Loss of Contrast
Camera lens do suffer from loss of contrast. This could be the result of poor lens quality, aging of the lens and bad handling.
When a new lens fails to perform best for tonal and color contrast, it could be because of lens flare or poor lens coating. Lens flare can be avoided by using a good lens hood or by blocking unwanted light entering the lens. However, if the lens has a poor coating of magnesium fluoride on its surface of elements, then the lens will not perform up to mark.
Lens coating provides farm for fungus. Any formation of fungus on a lens will make it soft and contrast will reduce. We can easily detect fungus formation on a lens. See through the lens against white back ground and fungus reticulated formation will be visible. When a lens is cleaned for fungus with anti fungus solution, some of the coating also is damaged and after a period, lens looses the contrast.
If the lens construction has used plastic elements, then these element turn opaque with aging. So over period of time, the lens will not be able to capture contrast as effectively as a new lens and finally, it will become useless.
Bad handling is one reason of loss of contrast, which can even spoil a brand new best quality lens. Finger prints on a lens is an example, followed by a layer of dust on the lens element. And if you try to wipe it clean with a cloth, microscopic scratches are formed. These scratches are a poor version of soft focus filter, which is permanent. Contrast and sharpness of the lens is lost.
Always take good care of the camera lens and store in in a place, which is free from moist and dust. To avoid fungus, keep it away from leather. Most correct way to store a lens is to keep it in an air tight container, which is transparent or translucent. Keep the lens in it without covers, so that light can enter in it. This type of environment, which is not dark, will not let fungus grow.
Read about these lens faults in detail:
Blur or defocus of lens: Blurring of an image due to lens fault
Chromatic aberration: How chromatic aberration or distortion affects the image quality
Diffraction: Scattering of light causing diffraction in an image
Field curvature: Field curvature aberration of a lens
Lens flare: How to take care of a lens flare
Optical distortion: Physics of optical distortion and how to check it
Spherical aberration: Optics of spherical and asherical lens
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