Confused about lens and focusing
 

Hey guys im hoping someone can help me out here
im new to the dslr world and im confused about some things
im looking at the t2i and my first lens being the Sigma 30mm f/1.4 ,however im confused about focusing with it ,from what i understand you have less depth of field when using a higher aperture and more with a lower one ,however ive seen a few people say that the effective distance thats in focus is only like 5 inches when using it at F/1,4 is this true?
another question about focusing is: ive seen quite a few videos where people are walking along side the subject matter ,and often they go out of focus as distances between subject and camera change ,is there a way to set the focus on a lens to infinity ,that way everything is in focus all the time no matter where you move in relation to what your shooting and even if you turn the camera and look at far off objects or close ones ,or is this just not possible ,hopefully some of you can clear this up for me :)

These concepts are interrelated.

A large aperture is smaller numerically. An aperture of F1.4 is larger (allows in more light) than an Aperture of F5.6. The smaller the aperture, the less light the lens is letting in. The smaller the aperture, the more DOF you have. Meaning the closer you have to infinity focus.

A related concept is the wider you are say have a selected 20mm focal length, the deeper the DOF, and the closer you get to an infinity focus. If you select a focal length of 300mm, it's going to be VERY difficult to get an infinity focus.

The problem is the larger the camera sensor, the harder it is to get an infinity focus. On a consumer camcorder, if you set an F16 aperture, you can pretty much bet that most things are going to be in focus. Even if you are well zoomed in.

So the way to get DEEP DOF on something with a large sensor like the HDSLRs is to use a smaller aperture (F8 or smaller), and to shoot wide, like 18-24mm.

Make sense?

cool thank you Perrone

The beauty of a digital camera is that you can just try this all out for yourself at no cost. It's simpler if you just take some still photos but you can quickly check out the inter-relationship between aperture, focal elnght & depth of field.