Lens Reviews Explained
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 wide range of focal length and aperture combinations.
The underlying black image is a highly simplified representation of the sharpness test chart. 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. You can operate these controls using the mouse or, in fullscreen and compare modes, 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.
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 x-axis is shared with the sharpness graph. 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.
Show graphs - Unchecking this box allows you to hide the graphs. On the right of the screen, a 'belt buckle' display illustrates the range of sharpness across the frame:
Measured focal length and T-stop display
At the bottom right of the graph is a new feature: a display of the lens's measured focal length range and maximum Transmission. For zoom lenses, you can see the T-stop at intermediate focal lengths, not just the extremes of the range.