I might have lost my cool a bit in the the "P.S." to the "What's a 'Fast Lens'?" post below. I apologize for that.
My problem is that I've been sensitized. I was the editor of a technical photography magazine for six years. Depth-of-field (DoF) is a subject that is deeply beloved of wannabe photo-tech writers, and few things could create instantaneous dismay deep in my editor's heart like opening a submission and finding yet another beautifully prepared, extensive, arcane article on DoF. DoF is perhaps the ultimate hell of photo-tech—graspable enough so many people think they can understand it (or think they should!), complex enough so that very few really can. Whenever the subject really gets going, great reams of material are spewed forth "explaining" it, always—always—strewn with considerable content that happens not to be true. Assuming any given reader engages with said great mass of verbal matter, he comes out the other side usually no more enlightened, and possibly considerably more confused. Finally, add to those conditions the simple fact that a technical understanding of DoF is not required for the successful practice of photography, and you begin to appreciate the extent of my exasperation with the subject.
(By the way, the other day I finally figured out why "grammar police" types can seem to be so fussy and cross. It's because they keep correcting the World, and the World keeps making the same mistakes anyway. If you tell your child three times to hang up the wet towel in the bathroom and he doesn't, you're not that irritated. When you've told him 700 times and you still come into the bathroom and find the damp towel in a moldy pile on the counter, you might well be incensed. So do you see my point? No matter how many times I remind people that "loose" is the opposite of "tight" and "lose" is the opposite of "win," people are still going to misspell it—because it's different people all the time, and there are an inexhaustible supply of them. It's like having 40,000 children, each one of whom needs to be trained to hang up the towel. The first three people I correct, I'm perfectly calm. By the 700th, I'm about to "loose" my mind. Anyway, you see the problem: educate one photographer about DoF, or ten, or a hundred—at however great a cost of time and effort—and you still have numberless millions to go. It's Sisyphean.)
In any event, I've just added more material to the end of the original post. Ctein chimed in. It's now longer. And hopefully a bit more clear. But it's beginning to flirt with becoming yet another of those great masses of verbal matter on DoF with which the literature of photography, and now the internet, is littered.
Anyway, I figured I might try to do a quick, painless end-around on the subject, and try to tell you what actually matters about DoF. I'm going to ignore the technical explanation: because That Way Lie Dragons; Abandon Hope All Ye Who Enter there; etc. I'm doing that on purpose, so please don't write to fill me in.
1. By far—by far—the most important factor in how much DoF you'll get is your distance setting. The farther the distance between the lens and whatever it's focused on, the more you'll get in the DoF.
2. Next most important: the focal length of the lens relative to the size of the film/sensor. Again, on principle, let's forget all the tech-talk about what's "really" going on and the scientific explanations and all that. All you need to remember is that the longer the lens, the less apparent DoF there will be in your shots. And, big jumps matter, little ones not so much; there's not that much difference between an 85mm and a 100mm, or between a 24mm and a 20mm. Very roughly speaking, when there's a 2x or 1/2x difference in focal length, you're going to have noticeably different DoF characteristics to learn.
3. Last: aperture setting. When I was teaching, all my kids had to memorize the following phrase: "The higher the number the smaller the hole the greater the depth of field." Can you see differences? Of course. But again, it takes relatively bigger jumps to really matter all that much. The other two factors matter more.
Photography is a visual medium, so let's take a look at a couple of examples of point 1, above. I'm saying that your distance setting is the biggest determinant of DoF. Take a look at these two shots:
In both of the above examples, I've focused on the tree trunk in the middle of the frame. I was standing about thirty feet away from it. There are some differences in the two pictures, mainly because most lenses are at their crappiest wide open and this lens is a fairly crappy little lens, so the lower picture looks a little worse. And the much closer branch-tips in the upper right are a little crisper in the ƒ/5.6 shot. But when your distance setting is 30 feet with a 28mm lens, the aperture isn't going to have very much effect on apparent DoF. No matter what the maximum aperture of the lens, you're going to have a hard time getting anything out of focus with this focal length lens at this distance setting.
Now look at the same two-aperture difference at a much closer distance setting, wider aperture first this time:
In this case, you can actually see a little difference in the blur, because you actually have some blur. In both of these, the focus was placed on the flower in the middle of the foreground, at the bottom of the frame. (I should have been a little more careful with exposure and framing. Sorry. Then again, if I made everything perfect, I'd never get anything done.) Now, obviously, there's some difference here. There should be: the aperture has been stopped down two stops. The flower is indeed a bit more "isolated" in the top, wide-open shot, and the blurry parts are indeed a bit blurrier. But just look back and forth between the house details at the top of the frame. There's some difference, all right....but it's not that much difference.
So in the top pair, you'd have a hard time opening up the aperture enough to introduce any blur; in the bottom shot, you'd have a hard time stopping down enough to eliminate all the blur. The point is that the top shot is pretty much going to have no blur, and the bottom shot is going to have some blur, with this camera and lens. And that's because of focus distance, mainly.
In any case, where DoF is concerned, don't sweat the small changes too much—whether this or that sensor is slightly larger or slightly smaller, whether this or that lens is ƒ/1.7 or ƒ/2.4, whether you're focused at 10 feet or 12 feet. When learning to manage sharpness and blur, it's the big swings that make more noticeable differences.
Hope this helps. But if it's just more verbiage added on to the great whacking pile of internet blather about DoF for you, well...at least you've got my sympathies!
Featured Comment by FS: "I am surprised that nobody has referred so far to this little gem: The Ins and Outs of Focus by H. M. Merklinger (Warning: PDF download):
"This is about infinity focus versus using DOF scales. The idea ist to focus on infinity, and then stop down so that the actual opening of the aperture is about the same size as the smallest object to b resolved."
Mike replies: I was looking all over for a current source for Harold's book yesterday, and was stymied. I had no idea the entire book was a free PDF download. Thanks!
Harold's methods are a bit quirky, but his two books are a fine brief introduction to the understanding of focus and DoF.