Range of Focal Length of Lens
Range of focal length of a lens is showed in terms of mm. This means that if a zoom lens is showing 18 to 75, its focal length can be set at any value, between 18 mm to 75 mm.
This range depends on the particular model of a lens. Higher range is good from a user's perspective, but higher the range, more it is difficult to correct it for various lens faults.
A zoom lens may have designing feature of internal zoom or external zoom. Internal zoom is a better design, in which the physical size of the lens does not change when we zoom in or out. Internal movements of lens elements change the focal length. In case of external zoom, the front barrel of the lens comes out or goes in when we zoom. There is no effect on picture quality or sharpness of an image but the first type of lens is more preferable, as chances of dust entering the lens is less.
Zooming of a lens can be twist type or pump type. Twist type is a better version, mostly given in a two touch zoom. One touch zoom has a pump type zoom action, which becomes loose over period of time.
Some zoom lens are designed with macro and close up shooting. Again, this is a good feature from a user's perspective, but a specially designed prime lens for macro photography is much more sharper, as compared to a zoom lens.
Avoid going for a lens, which has more range of focal length. Test it for its sharpness and other lens faults. Test it for its aperture and focusing mechanism. Shoot a test roll as showed on how to test a lens, which is for all apertures, different focal lengths and various focusing distance. If you are really committed to photography, then do not hesitate to buy the most expensive lens, that you can afford.
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
Sharpness of lens: Resolution and lens contrast - how to test it
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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