Fast and Slow Lenses
I’ve alluded to lens
selection a couple of times already, but lets take a look at some examples of
fast and slow lenses.
50mm f/1.8 Lenses
This is a fast
lens. Recall that the low f-number means that the diaphragm inside the
lens is very large when all the way open. These lenses are typically
pretty affordable and solid performers across brands of cameras. The 50mm
focal length is great for portraits on smaller DSLRs.
:
18-55mm f/3.5-5.6 Lenses
The reason for the range
of f/3.5-5.6 in the lens description is due to the zoom range. As you
zoom the lens out, the physics of the lens require a smaller aperture to be
used. As a result, when at 18mm, the widest aperture setting is f/3.5.
However, when zoomed out to 55mm, the widest aperture is limited to f/5.6.
Most of these kit lenses
are decent performers; however, you can certainly tell a difference in low
light. Often times, it is difficult to capture images indoors using these
lenses without also using flash.
In the earlier example
of the bride, if I were shooting with an 18-55mm kit lens that had a maximum
aperture of f/5.6 at 55mm, I would have to set my shutter speed to 1/25s to get
the same exposure, which would put me at risk of getting camera shake blur or
motion blur from the bride’s movement. This is a practical example of the
benefits of a fast lens, and why you often hear photographers concerned about
lens speed.
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