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Kirk 20D L-Plate Tests

Table of Content

Overview

Test Cases

SRF/MTF Results

100% Crop Results

Additional Tests

Conclusions

 

Overview

The objective behind this test was to see if the additional fixing point used in the Kirk L-Plate design is effective for reducing mirror slap vibration in portrait orientation  as claimed by Kirk. On the way we discover how difficult it is to get really sharp images in portrait orientation with modern SLRs.

Readers not failure with this product will find the photo of the additional fixing point on the Kirk L-Plate informative. A small L-shaped bracket goes through the strap fixing point and is screwed to the L-Plate to provide a second point of stabilization and vibration suppression.

 

All tests are conducted using a 20D with EF 200mm f2.8L II + 1.4X II at f8 using the timer release.

The same sharpening was used as for the normal lens tests, the TRL Sharpening Toolkit PS Action "Capture Enhanced Edge Masked USM" 0.7, 0, 300 with a 65% layer blend). This should be just enough to correct for the sensors sampling aperture.

SRF/MTF measurements using my standard lens test software.

Updated with additional tests comparing the L-Plate and TMA-A tripod ring at f5.6 and f8.

 

Test Cases

The test methodology and sequence was as follows:

a) Remove the second mounting point screw. The little bracket could not be removed without taking the L-Plate off but I did not want to disturb the test setup. The little bracket was loose and so not acting as a fixing just by removing the screw. A single auto-focus action was made here to focus the lens on the target.

b) Take a "No Screw" reference shot with mirror lockup at 100 ISO.

c) Take the "No Screw" sequence changing the ISO to get different shutter speeds.

d) Replace the screw without disturbing the setup.

e) Take the "With Screw" sequence, once more changing the ISO to get different shutter speeds.

f) Take a "With Screw" reference shot with mirror lockup at 100 ISO.

 

SRF/MTF Results

Below is the SRF/MTF 50% plot showing the sharpness of the above test cases.

 

Sample SRF plots are shown here:

No Screw 1/4 Sec
No Screw MLU Ref
With Screw 1/5 Sec
With Screw MLU Ref

 

100% Crop Results

No Screw 1/4 Sec
No Screw MLU Ref
With Screw 1/5 Sec
With Screw MLU Ref

 

Additional Tests

As some of the results were counter intuitive some additional tests were conducted using the Kirk L-Plate (with second screw fitted) and the TMA-A tripod ring (actually a 3rd party copy of the Canon part).

Here, in each test situation five test shots are made with a new AF action per shot, from either MFD or infinity focus. This is to remove the chance of a outlier AF operation spoiling the results.

The following test sequences were conducted with exposures of around 1/4 second at f5.6 or f8 on a soft (carpet) surface and a hard (ceramic floor tiles) surface respectively with the listed average MTF 50% spatial frequencies in lp/mm. Note these values are the average of the vertical and horizontal line sharpness.

    Landscape Portrait
    Soft Hard Soft Hard
MLU L-Plate 53.9 50.0 33.7 33.4
TMA-A 52.3 46.7 32.8 31.8
No MLU L-Plate 49.9 47.8 23.3 20.0
TMA-A 42.5 17.4 19.2 17.3

And plots of each test case, again the average of the vertical and horizontal line sharpness for the soft and hard floor surface respectively. The Landscape TMA-A No MLU Hard Surface case looks suspiciously low and may be an outlier.

There is not much variation across AF operations (Trials on the graphs) we can look at just the first instance of the soft surface case for brevity.

First the SRF plots and then the 100% crops, so we can see if the softening is indeed confined to one direction.

    Landscape
MLU L-Plate
TMA-A
No MLU L-Plate
TMA-A
    Portrait
MLU L-Plate
TMA-A
No MLU L-Plate
TMA-A

NB In the SRF plots the labelling indicates the direction of the test line, so the line resolution being measured is orthogonal to that. So FV is the vertical line measuring horizontal resolution and FH is the horizontal line measuring vertical resolution. In retrospect not the cleverest bit of labelling.

 

 

    Landscape
MLU L-Plate
TMA-A
No MLU L-Plate
TMA-A
    Portrait [1]
MLU L-Plate
TMA-A
No MLU L-Plate
TMA-A

[1] NB the test target was rotated through 90 degrees for the portrait orientation tests, so for landscape results the mirror motion is up-down and for portrait it is left-right.

 

Conclusions

So the alarming conclusion is the mirror slap is actually worse with the additional L-Plate screw than without it.

This is easiest to the see from the SRF/MTF 50% plot. This shows the 100 ISO with screw and the two MLU reference cases are very close within the test method repeatability. However the without screw case is consistently worse at all tested shutter speeds.

So counter intuitive was this result that the time stamps on the RAW files were checked to make sure there had not been a labeling mix-up. A visual check was also made of the RAW files at 100% where the with screw cases indeed shows slightly less sharp than the others. The setup or lens focus can not have been changed during the test because the two MLU reference shots were the first and last shots made, and these are very close.

How significant is the difference? It is visually small and might be missed if just pixel peeping, however it does show up very clearly in the SRF data. We can see the degradation in sharpness is dominated by that of the vertical line suggesting this is due to mirror slap.

Do I have a justification for these results?

Not quantitatively. But qualitatively possibly. 

Vibration in mechanically complex objects is not a straightforward thing, there will be many vibration modes, each with a different resonant frequency and damping factor. Clearly the intent of the second fixing point is to inhibit some of those vibration modes. However it is possibly that by doing this the damping factor for the remaining modes has been reduced increasing the magnitude and duration of vibrations in that case.

Note that the general sharpness in this test is fairly poor anyway, compare to the other tests of this combination where the 50% sharpness should be around 55 lp/mm in the centre at f8 and the test shows around 35-36 lp/mm. 

However the later additional tests (all around a shutter speed of about 1/4 second) show that this lower resolution in portrait orientation, even with mirror lock-up, is typical, even when the TMA-A tripod ring is used. No mirror lock-up degrades sharpness even further, and much more over the landscape case; in fact the no mirror lock-up portrait case is clearly soft.

As can be seen from the additional test SRF plots, and in the more degraded cases in the 100% crops, the sharpness is only degraded in the mirror movement direction. Note that, unlike the old FD film SLRs, modern digital SLRs have vertical-travel, focal-plane shutters rather than horizontal. So any vibration from the shutter operation can be expected to be in the same direction as the mirror movement.

That portrait orientation suffers a sharpness loss even with mirror lock-up enabled was probably the biggest surprise of this exercise. Another surprise was that a ring mount was not noticeably better than the L-Plate in either landscape or portrait orientations, in fact it was slightly worse.

Although a relatively long lens has been used deliberately to show up any degradation it is not exceptionally long. I routinely use a 200 or 300mm lens in portrait orientation for flowers. 

Balancing this, the tripod and head used are relatively heavy and solid, much more so than the tripod/head I use in the field. The combination used in the tests was the Benbo Tripod and Kirk BH-1 Ball Head. Total weight with the head is just under 4Kg and a folded length of 36 inches, just over 0.6 Kg of this is the head. The legs are 2-section tubular Aluminium about an inch in diameter.

In all tests no extension was used with the centre column, only the additional Hard Surface tests had any leg extension which was about 1/3; so the configuration used was also stable.

This shows the importance of testing and not making assumptions!

 

Last Updated 13/07/2008

All Content 2005-16 Lester Wareham All Rights Reserved     

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