them to relax their care in holding the camera steady.

Objectives of the study

This study sets out to measure the effectiveness of shake reduction and to determine whether there is any substance to the perceived problems listed above.

Test methodology

The immediate problem was to accurately measure motion blur in the image. Fortunately a simple and accurate method can be found in the way lens resolution is measured. ISO12233 describes the 5 degree slant edge method. Essentially the edge spread of a 5 degree slanted edge is measured. This is differentiated to derive a line spread function and then processed with a fast Fourier transform to yield the MTF. It is reasonable to assume that motion blur will degrade the image resolution and that the degradation is proportional to the motion blur. So by measuring changes in image resolution one can obtain a measure of the motion blur.

While MTF is familiar to experts in the field it does not convey any obvious or intuitive

meaning to the normal photographer. So instead edge spread of the slant edge method will be used. This is mathematically related to MTF but has an intuitively obvious meaning.In this study the words 'blur' and 'spread' are used interchangeably.

The package QuickMTF was used to measure edge spread.

Test conditions

A Pentax K7 was used with a Sigma 50mm F2.8 macro lens. Photographs were taken of a five degree slanted edge at the center of the image field (to avoid the effects of field curvature) at a distance of 1.85 m. The camera was checked for front and back focussing errors. None were found. Camera jpeg images were used with no further processing. The camera was set to Fine Sharpness 2 +3 (because that is the author's normal setting). The images were processed with QuickMTF and the 10-90% edge spread was measured in both the vertical and horizontal directions.

For the bench mounted photographs the camera was rigidly clamped to a large and heavy workbench while for the handheld photographs care was taken to hold the camera as steady as possible, just as if one was using a camera without shake reduction. A short rest was taken between every five photos (because preliminary tests showed that muscle fatigue quickly degraded the accuracy of measurements).

Test protocol

The first stage was to measure the best image blur obtainable at each aperture setting. This was done with the bench mounted camera (mirror locked up, remote control) and was the average of 10 measurements. Effectively motion blur has been eliminated and this forms the basis for comparison against the remaining measurements. This is also called static blur in this study.

In the second stage a set of 10 photos were taken with a hand held camera at each shutter speed between 1/6 sec and 1/800 sec. This was done with both shake reduction set to On and shake reduction set to Off in the camera. Care was staken to practice the good technique appropriate to hand held photography.

Motion blur was computed by computing the difference between the image blur and the static blur at the same aperture setting. Motion blur is defined as the increased image blur resulting from movement.

In this second stage it became apparent that variability in motion blur increased quickly as the shutter speed became longer. This is plotted below in illustration 3. It also became apparent that motion blur reaches a peak at 1/80 sec, seemingly confirming the many anecdotal reports.

So in the third stage of the study further measurements were done at 1/80 sec and F5.6. The results are shown below in Illustration 4. Here 20 bench mounted measurements were made in the vertical direction with no mirror lockup and these were compared to measurements with mirror lockup to determine the extent of sensor vibration caused by mirror slap.A further 20 measurements were made with the camera hand held and compared with the bench mounted measurements to determine the extent of camera vibration cause by mirror slap.

Discussion of Test Results

Illustration 1. Static image blur measured at various aperture setting on a rigidly mounted camera to eliminate motion blur. Mirror lockup and remote shutter release.

These measurements represent the best possible case where there is negligible camera vibration and sensor vibration. These measurements show the limits of optical and sensor resolution for this camera/lens combination.

As expected, the lens gives its best resolution in the F4.0 to F10.0 range. Above F16 diffraction starts to limit the resolution. Below F4.0 optical resolution declines quickly.

So what do these numbers mean in practical terms? When viewing the digital image at a 1:1 ratio on a monitor it is unlikely that an image blur of less than about 1.5 pixel can be seen.

This raises the question of Circle of Confusion when viewing the image as a print. If one accepts the maximum diameter of cicle of confusion of 0.018 mm (APS-C sensor) for a 10x8 print, this translates into maximum 3.6 pixel diameter circle of confusion on the Pentax K7 sensor.

The end result, though, is effectively the sum of static blur and motion blur. Motion blur, if large enough, can in practice, take the image blur above the circle of confusion. This study examines the extent of motion blur.

2.8 3.2 3.5 4.0 4.5 5.0 5.6 6.3 7.1 8.0 9.0 10.0 11.0 13.0 14.0 16.0 18.0 20.0 22.0

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Static image blur and its relationship to aperture

Pentax K7 Sigma macro lens 50mm F2.8

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Illustration 2. Motion blur with Shake Reduction On and Off

Here we measure motion blur by sutracting static blur from the image blur (at the same aperture setting).

Motion blur remains below 0.1 pixel at shutter speeds faster than 1/250 sec with SR both on or off. At slower shutter speeds the motion blur gradually increases to about 0.3 pixel at 1/125 sec, again for both SR on or off. At this point SR On and SR Off part company.

Motion blur with SR Off increases steadily, reaching 2.5 pixel at 1/13 sec, as expected, then increases sharply to 7 pixel after that. The shape of this curve is very dependent on the skill of the test subject.

With SR On we see a different story. First motion blur flattens out between 1/80 sec and 1/125 sec, reachin a local maximum of 0.39 pixel at 1/80 sec. After that it declines to about 0.1 pixel in the range 1/20 sec to 1/60 sec. From 1/15 sec onwards it increases to about 0.3 pixel motion blur and below 1/8 sec increases sharply to 1.6 pixel.

From this we can conclude that shake reduction is effective down to about 1/8 sec (with good technique). The moderate increase below 1/20 sec can be attributed to physiological tremor which has a major peak at 8-10 Hz, a smaller peak at 15-25 Hz and a minor peak at 3-5 Hz. The large increase below1/8 sec is probably caused by the subject's body sway

1/6 s1/8 s

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Motion blur vs shutter speed with Shake Reduction on and off

Hand held Pentax K7 with Sigma 50mm F2.8 macro lens

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Motion blur = Image blur – Static blur

Static blur measured with the same aperture and test target with camera clamped to a heavy workbench using Mirror Lock Up and remote shutter release.

Average of 10 measurements made at each shutter speed in both X and Y directions.

where the amplitude exceeds the sensor correction range.

It also can be concluded that the local peak at 1/80 sec is a continuation of the general upward trend in motion blur that started at /320 sec.

It is probable that the sensor correction circuitry has a low pass filter that starts to act at about 1/125 sec and comes into full play at 1/60 sec.

From this it appears that there is no harmonic resonance causing sensor vibration as many have speculated. It is simply the effect of a low pass filter without an abrupt corner frequency that starts to reduce motion blur at an approximate shutter speed of 1/80 sec. To clarify this more measurements were made at a shutter speed of 1/80 sec. See illustration 4.

Illustration 3. Variability of motion blur with shutter speed.

From examination of the individual measurements it became apparent that measurement variability followed the same pattern noted above. This is plotted in Illustration 3 and the same conclusions as above apply. With Shake Reduction Off a larger proportion of the photos are affected by a high level of motion blur.

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Variability in motion blur vs shutter speed with Shake Reduction on and off

Hand held Pentax K7 with Sigma 50mm F2.8 macro lens

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Variability was calculated by computing the standard deviationof the measurements and multiplying by two. 95% of allvalues fall within this range.

Illustration 4. Image blur measured at 1/80 sec

Now we examine the response of the system in more detail at 1/80 sec.

Here we see that the base or static blur is 1.11 pixel. This increases by 0.21 pixel to 1.32 pixel if the mirror is not locked up but the camera is still rigidly mounted, eliminating camera vibration. From this we can conclude that there is a motion blur of 0.21 pixel in the sensor caused by mirror slap. This is the same whether SR is On or Off because with no movement in the body SR will send no corrective signals to the sensor.

When the camera is hand held (with SR On) the motion blur increases by 0.43 pixel to 1.75 pixel. From this we can conclude the motion blur caused by uncorrected camera vibration is 0.43 pixel.

Bench mounted with MLU

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Image blur in vertical and horizontal directions at 1/80 sec

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Vertical blur caused by camera vibration with SR Off = 1.0 px

Vertical blur caused by cameraVibration with SR On = 0.43 px

Vertical sensor blur caused byMirror slap = 0.21 px

Base image blur without sensor orCamera vibration = 1.11 px

Average of 20 measurements inboth X and Y directions1/80 sec at F5.6

How do we know this is camera vibration and not hand movement? Because the increase only takes place in the vertical direction and not the horizontal direction.

However if SR is turned off the camera vibration increases motion blur by a much larger 1.0 pixel to 2.32 pixel.These results confirm the earlier conclusion that there is no harmonic resonance at about 1/80 sec causing sensor vibration.

Conclusions

1. There is no evidence that sensor movement causes unsharp photos. Motion blur attributed to sensor movement is no more than 0.21 pixel, a negligible amount.

2. There is no evidence that mirror slap resonance causes unsharp photos around 1/100 sec shutter speed. The small local peak in motion blur is probably a property of the low pass filter in the sensor correction circuitry. A suggested improvement would be to raise the cut-off frequency of the low pass filter.

3. Shake reduction is very effective provided that the photographer pays due attention to good technique for hand-held camera shots. These results demonstrate a 4 stop gain (Pentax claim a 2.5 to 4 stop gain).

4. Measuring motion blur is simple and accurate. There is no reason why camera review sites such as Dpreview.com should not adopt a more objective testing procedure.

Caveats and limitations

The study was done with one camera and one test subject (the author). The results might not be generally true for other cameras and other test subjects. Many hundreds of photos were measured and that experience showed the results were accurate, repeatable and consistent. As an example, 10 measurements of motion blur at 1/80 sec (averaged in the X and Y directions) gave a result of 0.97 pixel. The measurements were repeated 5 days later, using a sample size of 20 (in both X and Y directions) and this gave a result of 0.98 pixel which is in close agreement. The author is therefore reasonably confident that the results are generally applicable and reproducible. It may be that some cameras have a defect that increases motion blur in which case the author is the lucky possessor of a good camera.

Further study

This study should be repeated with another K7 camera and test subject to confirm these results. To repeat the study with other brands of camera would also be very informative.

Early on in the study the author found that muscle fatigue quickly degrades image sharpness when taking hand held photographs. This should be studied further.

The prior expectations of the photographer may also have a large bearing on the outcome. It is hypothesised that Shake Reduction technology leads photographers to relax their standards of care and this in turn is a cause of unsharp images. A study to test this hypothesis should also be carried out.