Newb - Fast glass, low light, depth of field

[eleazar]

I have heard many people throw these terms around and I am a little confused. I have heard that if you have a low aperture lens like a 1.8, you can shoot in low light conditions. I understand the concept behind this, however, don't you always end up with a shallow depth of field every time that you select a low f-stop?

So I am confused when people say these types of lenses are best for low light and or sports because it seems like the focus will be off like you see in those portraits where the eyes are in focus but the ears are not.

What am I missing?

[OsmosisStudios]

Here's the thing with DoF: it's small, but it changes.

With my 50mm f/1.8, if I shoot wide open (ie at f/1.8) at a distance of 1 foot, the DoF is a fraction of an inch. At 10 feet it grows to just over an inch. At 20 feet it's already at several inches. The further away you focus, the deeper your focus becomes. There are several calculators that will help you figure out your depth of field at an aperture given a subject distance.

The reason sports photogs use faster (larger aperture) lenses is twofold: one is to get the short shutterspeeds they need to stop action (especially at night), and the second is to isolate the subject. At the distances most photographers are shooting sports, they've easily got a foot or two to work with, and the background is so far away that it turns into a giant blur. So the player is isolated against their surroundings.

Depth of field is mainly aperture dependent, but it also depends on the focusing distance youre using (ie how far your subject is)

[eleazar]

This makes complete sense. Thank you so much. I feel like that "something" just clicked and I am starting to get all of this.