I have a 50mm F1.4 prime lens that I’ve only used a couple of times. I keep meaning to take it out on a walk one day but I hate being limited to that focal length. For those not in the know, a prime lens means there’s no zoom to it. You want to zoom in, you do so by walking closer to your subject. Wanna zoom out, walk backwards. All you got is a 50mm lens. My street lens is a zoom with a range of 24mm on the wide end to 105mm on the zoomed end. Anyway, this is a shot of the base of a small moose lamp in my living room. At F1.4 (the widest aperture) the depth of field (amount in focus) is very narrow. I was trying to focus on the moose’s eye but missed by a tad. I should have stopped down to F2.0 or F2.8. The image would have been sharper and a the depth of field (DoF) wider (meaning more of the moose’s head would have been in focus). But I was interested in seeing just how narrow the DoF would be at F1.4. It’s pretty darn narrow.
The more you stop down (make the opening of your lens smaller by increasing the F stop number), the wider your depth of field. When I shoot in the street I often shoot at F5.6 or F8, which gives me a good depth of field. If I want to ensure my entire scene if in focus (subject and background) I’ll shoot at F11 to F16. Landscape photographers often shoot at F22, since they are dealing with objects in the foreground as well as distant mountains and clouds. At F22 pretty much everything is in focus. For portraiture, you want to be shooting at a wider aperture (smaller F stop number). I’ll often shoot at F5.6 down to F2.8, depending on my distance to my subject and the focal length. I want my subject to be in sharp focus by the background behind him/her to be slightly or sometimes heavily out of focus.
So, the rule here is simple. The larger the f-stop number (F11-F22), the narrower the lens opening and the broader the depth of field (amount in focus). The smaller the f-stop number (F1.4-F5.6), the wider the lens opening and the narrower the depth of field.