Lens test procedure

 

These lens tests are carried out to look at the performance of a specific lens at different apertures and focal lengths. Remember that only one lens is tested so the results are only valid for this lens. Each lens is tested both in the field and using specific lens test charts.

The lens chart and the method used to test the resolution of the lens is the Norman Koren lens testing procedure. Ten 5mm charts were printed at 5 x 25 cm on an A4 Epson Glossy photo papers. These were then mounted on a YYxZZcm card board. Two were placed in the centre, one at each corner and one at each APSC-corner., see figure. The camera was mounted on a Gitzo 1558 carbon fibre tripod with an Arca Swiss B1 ballhead. The charts were photographed at 25x magnification, meaning that the lp/mm displayed on the charts has to be divided by two to give the right value. This means that the maximum lp/mm is 100.

The lens was focused manually using the focus magnifier on the Sony A99 on the centre resolution chart. Two exposures were taken at each full aperture stop using the 2 sec self-timer drive mode which gives a delay to reduce vibrations caused by handling the camera. The exposures were taken at ISO 200. The RAW files were converted to TIF using Adobe Lightroom 4 and the sine-pattern of the resolution charts were analysed using ImageJ and sfrcalc, see Norman Koren's site . The results were analysed and plotted using Matlab.

Measurements were made at the all the ten resolution charts. The results for the two centre charts are shown together with the mean of the four APSC-corners and the four outer corners. The standard deviation for the outer corners is also shown. A large standard deviation can be an indication of a decentered lens. These results were calculated for each full aperture stop.

Making sense of MTF

MTF (Modulation Transfer Function) describes a lens ability to resolve finely spaced black and white lines printed on a test target. As the lines get closer together they start to blur and blend together as the limits of the lens's resolving ability are reached. When the difference between black and white (the level of subject detail), which is near 100% on the target, reaches only 50%, then that frequency is the critical lens resolution at MTF 50%.

MTF is usually plotted for multiple levels of subject detail (Y-axis) at a number of points from the optical centre of the lens to its periphery (X-axis). The resolving power of a lens is expressed as line pairs per millimeter (lp/mm).

Resolution: MTF 10%

The minimum threshold of visibility corresponds to a variation in light and dark of about 10%. Thus, the MTF 10% critical values presented in these tests are reasonably comparable to evaluating a test chart to see where you can just make out the line pairs on the test chart when inspecting it under a microscope.

Contrast: MTF 50%

In practice, though, the 10% MTF does not correspond to the perceived sharpness of a lens very well. There is general agreement that perceived image sharpness is more closely related to the spatial frequency (lp/mm) where MTF is 50% (i.e., where contrast has dropped by half) than to resolution alone.

Details of calculating MTF:

1. The 5mm Koren 2003 lens test chart designed to be printed at 25 cm long (50X magnification) was downloaded from the Koren website and printed on glossy paper with a Epson 2700 printer. Charts are trimmed and mounted on the test chart as shown:

2. The chart is photographed at a working distance that is close to 1/2 the recommended distance so that the entire chart can be photographed for resolution and determination of 50% MTF. The method was developed for full frame cameras and lenses.

3. The 16-bit tif files are opened in image analysis software, ImageJ, to analyze the sine patterns on the chart (top band)

Click on "File" and then "Open" to select and open the tif of interest. A 16-bit grey scale image will be displayed.

4. Click on the "magnifying cursor symbol" to fill the window with the Koren chart image and click on the "hand" icon to move the chart image into the middle of the window.

5. Click on the line icon and draw a straight line through the upper sine pattern bar on the Koren chart.

6. Click on the "Analyze" menu and select "set scale" and calculate "known distance" based on sensor size and distance in pixels and enter results, set "units" as "mm".

7. Click on "Analyze" again and select "Plot Profile."

8. A sine wave pattern will be generated and displayed.

9. The file is saved.

10. The MTF is calculated using sfrcalc.

11. The data are plotted using a Matlab script.

The whole process is organized in a separate Matlab script with a GUI. The script organizes all the data from different cameras and lenses in different files and plots the results.

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