The best way to test your lens sharpness is to use a test card, like the one we’ve used here. To get ready to test your lens, you need to ensure that your camera back is parallel with the chart, and the middle of the lens is aligned with the chart’s centre point. The chart also needs to be evenly lit.
The ideal solution is to position two lights either side of the chart, but soft, diffused window light will do the job. Set your camera to shoot raw files and use the lowest ISO setting for the best quality results.
Shoot your chart at each aperture setting, then open the images to compare the results. The most obvious differences between the results at different apertures will be at the edges, so zoom in to 100% and look closely at the detail resolution in this area. By checking each image you can determine your lens’s best aperture for the sharpest results.
You can also check the amount of fringing (purple/yellow or red/cyan) visible at the edges of the star, and distortion (bending of the lines on the edge of the chart). Both of these can be corrected using the Lens Correction options in Photoshop CS2 or later.
Here’s how to test your lens’ sharpness step-by-step.
Step 1: Position your chart
To get started, mount your free test chart on a wall or board, so that it’s exactly vertical. Ideally, you should check that it’s straight with a spirit level, and also make sure that the chart is level horizontally. Next, place your camera on a tripod as near to the chart as possible.
Step 2: Set up the camera
Adjust the height of your tripod so the middle of the lens lines up with the centre of the test chart, and make sure that the camera is level. Move the camera and tripod away from the chart to a position where the chart just fills the frame at the focal length that you want to test.
Step 3: Key camera settings
Now make sure again that the back of the camera is parallel to the wall, and manually focus carefully on the centre of the test chart. Set the camera to its lowest ISO and switch to Aperture Priority shooting mode. Set Exposure Compensation to +1EV and take a test shot.
Step 4: Try different apertures
Take a series of shots at different apertures, ideally using a remote release or the self-timer. Start at your lens’s widest aperture, then take a shot at each full aperture down to the smallest. Don’t move the camera during the sequence. You can now compare the results on screen.