APERTURE

http://www.digital-slr-guide.com/maximum-aperture.html

1 – Natural Light Photos
No flash with a max aperture of f/2.8
A digital SLR camera sensor needs light to create an image.

There are two ways that it can get the light that it needs: it can keep the shutter open for a long period of time or open the aperture as wide as possible.

Let's say that you hate taking photos with flash, because you know how washed out they look.

You're indoors and your camera tells you that with your aperture set to f/4, your shutter speed will have to be 1/30 of a second. That's not going to be fast enough to take a shot of your child rampaging in the living room.

You're going to have to use the flash, or get the kid to sit VERY still.

If you had a lens with a maximum aperture of f/2.8 (which lets in double the amount of light of f/4) then you can use a shutter speed of 1/90, which means that you can get clear photos without the flash



2 – Fast Shutter SpeedsA fast shutter prevents motion blur
You're taking photos on an overcast day of a flower in the backyard. A strong breeze is blowing, and that flower is moving all over the place.

You set the aperture on your lens as wide as it can go (let's say f/5.6), and check the shutter speed. The camera tells you that you can use a shutter speed of 1/125 of a second.

That's not fast enough to get a clear photo of the flower. You're stuck.

If you had a lens with a wider maximum aperture you could get the shot. With a wider aperture (say f/2.8) your shutter speed could be 1/500 of a second.

Since the wide aperture lets in so much light, the shutter doesn't have to stay open as long.


3 – Reduced Depth of FieldWide aperture = shallow depth of field
Lenses with wide maximum apertures have shallow depth of field.

Without getting into a discussion on depth of field (that could take awhile), I will just refer to the photographic example at right.

This shot was taken with a lens set to f/3.2. Notice that the background is completely out of focus even though the ball in the foreground is clear?

This is shallow depth of field.

Nature and portrait photographers use this all the time to reduce the impact of a busy background.

If you have a lens with a maximum aperture of f/5.6, you can't reduce the depth of field as much as you can with a wider aperture.