Focusing

Leeamcd

Moose love the forum and thanks for all the time you've put into it. I've been venturing into aperture priority lately while shooting and I noticed that my aperture increases and decreases automatically as I zoom in and out depending on the distance that I'm shooting at. For instance with my 18-55mm kit lens I can only get down to f3.5 while shooting at 18mm. I'm still a little cloudy on depth of field but can you explain why this happens and how depth of field relates to aperture? I think the amount of light being let in due to a more wider depth of field has something to due with it but I'm not sure. 

Thanks!

bpress54

Depth of field (DOF) is the area that will be in focus at a given f-stop. Low f-number means a wider aperture, consequently this gives a shallower DOF. Conversely, higher f-numbers mean a narrower aperture, giving a greater depth of field so more of what you're shooting will be in focus. For example, let's say you're taking a pic of your friend standing in front of a tree. If you stop down (widen the aperture) your friend will be in focus but the tree will be blurred. This is preferable to stopping up (narrowing the aperture), because if you have too much DOF the tree will appear to be growing out of your friend's head. DOF doesn't relate to the amount of light being let in, but to the way the light rays are focused on the lens depending on the aperture. In any case, your 18-55mm lens isn't capable of opening any wider than f3.5. Hope this makes sense.