Lens Reviews Help
Sharpness and Chromatic Aberration Display
This is the opening view in a lens review, and looks the most complex. It is designed to display a how the sharpness of a lens varies across the frame at a comprehensive range of focal length and aperture combinations.
The underlying black image is a simplified representation of our sharpness test chart, showing the locations of the slanted-edge test patterns from which we measure our data, and a series of checkerboard patterns which give visual representations of how this looks in practice. This is overlaid by a colour gradient, indicating the measured sharpness across the frame, and ranging from blue for best to magenta for worst. On the right of the chart display are two graphs, ‘Sharpness (MTF-50)’ and ‘Chromatic Aberration’.
Along the lower panel are the user controls, which allow you to select any tested combination of focal length and aperture. We measure at all of the focal lengths marked on the lens, using whole stop aperture increments for zooms, and third stop increments for primes. You can operate these controls using either the mouse or the cursor keys (up/down to change focal length, and left/right for aperture).
This displays a value known as MTF50, which is considered to correlate well with perceived sharpness. The y-axis is in line widths per picture height, and the x-axis represents the distance from the image centre along the frame diagonal. This choice of scale has been specifically chosen to allow direct comparison between camera systems with different sensor sizes and aspect ratios.
To make it easier to visualize what point in the frame any specific measurement corresponds to, a thin red circular line follows your mouse as you move it over the display to show the radius being viewed, with a corresponding vertical red line on the sharpness graph.
Chromatic Aberration graph
Here we display the lens's chromatic aberration characteristics, defined as the amount by which the red- and blue-channel components of the test patterns are displaced from their 'correct' position (using the green channel as the reference). The y-axis shows the width of colour fringing across the frame, and the higher the value, the more visible fringing will be. The chart also predicts the colour of fringing which will be produced; the red line indicates red/cyan fringing and the blue line blue/yellow fringing, with a combination of the two resulting in green/magenta fringing. The shapes of the chromatic aberration profiles are also important; the closer they are to linear, the easier correction is likely to be in post-processing.
Just as important as the graphs themselves are the checkerboard patterns, which provide visual indications of what any specific combination of sharpness and chromatic aberration actually looks like. Hover your mouse over one of the checkerboard positions on the chart, and up will pop a 100% crop from the actual test image which was used to generate the data. We want to encourage you to explore this feature extensively, to see for yourself exactly what any measured point in the frame really looks like. However we would also point out that these patterns, with their high contrast and sharp white/black transitions, have been chosen specifically to represent a worst-case scenario for the lens’s imaging performance; real-life images will rarely look quite like this.
Show MTF-50 & CA graphs - hiding the graphs allows you to see checkerboard patterns across all four diagonals. On the right of the screen, a 'belt buckle' display illustrates the range of sharpness across the frame: