Aperture of Lens
Aperture of lens can be set with mechanical (rotation) movement of a ring, or in case of a DSLR camera, it can be set with electrical signals. These signals are to be controlled with a button, which is provided on the camera body.
Mechanical aperture setting is done with a ring, provided on the barrel of a lens. Aperture numbers (1.4, 2, 2.8 and so on) are engraved on the ring and a marker is there on the barrel. We can manually turn this ring to set the aperture of a lens.
Most of the average quality lens has a notch at one step increment, so the aperture setting can be only at full aperture value, which is either f 2.8 or f 4 or f 5.6. We cannot set it for in between aperture value.
Better version can be set at half stop increment. This is very useful when we have to either over expose or under expose a frame by half stop. Another advantage is, because of the lock, accidental shift or turn of the ring is not easy.
Professional version rings are step-less to give full control over the exposure to set it at any in-between aperture value. Yes, accidental shift is possible but a professional photographer can handle it without any problem.
Check the ring rotation, which should be smooth.
Some modern DSLR camera lenses does not have aperture setting ring, but the setting is done with a button on the camera body. The button gives electrical signal and when we click, the lens aperture is stepped down to pre-set value. Information of setting is displayed in the view finder window and on the LCD screen. Most of cameras have an option of either one step, half step or one third step increment in aperture setting. Inspect the electrical contacts for any damage or corrosion.
If the camera has a depth of field preview button, press it and check for a smooth movement of aperture blades. In a rare case, blades will not move smoothly, which is most probably the result of bad handling while servicing of the lens, or some foreign particle is stuck between the aperture blades.
Shoot a test roll for all aperture settings and enlarge prints to check, at what aperture, the lens starts giving diffraction. This is how we come to know about the limitations of aperture of lens.
Read about this:
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
Sharpness of lens: Resolution and lens contrast - how to test it
Zoom ring: Ring on a lens barrel for zoom and focusing
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