Nikon Chat: Nikon Digital Cameras and Equipment

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What does the “f1.8″ on the Nikkor 50mm f1.8 lens mean?

Okay, I’m pretty confused, as I’m pretty new to DSLR photography. Especially clueless about all those numbers that you can find on lenses…e.g on the Nikkor prime lens, the 50mm f1.8. Okay, for starters:
a) What does the f1.8 mean? I assume it’s the aperture…correct?
b) If so, does this mean that this lens has a fixed aperture of f1.8? Or can it also go to smaller apertures/bigger f-stops i.e f8, f11, etc?
c) How does aperture affects depth of field? And why is DOF important?

Depth of field can be used to seperate a subject from its background, so by using a shallow depth of field (like f1.8) the zone of focus is pretty narrow, with a portrait shot this would mean the persons eyes and mouth are nice and sharp, by the time you get to the ears they may be going slightly soft, and the background and foreground may be totally diffuse drawing attention to the subject more than their environment.

For a landscape shot you may use a small aperture so that a lot more of your image appears nice and sharp.

Apart from aperture, the focal length of the lens affects the perception of depth of field.

If you had a 28mm lens, a 50mm lens and a 100mm lens all set at say f4, the 28mm image would appear to have the greater depth of field, the 50 less, and 100mm even less again.

This would also affect things like perspective.

Note that the depth of field appears to be further reduced with any lens the closer you focus to the camera.

The other function of the aperture is to control the light. F2 will let in double the amount of light as f2.8, so if you are shooting in low light you might want to use a wide aperture to allow more light in.

If you were shooting a fast moving subject you might want to use a wide aperture to let in more light so that you can increase the shutter speed (decrease the exposure time)

If you wanted to use motion blur as an effect (i.e a river or waterfall) then you may use a small aperture so that you can have a slow shutter (long exposure time) without the increased light from the long exposure burning the image out.

It is also generally the case that most lenses don’t perform at their best at the fastest / widest aperture, so although a lens may have f1.8 images taken at this setting may be soft at the edges, or have less resolution detail (regardless of the cameras MPs) as such usually you would stop a lens down to f2.8 or f4 when the quality picks up, although this is less of an issue with cropped sensor DSLRS.

If you were using the kit lens (usually an 18-55 f3.5-f5.6) then you would really want to stop down to f8 to get anything decent out of the lens, so immediately there is a benefit to using a faster aperture lens, even if you seldom use it wide open (at its maximum aperture)