Red-Blue Ratio – Aperture Sharpness - Reikan FoCal Help

Red-Blue Ratio – Aperture Sharpness

(See Chart Features for general chart features - changing scales, copying to clipboard etc)

The Red-Blue Ratio gives a view of the image quality difference between the red and blue colour channels.


  • Red markers – Red-Blue Ratio result

A positive value indicates the red channel is sharper.

A negative value indicates the blue channel is sharper.

Basic Interpretation

Ideally, a lens will focus all colours to the same point on the camera sensor, but in reality, this does not happen perfectly. More expensive lenses with dispersion correction elements (aspherical, fluorite etc) will do a better job of focusing the various wavelengths of light.

The Red-Blue Ratio chart shows the difference between the sharpness in the red and blue channels. A point above the zero line has a sharper red component, and a point below the zero line has a sharper blue component.

Note that this chart does not indicate the absolute sharpness, just the ratio between the red and blue values.

Astigmatism Factor – Aperture Sharpness | This Page | Red-Blue Quality – Aperture Sharpness