We measure the Modulation Transfer Function (MTF) of a camera (body and lens) according to the ISO 12233 standard SFR method (see MTF measurement definition). The target is a pattern of white and black squares tilted at a 5° angle and filling the field of the camera.
The target designed by DxO Labs is produced using a high-resolution printer to achieve sharp transitions between black and white areas without aliasing. We attach the target support to a frame made with aluminum profiles to provide the necessary rigidity to the target assembly.
We illuminate the target uniformly, with halogen lights filtered to produce a daylight color temperature of 5500K.
To guarantee absolute stability and prevent any motion blur, we fasten the camera on a geared head fixed to a heavy-duty studio stand. A graduated rail on ball bearings permits very accurate adjustment of the distance between the camera and the target.
To further ensure that camera vibrations do not affect the measurements, we use the reflex mirror lockup function (when available), and release the shutter with a remote control or self-timer.
Before shooting, we set the parallelism between the sensor and the target planes using a mirror flush against the target: Perfect alignment is achieved when the reflected image of the lens appears in the center of the camera viewfinder.
We select the lowest actual ISO speed of the camera to acquire images with a minimum level of noise. We set the exposure so that the white squares of the target are just below sensor saturation in RAW format, to ensure that the entire dynamic of the sensor is used. Of course, we deactivate all sharpening options and stabilizing systems of the camera or lens.
For each focal length and aperture of the lens, we take pictures at 60 different focusing positions around the focusing point set by the camera's autofocus system. We then use the sharpest image to measure the camera’s MTF.