Megapixel suitability for test charts

March 1, 2018
March 1, 2018

Megapixel suitability is based on analysis of Modulation Transfer Function (MTF) obtained from a chart image captured using a 1:1 magnification lens (Canon 65mm f/2.8 1-5x macro) and a 6.5µm pixel size sensor (Canon EOS 6D). Conversion to megapixels is based on the Imatest Chart Quality Index (CQI) calculation which determines sensor height suitability using the equation:

2 * MTFxx (cycles per object mm) * vertical chart height (mm) where xx is 90, 70 or 50.

In order to consider performance throughout the camera’s range of expected sharpness, we photograph slanted-edges of transmissive and reflective substrates and take the sum of:

  • MTF90 (the spatial frequency where MTF is 90%) multiplied by 0.5,
  • MTF70 multiplied by 0.35, and
  • MTF50 multiplied by 0.15

Megapixel suitability calculations assume that:

  1. the lens is of high quality
  2. that the chart fills the vertical field of view (vFoV) of the camera system
  3. that the sensor aspect ratio is 3:2.

For 16:9 aspect ratio sensors (with pillarboxed framing, if applicable), multiply the megapixel suitability by 1.185.
For 4:3 aspect ratio sensors (with left/right sides of chart cropped), multiply megapixel suitability by 0.889.

Charts can be suitable for significantly higher megapixel counts if the minimum resolvable feature size of the lens is larger than the pixel size or the chart fills less than the full sensor vertical field of view.

Additions to Imatest 5.1

Imatest 5.1, to be released in April 2018, has some important enhancements that increase the megapixel suitability of most Imatest charts by up to a factor of 2. The MTF for most charts, which is a function of the chart media and printing technique, has been measured, and the measurements have been fit to a simple two-parameter function which can be used to correct MTF measurements by deconvolution (by dividing the measured camera MTF by the chart MTF function projected on the image sensor). The correction can be applied by entering an MTF correction file into the settings windows for any MTF calculations. For more details, see Compensating camera MTF measurements for chart and sensor MTF.

Chart Quality Calculator that uses the new MTF functions is also available. It provides a clearer and more accurate estimate of MTF suitability (including the expected MTF loss from the media without and with the correction) than the older Chart Quality Index.

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