I had little understanding about the optical qualities of my lenses, especially when used with a full frame camera body. Because of hands on experiences I decided to learn more about my lens’s optical characteristics and began analyzing the image quality for each of my lenses. In this context, “Image quality” refers to sharpness of the lens. Specifically, I studied image quality degradation from the center to the outer image area and across the zoom and aperture range for APS-C and Full Frame sensors.
An APS-C sensor camera effectively crops a portion of the outer frame area. This is true for lenses not specifically designed for APS-C sensor cameras. How does this impact image quality? Most lenses are sharpest around the center and sharpness degrades progressively towards the outer edge of the frame. This is one reason why lenses designed for a full frame camera are more expensive. They must be designed to have an acceptable sharpness from center to outer area.
The image sharpness also varies depending on zoom level and aperture permutations. Even with today’s sophisticated 3D CAD-CAM design tools, the main disadvantage of zoom lenses is that their image quality varies across the zoom range. The variations can be substantial enough to be noticed on prints. This is the reason why even today some of the professional photographers still prefer Prime Lenses. Because of their simpler optical design, sharpness only varies across the Aperture range.
There are not many labs that do professional lens sharpness measurement and publish the results to the public. Here are two examples I trust with their analysis:
The-Digital-Picture:http://www.the-digital-picture.com/ and,
Optical Limits (Previously known as Photozone)https://www.opticallimits.com/
(I have no association with these test labs)
After analyzing my lenses with these tools, I better understood their optical characteristics. This knowledge assisted me in the decision which lenses I wanted to own. As a result, my current lenses fulfill my expectations for a more consistent image sharpness across the frame and zoom/aperture range.
I shoot in RAW and can correct other image quality deficiencies like distortion, vignetting and chromatic aberration with Lightroom or Photoshop. However, correcting the native sharpness of an image is not practical. High quality lenses are more expensive and at the end it depends how much you are willing to pay for a high-quality lens.
However, good equipment is only the second most important factor in taking great pictures. The overwhelming factor in taking compelling pictures is the person standing behind the camera.
The “Lens Cheat Sheet”: I have created my own “lens cheat sheet” to squeeze the best possible images out of the glass I own. A small laminated document has become a companion on all my photo shoots. When I am out in the field taking pictures this quick pocketguide together with the DOF calculator have become my “best friends”. Below is an example of lens sharpness tables I created for a few of my lenses.
Notes in how to interpret the charts. The assigned image quality values for a lens have assigned four color codes. For example, “dark green” for the EF 16-35mm f/4 USM L IS means it is the best possible image sharpness for this lens. I created two groups named “B” and “C” and associated sharpness numbers to a zoom level. They stand for Image Frame Center “C” and Border “B”. Further, to establish sharpness values I used an average between Mid-Frame and Border sharpness. Center sharpness is seldom an issue.
Franz Zihlmann
Lens sharpness tables
