How To Test The Lenses

• Sharpness.
  Sharpness is a combination of resolution and image contrast. Resolution and contrast are directly linked and when one goes up, the other goes down. Use Modulation Transfer Function (MTF) analysis to measure sharpness, as do lens manufacturers. Result are shown as % MTF at 24 lines-per-mm for full-frame lenses and at 36 lines-per-mm for APS-C to adjust for the crop factor, so sharpness is generally slightly lower than full-frame, in line with actual use. Multiple readings are taken and averaged, and edge readings are taken from points 1-2mm from the sensor edge. Peak resolution shows maximum lpmm at 20% MTF, which is the lowest level where light and dark tones can be reliably measured., though the difference is faint.
• Distortion.
  Distortion makes straight lines towards the frame edges appear curved. It changes with focal length and is less noticeable on APS-C. Barrel distortion lines curved outwards (indicated as a positive percentage value, like +15%); pincushion distortion curve inwards (indicated as a negative value, like -0.5%). The lower the stated figure, the better: 1% or less is good, 2% or more being increasingly noticeable.
• Vignetting.
  Vignetting is darkening towards the corners. It changes the focal length and is also less prominent on APS-C. Vignetting reduces quickly as the aperture is closed down. It’s easily removed in post-processing, but when it’s strong this result in greater noise. Vignetting is measured in Exposure Values (1EV equals one stop).
• Chromatic Aberration.
  Also known as CA colour fringing, is usually only slightly reduced at higher f/numbers, and is more noticeable on APS-C format due to the crop factor. Distortion, vignetting and CA can all be substantially reduced or eliminated with post-processing software.
• Autofocus.
  Autofocus speed and accuracy is a game of two halves. Half camera, half lens. On the lens side on things, most of the differences related to the mechanics and build quality.
• Image stabilisation.
  Use an oscillating platform, custom-made to hold a DSLR body in place and mimic handholding characteristics, and in our testing it has proved very realistic. The platform’s great virtue is that it’s an absolutely standard test.

Resource: Digital SLR Photography Issue 47