Tag Archives: spatial frequency

Bayer CFA Effect on Sharpness

In this article we shall find that the effect of a Bayer CFA on the spatial frequencies and hence the ‘sharpness’ captured by a sensor compared to those from a corresponding monochrome imager can go from nothing to halving the potentially unaliased range based on the chrominance content of the image projected on the sensing plane and the direction in which the spatial frequencies are being stressed.

A Little Sampling Theory

We know from Goodman[1] and previous articles that the sampled image (I_{s} ) captured in the raw data by a typical current digital camera can be represented mathematically as  the continuous image on the sensing plane (I_{c} ) multiplied by a rectangular lattice of Dirac delta functions positioned at the center of each pixel:

(1)   \begin{equation*} I_{s}(x,y) = I_{c}(x,y) \cdot comb(\frac{x}{p}) \cdot comb(\frac{y}{p}) \end{equation*}

with the comb functions representing the two dimensional grid of delta functions, sampling pitch p apart horizontally and vertically.  To keep things simple the sensing plane is considered here to be the imager’s silicon itself, which sits below microlenses and other filters so the continuous image I_{c} is assumed to incorporate their as well as pixel aperture’s effects. Continue reading Bayer CFA Effect on Sharpness

How to Get MTF Performance Curves for Your Camera and Lens

You have obtained a raw file containing the image of a slanted edge  captured with good technique.  How do you get the MTF curve of the camera and lens combination that took it?  Download and feast your eyes on open source MTF Mapper by Frans van den Bergh.  No installation required, simply store it in its own folder.

The first thing we are going to do is crop the edges and package them into a TIFF file format so that MTF Mapper has an easier time reading them.  Let’s use as an example a Nikon D810+85mm:1.8G ISO 64 studio raw capture by DPReview so that you can follow along if you wish.   Continue reading How to Get MTF Performance Curves for Your Camera and Lens

What Radius to Use for Deconvolution Capture Sharpening

The following approach will work if you know the MTF50 in cycles/pixel of your camera/lens combination as set up at the time that the capture you’d like to sharpen by deconvolution with a Gaussian PSF was taken.

The process by which our hardware captures images and stores them  in the raw data inevitably blurs detail information from the scene. Continue reading What Radius to Use for Deconvolution Capture Sharpening