Today’s Question: I’m trying to evaluate the sharpness of several lenses, but I don’t know how much I should be able magnify an image and still have very sharp edges. In other words, I can look at an image using the “fit” setting in the Navigator in Lightroom Classic and the image looks good. But if I enlarge it to, say, 200%, it looks soft. Is that normal when enlarging an image that much, or is that because the lens is not very sharp? At what magnification should I view an image to critically judge the sharpness of an image/lens?
Tim’s Quick Answer: I recommend using a 100% zoom setting when evaluating sharpness for a photo. However, to test the sharpness of a lens I recommend photographing a lens resolution test chart rather than evaluating standard photos.
More Detail: The reason a 100% zoom setting is preferred when evaluating the sharpness of an image is that at 100% one pixel in the image is represented by one pixel on the monitor display. The reason the image appears softer at a higher zoom setting is that multiple pixels on the display are being used to display each pixel in the image, and so there is blending introduced along the contrast edges in the image.
When evaluating lens sharpness, you could simply use a relative comparison between test images captured by different lenses. In other words, you would simply be determining which lens was sharper than another, without actually quantifying the results.
If you want to get more detailed in your evaluation, you can use a lens resolution chart. However, a calibrated lens resolution chart can be rather expensive, ranging from several hundred dollars to more than one thousand dollars (https://bhpho.to/3KDurGL).
There are some less expensive (though less precise) options that can work well. For example, the DGK Color Tools test chart (https://bhpho.to/3kBb2LK) sells for about $16. You can also use printed text as a reasonable alternative in place of a test chart, comparing the sharpness of the edges of text from one photo to the next.
By using a single lens chart or other target for each lens you’ll test you can get more accurate and meaningful results. And if you want to be particularly detailed in your evaluation you can use a lens resolution chart that enables you to measure the actual effective resolution of a lens (generally represented by a number of line pairs per millimeter in a test target).