Sharpness of Lens
Optical design gives us sharpness of lens. It is the result of precisely calculated surface curvature of lens elements, its internal distance and quality and purity of the material. Any well designed and manufactured lens render sharp resolution in the image. This sharp image also should have good contrast ratio. A lens is further treated for improving contrast.
There can be a lens with good sharpness and poor contrast. The result will be flat and muddy. On the other end, there can be a lens with poor sharpness and good contrast. Again, the result will be foggy. A good lens must have both features.
Do not get manipulative image with smaller aperture, when sharpness of a lens is to be tested. Shoot a test roll at widest aperture. Select a flat surface with texture, as a subject. Shoot at different distance and if it is a zoom lens, then shoot also at different focal lengths.
Inspect the image for sharpness and contrast at various sections. Each square inch of the image at any place must be sharp. If at all, there is any problem in the lens, then you will be able to notice.
I have come across a good lens, which was giving me sharp image on right side but out of focus on the left side. And other image was sharp on left side, but out of focus on the right side. This was because of a manufacturing defect, where one of the internal element of the lens was not properly tighten on its mount. Upon camera movement, the element would tilt at different position and focusing was shifting.
When we thoroughly check a lens methodically for its performance, then we can find out the real quality and sharpness of lens.
Read about this:
Aperture of lens: What we should check in aperture setting of a camera lens
Built, finish and weight of a lens: Why finishing and weight of a lens is important
Focusing mechanism of a lens: Different type of focusing mechanism of a camera lens
Lens faults: Various types of faults a lens can have, and how to check it
Other information on the lens: Indication and information given on a lens barrel
Range of focal length: Range of zoom lens in terms of focal length
Zoom ring: Ring on a lens barrel for zoom and focusing
Invitation to equipment manufacturers: Camera and lens manufactures can send their product article along with photographs.
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