Lens faults on optical part has to be checked methodically. Any lens can have a number of optical faults, which must be tested. A good designed lens will have less faults, however, with each correction in optical design, the cost of the lens is increased. If we are looking for a cheaper lens, then it will also work well at certain aperture, focusing distance and focal length. Testing of the lens will allow us to use it at its best settings.
A lens can have blur or defocus, chromatic aberration, diffraction, field curvature, lens flare, loss of contrast, optical distortion and spherical aberration. Each fault is because of bad design, glass material, lens coating and bad handling, which is corrected in good quality lens. It can also have a manufacturing defect (in a particular product), which is to be checked.
Take a test shoot at all aperture settings, various focusing distance and focal length. This means that shoot an object at infinity at wide angle, with all apertures, in between wide and normal focal length at all aperture settings and so on. Then again shoot an object at ten feet distance at all focal lengths and apertures and again for an object at five feet and for an object at closest focusing distance. Keep full record of images and be sure that the camera is well secured on a sturdy tripod. Aperture priority mode is a better option, if we are shooting in a bright day light. Keep ISO settings at lowest, so that noise artifact in the photograph is minimum.
Now enlarge the prints or if we are shooting with digital camera, then view the result on a computer screen at least 400% zoom.
Check for each fault of lens, which has been described 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
Loss of contrast: How ability of capturing contrast of a lens is lost
Optical distortion: Physics of optical distortion and how to check it
Spherical aberration: Optics of spherical and asherical lens
We will be able to find out, at what point, the lens is performing at its best and at what point, it starts a break down because of lens faults.
Yes, be sure that the object we have selected for shoot is not moving.
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
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
Sharpness of lens: Resolution and lens contrast - how to test it
Zoom ring: Ring on a lens barrel for zoom and focusing
Also read about red eye effect in a photograph, what is the reason and how to avoid it
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