Understanding Depth of Field

 

Going Beyond Auto - Understanding Depth of Field

To put it simply, Depth of Field is a range of distances in a photograph that appear to be in sharp focus.  When you focus on a point in front of the lens, there is a plane parallel with the camera’s sensor that will be in focus.  However there is a distance in front of and behind that plane that will appear to be in focus and that is what is referred to as the Depth of Field.  Notice that I said that it appears to be in focus.  There is a term called Circle of Confusion that describes this area because as I mentioned, there is only one plane that is actually in focus.

To really gain a full understanding of Depth of Field you will need to understand mathematical formulas, know the Circle of Confusion, the hyperfocal distance, the focal length, and the aperture.  I don’t know about you, but that does not sound fun at all!  I don’t want to think about all these equations and numbers when I am taking a picture, I just want it to turn out the way I want it to!  If you really want to know all of that, there are plenty of places on the internet that go into extreme detail on the subject as well as a few thousand books.

We aren’t going to do that on Going Beyond Auto.  The goal of this site is to take your photography to the next level…not scare you away from cameras all together.  The good news is that you don’t need to know all those numbers in order to take a photograph you are proud of.  You do need a basic understanding of how Depth of Field works, and that is the goal of this post, so let’s get started.

Circle of Confusion

You don’t really need to understand this section, but it is worth mentioning.  If it doesn’t make sense, don’t stop reading, just scroll on down to The Basics section for the information you really want.

You may have heard the term Circle of Confusion, or perhaps saw it somewhere and wondered why photographers were so confused?  Well, this has everything to do with depth of field and how the focusing system on your camera functions.  Regardless of your aperture setting, precise focus is only possible at one distance, or the focal plane.  That is where your camera’s focus system has established focus for your picture, because you asked it to.

Say that spot that you focus on is a single point.  When you take a picture of that point, it will show up as a point.  Simple enough, right?  At any distance before or after that focal point, you are out of focus.  Still makes sense.  Points that are out of focus will take on the shape of the aperture, which as mentioned could be a circle, or close to it, depending on the quality of your lens.  When those circle points that are out of focus are so small that they look like the point that is in focus, it is considered acceptably sharp.  However, the further away from the focal point, the larger those out of focus circles become.  The largest and furthest circle that looks in focus determines the circle of confusion.

That makes sense, but what exactly does that have to do with Depth of Field and my pictures?  Basically, what it means is that focus shifts from that one sharp point to a point that is out of focus gradually.  There are no hard transitions.  The further away from the focal point you move, the further out of focus you are.  This circle of confusion helps us determine the depth of field.  While it may look like everything inside the depth of field is sharp and in focus, the reality is that only one point is in focus.  However there is a circle of confusion in which you cannot distinguish between what is in focus and what is not.  Confused yet?

The circle of confusion however depends on the size or magnification of the final image and the distance at which you are viewing it.  That makes it more confusing.  I am not going to go into the numbers, this just serves to demonstrate how focus is established, and how we determine what is acceptably sharp in our pictures.

The Basics

As I mentioned earlier, you don’t need to know the math behind this.  I just barely scratched the surface with the Circle of Confusion, and there are many other factors involved in determining depth of field.  However, the goal is to take a picture and have it look the way you want it to, so here are the basics.

Aperture and Depth of Field

Aperture plays a big role in determining the Depth of Field of your photo.  If you forget everything else on this page, remember that a bigger aperture will give you a shallower depth of field and a smaller aperture will give you a deeper depth of field.

For instance, below I have taken a picture of some flowers at a distance of around 2 feet in front of the lens with different apertures.  As you can see, the wide open aperture of f/4.0 on this particular lens yields a very shallow depth of field and gives a nice out of focus background.  With each stop down of the aperture, the depth of field becomes deeper and the background gets more focused.  Please note that the background is never completely in focus due to the fact that I am so close to the lens, more on that in a second.

Going Beyond Auto - Depth of Field Comparison

Going Beyond Auto – Depth of Field Comparison (Click for Larger Version)

If you remember from Aperture – Part 1, each stop down in aperture results in half the amount of light passing through the lens.  So to compensate for half the amount of light, I will have to slow down the shutter speed by half as seen above.

If you want a shallow depth of field (blurry background), then start at f/5.6 and open it up from there as your lens allows.  If you want a deeper depth of field (more of the image in focus), then start at f/8 and close it down as much as possible depending on the amount of available light.

Distance and Depth of Field

In the last section I mentioned that I was about 2 feet away from the flowers, and that plays a big factor in depth of field as well.  The closer you are to the object that you are taking a picture of, the shallower the depth of field.  In the previous example, I was approximately 2 feet from the flowers at a focal length of 100m on my Canon 24-105mm f/4L lens(Sponsored Link).  If I were 10 feet away from the flowers, they would be smaller, but the depth of field would be deeper and I would probably appear to have sharp focus closer to f/8.

There are a good number of charts that show you how the angle of the light rays passing through the aperture and hitting the sensor determine the depth of field based on distance, but I have an even easier way to illustrate this for you.  Hold your hand up at arm’s length and look at it.  You will find that you can see everything around it clearly.  Move your hand closer to your face by about half this distance and those same objects that you could see clearly are not as clear any more.  Move it even closer, about 6 inches away from your face, and everything else is pretty blurry.  Your camera operates similarly in terms of depth of field.  When you are closer to your object, even at small apertures, you will have a shallower depth of field than when you are farther away.

When you are taking a landscape picture, because you are capturing a wide area that is far away from you, starting around f/8 will give you the deep depth of field you are looking for.  Of course f/11 and f/16 will give you an even deeper depth of field if lighting allows.

Focal Length and Depth of Field

Focal length and depth of field are and are not really related.  If you take a picture of an object that is 5 feet away from you at 24mm you will have a deep depth of field.  If you switch your lens to 200mm and take a picture of the same object at the same 5 feet away you will have a shallow depth of field.  That is how you can say that the two are related, however the main difference is magnification.  Obviously I am going to have a wide shot at 24mm, but I will be much tighter and closer at 200mm without moving the camera at all…that is magnification and it results in a shallower depth of field.  If you want a closer photo with a shallower depth of field, go ahead and switch to that telephoto lens.

The two are not really related though, because if you compose the same scene with identical apertures but at different focal lengths, the depth of field will be the same.  The key is that to obtain that same wide shot using a 200mm lens instead of a 24mm lens, you will have to put more space between the camera and the scene, but the depth of field will be the same.  To prove this, using a 24mm f/4 lens at a distance of 5 feet will yield a depth of field of 3′ 6″.  To achieve that same 3′ 6″ of DOF with a 200mm f/4 lens you would have to be approximately 12.5 feet away from the subject instead of 5.  Both of these photos will be roughly the same, the only difference is how far away you were when you took them.

Where to Focus

Understanding the way your camera and lens focus will help you choose the correct focal point for the best possible image.  Remember those charts I briefly mentioned?  If you put your camera on a tripod and then look down at it, you will gain the understanding you need for choosing the correct focal point.  If you draw a straight line in the direction the lens is pointing, that is where you composing your shot.  Now, if you draw 2 angled lines from the lens outward, that is the area your camera sees.  The angle of these lines is determined by the aperture, focal length, and distance to subject.  You will notice that these lines get further apart from each other the further you are from the camera.  This tells you that the camera can focus on more behind your subject than it can in front of your subject.  Don’t worry, I’ve drawn this out for you here:

Going Beyond Auto - Depth of Field Diagram

The most important thing to remember is that you need to focus roughly 1/3rd of the way into your scene.  Whatever your depth of field is at this focal distance, 1/3rd of that depth of field will be between your focal point and the camera and 2/3rds will be behind your focal point.  This is most important when you are taking a portrait of somebody.  If you focus on their eyes, the front 1/3rd should include their nose while the back 2/3rds should include the rest of their head, or at least behind their ears.

What You Need To Remember

You now have a basic understanding of depth of field and how to achieve the look you want in your photos.  Some of this information may be too much, and for some of you it may not be enough, the goal is too keep it simple but help you understand how DOF works.  Here is a brief recap of the important items from this post to help you take your photography to the next level by understanding depth of field.

In general, the closer you are to your subject and the bigger the aperture, the shallower your depth of field will be and you will have a blurry background.  The further away from your subject and the smaller the aperture, the deeper your depth of field will be and the sharper your entire photo will be.  So if you want a blurry background, open up your aperture and get close (with the camera or with a telephoto lens).  If you want a sharp photo all around, close the aperture down and back up from the subject.

Using these basic tips will help you take the picture you want with the results you expect.

 

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