Learning about Lenses

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.

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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.