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Articles

2007-09-30
 

How do we test the lenses?

2. Evaluation criteria

BUILD QUALITY – We evaluate whether the body build is solid, how it fits to the camera, if it has a comfortable grip, if its elements move smoothly, as well as the overall weight and bayonet construction.

PICTURE RESOLUTION – The test chart picture in RAW format is converted (without any editing) to PPM/TIFF raw format by using the dcraw program. Next, using the Imatest program, we determine the MTF function courses based on spatial frequency (lpmm and LW/PH). The resolution is measured in vertical and horizontal values as well as in the center and at the edge of the frame (see the picture below).

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How do we test the lenses? - Evaluation criteria


CHROMATIC ABERRATION – Is measured by the Imatest program in points at a distance of 70% from the center of the frame (see the picture below). The measurements are made only for latteral chromatic aberration. We assume that all results below 0.04% are a negative aberration, 0.04 to 0.08% are a small aberration and that 0.09 to 0.14% are an average aberration, while from 0.15 to 0.20 is a large aberration and anything higher than 0.21 is a very large aberration.

How do we test the lenses? - Evaluation criteria


DISTORION – Is measured by the Imatest using the two diagonal ISO test chart lines (see the first picture below) as well as on an independent additional test chart specially prepared for distortion measurements (see second picture). The results are consolidated.

How do we test the lenses? - Evaluation criteria


How do we test the lenses? - Evaluation criteria


COMA AND ASTIGMATISM – Coma is measured using the pictures of the star or diode, placed in the middle or in the corner of the frame. To measure the astigmatism we compare the horizontal and vertical MTF50 values.

How do we test the lenses? - Evaluation criteria


VIGNETTING – We take a picture of a white and evenly lit space and then measure the vignetting (the light fall off in the corners of the frame) by using the Light Falloff option in the Imatest program. The light fall off is expressed in percentages and diaphragm values. We usually take four to five pictures at each diaphragm value with the camera being turned 90 degrees after each picture.

GHOSTING AND FLARES – We take pictures in bright contrasting light (sun, moon or lamps) and we evaluate the performance of the antireflective coatings with the unwanted light artifacts.

AUTOFOCUS – We evaluate the speed and loudness of the sharpness adjusting mechanism as well as the number of times the lens failed to accurately capture the target subject (for this we take into account the result in which the measured MTF50 value is more than 20% worse than the maximum achieved rate for the given diaphragm value). We also check if the autofocus has any tendency towards front or back focusing.

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