A Simple Guide to Depth of Field

in this video we're going to cover our concept called depth of field you'll see this written as DOF first we're going to understand what depth of field actually means and then we're going to look at how it works and then we're going to look at how to control it so the first thing let's see what it means so let's say we have our camera here and we've got our lens and we have this area that the camera can see now in that area we might have a tree here and maybe some apples and what we're actually going to take a photograph of is the tree that means that this area here that contains the tree this distance from the camera to this distance of the camera is the area that we're interested it's the area we want to be in focus so this area is called the field this is what we're taking a photograph of what we're interested in is the depth of this field so that's what we're looking at when we say depth of field is the depth of the field we want to know how can we control it can we make this deeper shallower in other words what if we wanted to make it a deeper depth of field meaning we want to include the apples or make it shallower so maybe we only have part of the tree in focus and leave the rest of it out of focus so let's take a look at what's happening here we have our film or sensor we have our lens and we have our subject which in this case is a tree and this camera is focused on this tree that means that light from the tree is going to hit the lens and get refocused back here onto the film so that means that this point here on the tree is getting refocused perfectly back to this point on the film or sensor does it mean that this is the only point in focus on the tree it just means that everything from this distance from the camera is in focus really any point along this line when we're focused here any point along this line is going to get refocused perfectly back to here on the film now we're just going to use one point here just for demonstration purposes because it's easier to understand what's happening with one point than to try to do it with all of them so this point here that we're focused on as we get closer to it gets more and more in focus and as we get farther away from it it gets more and more out of focus so looking at what's happening here the eye as the light rays are spreading apart it's getting more and more out of focus our eyes are a little bit forgiving and they can spread apart a certain amount before we consider it to be out of focus so there's kind of a limit here on either side where we consider okay that's too out of focus this is too out of focus and you know it's a little bit exaggerated for this diagram but you know that's just what we're doing here so this area here as the light spreads apart it's getting more and more out of focus in that distance here from say here back to here this is our depth of field it's the area where okay now let's start to be in focus and then it starts to go out of focus again and again because it's for any point along this line this is really you know this whole area in here is what becomes in focus now we want to know how can we control that well if we want our a wider depth of field we need to make sure that this light is not spreading out too fast if we want a shallower depth of field we want to make sure the light spreads out fast enough so that it by this point it's now to spread out or by this point it's now to spread out for us to consider in focus so let's take a look closer in here and see what's actually happening and then we'll understand how we can control it so taking a closer look at our area of focus we have this area here around the point that we're going to be focusing on here now as the light spreads out we have a limit here where things are still considered InFocus this is our depth of field here this is the area that's in focus now bear with me here if you consider that this area that's in focus like toothpaste if weirdest if we were to squish these lines together push them this way in this way this way and this way well then we end up with lines that look like this well what happens to the toothpaste it squeezes up the tube right now this is the area that's in focus and it makes sense because you know the light is not spreading out quite so fast so it's not going to get out of focus quite so fast which means that a wider area is going to stay in focus same thing if we pull these lines closer together and we end up with lines like this now the light is spreading apart really fast the toothpaste squeezes back up the tube and now we end up with a much shorter depth of field or much shallower depth of field here we end up with a wide depth of field so what we really are wanting to focus on is how can we control how fast that light is spreading apart from this point we're focusing on if we can make it spread apart slower then we can get a wider depth of field if we make it spread apart quickly we get a shallower depth of field so let's take a look at that there's three main ways we're going to have control over this so in the interest of time here I've gone ahead and pre-drawn some things either the different types of depths of field we're going for our normal standard depth of field nice wide depth of field and our shallow depth-of-field and then our basic example setup we've been using to show how this works now what we want to look at is what we can do to control our depth of field and achieve either this depth of field or this depth of field so in this first example we're going to leave our subject where it is and what we're going to do is we're going to move the camera much farther away from our subject we're going to move away from it and as you can see what this does with the light this now causes the light to spread apart much more slowly and we end up with the toothpaste squeezing down the tube here and we end up with a very wide depth of field now again remember that when we're using these examples here we're not saying that this is the only area in focus we're saying that this area from this point forward all the way back to this point just because we're dealing with this point here but it could be any of these points along the lines so this depth of field is really you know this whole area here so as you can see if we move the camera away from the subject we're going to achieve a wider depth of field now in this example down here we're going to move the camera very close to the subject and we'll see what that does we now end up with a very shallow depth of field because the light is spreading apart much more quickly and we get our nice shallow depth of field so again if you want a wide depth of field move away from your subject if you want a shallow depth of field move much closer to your subject we're going to list this first method up here so we can keep track of these and this is distance to subject okay let's look at the next method here so in this method again we're going to leave the subject where it is now before we continue here I want to explain something this distance from the lens to the film is called focal length now I'm going to do a whole separate video about focal length because it's really its own subject that could have its own explanation but you should know that that's what this is called the distance from the film to the center of the lens is the focal length and you'll see this measured in millimeters so for instance you know your lens might have a rating saying it's a 28 millimeter lens or a 35 millimeter lens or it could be a 300 millimeter lens and what that's referring to is this distance and we're going to use a change in the focal length to adjust our depth of field so let's see how that works so we're going to leave the film where it is for both of these examples we're going to leave the film in the same place and leave our subject in the same place so we're not actually moving any closer to our subject or farther away what we are going to do is we're going to adjust the focal length so if we want to Y depth of field we want to have our lens closer to our film so that we end up with a nice wide depth of field so as you can see we'll have a nice wide depth of field here toothpaste squeezes down the tube as these get tighter together and we end up with our wide depth of field now we're going to move the lens a lot farther from the film and as you can see that's now going to achieve a nice shallow depth of field because the light is going to be spreading apart much more quickly than before with a you know my short focal length so here's our focal length is nice and long to achieve a shallow depth of field and nice and short to achieve a wide depth of field now again this is a very exaggerated example of what's going on just so you can see but this is actually what's happening so let's list that over here up in our second way we can adjust our depth of field here which is focal length okay so let's take a look at the third method of adjusting our depth of field for this example we're going to leave the subject where it is and we're going to leave the film where it is and we're going to leave the lens where it is okay so if we're leaving all that the same what is there to change well this is talking about a subject that we haven't covered yet which is something called the iris the iris is a mechanism inside the camera that opens and closes a hole to let light through from the lens to hit the back of the camera and so you might have this mechanism in here with a hole that can get bigger or smaller the hole itself is called the aperture the mechanism that creates that hole is called the iris so here we have the iris at this size and it lets the light through and we get our depth of field now what happens if we have our iris here but instead of a nice big hole now we're just going to open it up just a little bit we're only going to open up this little hole here and let some light through well what happens well our light can only go so far in terms of spreading out and as you can see what we end up with is this nice wide depth of field that makes sense so by closing down this aperture we end up with a wider depth of field when we open it back up we end up with a shallower depth of field now the size of the aperture is often referred to as an f-stop and you'll often see this on your lens as either f-stop or f-number or on the display on your camera and this is referring to the size of that hole the way the rating scale for an f-stop actually works in Reverse to the size of the hole meaning that if you have an f-stop of 1.4 for instance you're looking at a very wide open aperture then you go up the f-stop scale to say 2.8 and now it's closed down a little bit more than 5.6 then you know 11 these are examples of f-stop numbers you may see the higher up the f-stop scale you go the smaller the aperture and this is important because you can think of it as the higher the f-stop the wider your depth of field because if we go all the way up to 11 here we're getting a nice closed down iris with a small aperture so we get a nice wide depth of field so think of it this way we have a large aperture to a small aperture okay and then we have a small depth of field to a wide depth of field okay so let's mark this down as our third method here for adjusting depth of field which is the aperture or f-stop so we have our subject distance we have focal length and we have aperture or f-stop so if we want to look at what these do if we want a nice wide depth of field we want our subject far if we want two shallow depth-of-field we want our subject close if we want a wide depth of field we want a short focal length for a shallow depth of field we want a long focal length so for aperture if we want a wide depth of field we want a small aperture which of course means a large f-stop and if we want a shallow depth of field we want a large aperture which of course means a small f-stop so I hope this video has given you a good overview of depth of field and what it means and how it works if you have any questions feel free to leave it in a comment and I will answer it as best as I can we glossed over a lot of things here there's lots of details and exceptions and so on and so forth about how this all works and that those are subjects for later videos but I hope this has helped give you a good understanding

Channel: Dylan Bennett

Views: 676,134

Keywords: photography, video, depth, of, field, dof, explain, explained, explanation, simple, simply, iris, aperture, focal, length, focus, guide, tutorial, lens, lenses, blur, blurry

Id: 34jkJoN8qOI

Length: 16min 29sec (989 seconds)

Published: Wed Feb 29 2012

Reddit Comments

[u/rumphy]

This would have been super helpful when I was starting out. It took me way longer than I'd like to admit to figure out why this happens, I just knew that it did.

[u/clickbaitcomment]

Thought this was from khan academy for a second haha