Understanding Depth of Field - It's Not All About Aperture

Understanding Depth of Field - It's Not All About Aperture

Understanding your fundamentals is, well, fundamental to photography just like it is in anything else. In a previous article, I discussed the basics of aperture and exposure. Now, moving forward I want to address one of the key elements of aperture which is depth of field. All variables in photography have a give and take, and with your aperture as we gain light we also lose depth of field. But aperture is not the only variable the affects depth of field, and in this article we will take a look at those other variables.

First things first, aperture does affect depth of field and in a normal shooting scenario aperture is your first consideration (if not only for most) when considering your depth of field. But there are times when what you think should be happening doesn't, and also there are times where you may not have the option to shoot at f/1.4 and still want a shallow depth of field. Before we look at those other variables that affect depth of field, let's establish what depth of field is.

What Is Depth of Field?

When you focus your camera on a subject, your camera will establish a plane of focus — basically an imaginary plane is set and the things on that plane are “in focus” (more specifically they are at the point of critical focus). I put that in quotes because around the plane of focus (in front and behind) there is still an area of acceptable focus. How long this area of acceptable focus is (or how deep, meaning area from front to back) is your DoF or Depth of Field.

As you increase your aperture number (close down to a smaller opening, or a larger number), say go from f/2.8 to f/4 or from f/4 to f/5.6, the invisible area in front and behind the plane of focus will get larger. Every time you move up in your aperture setting that area of focus in front and behind the PoF (Plane of Focus) get deeper as to include more in that area. So you focus on a person you are setting your critical focus area on them and as you move your aperture setting to larger numbers, you then start to increase how much in front and behind them will also be in focus, you are increasing the depth of what is in focus or in other words the Depth of Field.

The above graphic is meant to be a simple visual representation of depth of field and how it increases along with your aperture setting and not an exact mathematical representation.

Side note: when focusing on subjects relatively close to the camera the plane of focus tends to be about 50% percent in front of the subject and 50% behind. As subject get further away from the camera, the plane of focus will shift so that is it closer to about 1/3rd in front of the subject and 2/3rds behind the subject.

What Else Affects Depth of Field?

The main element, other than the aperture setting, that affect depth of field is distance. More specifically, the distance from the camera to the subject. As you move closer to your subject, the area of the image that is in focus gets smaller. When I teach my Introduction to Digital Photography course, most of my students begin with kit lenses that have variable apertures which makes choosing f/2.8 or wider impossible and makes the "widest" aperture to be somewhere between f/3.5 to f/5.6 with most students shooting at f/5.6. We do a project every semester that requires them to create shallow depth of field. Students are still able to successfully create shallow depth of field images mainly by using the principle that getting close to your subject will create that shallow depth of field look that everyone loves.

The set of four images in this articles header illustrate how the same aperture setting (all of them at f/2.8) can create a different depth of field. The closer the camera is to the subject, the less is in focus behind the subject. As we pull back you start to see that more and more of the playground rings are in focus.

These images above were both shot at f/5.6. The can is about two feet in front of the camera, so by keeping the subject relatively close to the camera and by keeping the background elements further away, we can achieve the shallow depth of field look even at mid-range aperture settings, like f/5.6.

There really isn't one set formula, just because you are at f/4 doesn't mean your area of focus will be a certain depth and just because you are one foot away from your subject also doesn't mean your area of focus will be a certain depth. Your aperture setting in conjunction with your distance from the camera to the subject affects how much of your image is in focus — it affects how deep from front to back the plane of focus is. Also the area of in focus and the area of out of focus are two different elements, in my opinion. Distance from camera to subject affects the area of in focus, but distance of foreground and background elements affects the areas out of focus. Really the test images with the Check soda can are more about the out of focus elements rather than the in-focus elements. This separation of what determines the plane of focus and what determines the qualities of the out-of-focus areas leads me to my next point.

Let's Have A Debate

One of the most unsettled facts in photography is if focal length affects depth of field. Different textbooks give completely contradictory information that are presented as clear fact. Different blogs and books will tell you different things. So, what do you think? Does focal length affect depth of field?

My answer in short: No! But, it's really not that simple. The possible reason for why you may find different answers to this question is that when you test the theory, the in-focus objects look the same, but the out-of-focus areas of the image look different. Keep in mind that when you zoom in, the out-of-focus elements that are nearer to the lens may be eliminated from the frame, but also I think at times the out-of-focus areas with longer focal lengths appear a bit softer as well — this may be where many will disagree. Do your own tests and post the results!

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32 Comments

Patrick Hall's picture

I always tell everyone that aperture, focal length, and distance between sensor and subject matter are the 3 things that affect DOF. Assistants are always shocked when I shoot my ring shots at f/22 - f/56 and then they see images that have very shallow depth of field. Also, in the helicopter article about to be published you can see how I was able to make all of New York City in focus while shooting at 300-400mm wide open at f4. So while they all 3 affect depth of field, I always say that focusing distance is the biggest factor in how sharp or blurry a scene will be.

Tony Northrup's picture

Technically, only two things change DoF: magnification and f/stop. However, focal length and subject distance are easier for us to comprehend, and you can use them to calculate the magnification.

DoF tells us how much detail their is in out-of-focus areas... and if you keep the subject size the same, 24mm and f/2.8 has the same DoF as 200mm and f/2.8.

But nobody *really* cares about that. We care more about how blurry the background looks, and that's dependant on the angle of view. A smaller angle of view might contain the same amount of detail as a portion of the background from a wider angle shot, but because we're seeing less of the background in our photo, it appears to be less sharp.

I guess I'm saying that I wish we would stop talking about DoF, since it has this technical but completely impractical definition, and start talking about background blur.

The lens magnification depends on the lens focal length and the distance from the lens to a subject. Have no idea how to insert a formula here but in words it's the focal length divided by the difference between the focal length and the distance to the object. Longer the lens (higher the focal length), greater the magnification. Closer the object, the greater the lens magnification.

So you could say that DoF depends on distance to object, focal length and the lens aperture, right? You can control the distance to the object, you can change the lens and change the focal length. So why talking about the magnification at all? :)

The Reason Magnification is important (Equal Framing of subject in view) is that when you achieve that, IN most cases, Then Focal Length and Distance to subject cancel each other out and the two images will have the same DOF and the only thing then that changes DOF is Aperture. The one exception is when one of the distance to subjects is close to Hyperfocal distance. Then it throws the whole thing out of whack. Now this doesn't mean the two images shot at different focal lengths and different distances will look alike because you will have two different angle of Views and also the Longer lens you will see "Perspective Compression" meaning in the image with the more telephoto lens will appear that the background is closer and more out of focus even though the DOF is in fact the same

Lens magnification is not related to projection size at all. :)

The former is a function of focal length and distance to a subject, the latter is a function of lens angle of view and how close the projection is to the lens (both are fixed for a given prime lens). In the case of DSLR they just happen to have a common variable affecting both of them -- focal length.

What you describe above is true only if you take an image of a flat surface. In the case of 3D object, it's not true at all Your logic kinda works when you compare 21mm lens to 35mm, but is completely off in case of comparison of a wide angle to a telephoto lenses.

Look at the images here ( https://luminous-landscape.com/dof2/ ). But don't read the article, it is wrong. Compare 400mm to 200mm images. The 200mm image has much smaller circle of confusion than that of 400mm. Can you see this?

If not, let's stop right here. :)

Hullo Jerry, apologies for butting in in your question to Tony.

Magnification is when you achieve augmentation of the size of a given object/subject only in appearance, but not in its real physical size. We may achieve that in photography by playing with the way that we frame a subject in relation to its most distant point in the background (usually, the horizon line is fine enough for a baseline).

More specifically in this case, magnification happens when we physically zoom in - actually coming closer to the thing that's being primarily shot and, depending on how we set the backdrop to 'lock' with the subject, we achieve that effect of enlargement - even though the physical size of the subject is actually smaller than it looks in the composition.

In summary, to magnify the subject is to make it appear bigger than it physically is by means of employing a closer composition, highlighting the object/subject in relationship to the farthest background point of reference. Naturally, the choice of different lenses will also have an effect on that.

You know focal length (the distance between you and the focused subject) and you know depth of field (the distance between the subject and the farthest reference point in the backdrop), right? We get magnification when we warp and distort these two; more with the first, than with the latter. ;)

I reckon this is was Tony meant with his articulate elaboration; I hope it can help you as well.

These two posts have some pretty clean and objective descriptions/examples of magnification as well. Have fun:

http://digital-photography-school.com/photography-101-lenses-light-and-m...
http://www.scalemodelguide.com/construction/tools/use-magnification/

Regards.
San.

Look at any decent camera, there are markings on a body like crossed out circles. The longer line marks location of the focal plane of the camera. The focal plane is an imaginary plane (has 0 depth), drawn through the lens focus perpendicular to the axis of a lens. You can't talk about DoF like it's the same as the focal plane. Dohhh...

Lance Nicoll's picture

Alex, when I say Plane of Focus I am simply referring to an invisible plane that includes what is in focus. Depth of Field is by definition the distance between the Plane of Focus or the distance between the things that are in focus in that shot.

Using them interchangeably is like using Meanline and X-Heigh interchangeably in graphic design - one is the thing the other is the distance to the thing but we'd understand the basic same function of them.

Consider also that when teaching Depth of Field you are teaching usually to someone that is very new to Photography, so I try to keep things and explain things in ways that are simpler to grasp initially.

How would you categorize the difference between the two terms, in your words?

Lance, that is exactly my issue here. This is just wrong way to teach such important concepts like DoF, especially to beginners. When beginners get confused by misconceptions like "cropfactor influencing DoF", "magnification influencing DoF", "DoF is a plane but with depth", "manipulation of objects in the frame could affect DoF" (this assumed in you article) etc. they have no chances to get as good as they could've been having they correctly understood the concept DoF or any other fundamental principles related to photography.

I strongly believe, it's one of the reasons of why there are so many people taking snapshots and (sorry) pretending they are art makers. Fuzzy concepts create fuzzy images where even creator can't explain what are they about. :)

If anyone really wants to straighten out the DoF concept for themselves, they should try to find out WHY APERTURE effects DoF. Once you know what is going on with aperture, rest will be easy, obvious and clear of misconceptions.

Do you guys know what is the circle of confusion? It's a bunch of photographers sitting at a round table and discussing DoF!

:D

Lance Nicoll's picture

I agree that the essential step is understanding why aperture affects DoF - agree with you there, but I would also say its important to know that its not the only thing and the distance is a component. Its why my students are able to create shallow depth of field shots with their kit lenses at 5.6, using the knowledge that distance is a factor.

What do you think are the major misconceptions associated with DoF?

My idea was that if you understand WHY aperture effects DoF, you'll be able to understand the concept of DoF in full. DoF is affected by aperture, distance to an object you are focusing on, lens aberrations and resolution plus the output resolution (billboard, gallery print, 6x4, web etc.). The first two are related to the way lenses work and effect real DoF. Rest of them control perceived DoF.

What you describe above is perceived DoF. You can't change actual DoF of a 5.6 lens, but having knowledge of actual DoF of the 5.6 lens you can arrange the subject and the background the way that in the image the perceived DoF will be seen as much shallower than it is in reality.

Does it make sense?

Nobody should mix actual DoF with DoF perception in a final print. This is one of major misconceptions. Few more are in my post above. You can add to the list the idea that you can compensate DoF and focal length differences by getting closer to the subject.

Ah, almost forgot. The hyperfocal distance. Who can explain what is going here? It's simple if you understand that the aperture controls the circle of confusion and that smaller object gets in a projection, sharper it LOOKS for a given focal length. Look at the magnification formula I showed before. Shorter the focal length and further the object is, smaller it is in the projection. So for a wide angle lens, magnifications drops with distance faster than for a telephoto. When you focus at a distance when further "edge" of DoF has relatively similar circle of confusion as the objects at that distance you see all of them as equally sharp. Hence the hyperfocal focusing effect. This is again, perceived DoF not actual.

Hope all of this makes sense...

You are using "Plane of focus" and DOF interchangeably and they are not. Plane of focus is an invisible plane that extends up and down and left and right from the "Point of Focus" any "Defocusing" that is at the edge of the "Width" (DOF is Depth not width) at the edge of the width is caused by Field Curvature of the lens not DOF
As to your Debate. 4 Things determine DOF : Aperture, Sensor/film COF size, Focal Length and Distance to subject. However in Practice, Focal Length and Distance to subject (distance to background from subject DOES NOT affect DOF only the look) But Focal Length and Distance to subject will usually Cancel each other out because as Tony Pointed out...we Move. So for Equal Framing/Magnification we have the same DOF....EXCEPT..when one of the distance/focal length Combinations is Close to Hyperfocal Distance an example would be 25mm f/11 10 Foot from subject, or 200 mm f/11 80 foot from subject (equal Magnification) The Former would have infinite DOF while the later would be 21' (on a crop sensor)

Lance Nicoll's picture

things in an image are not in or out of focus from left to right, generally, its front to back. The plane of focus exists on the same plane as the sensor, how much information will be included in that plane is determined by the DOF - I would see an argument to say that the PoF is a plane that only includes critical focus and DOF is the distance from the front to back of acceptable focus.

But, I am really more referring to the idea of the invisible plane or invisible area that is in focus not so much the technical definition of PoF - introductory courses where this info would be taught require for me to explain things in ways someone new to these concepts would understand, the PoF term is not really a basic term, at least I don't think so.

So when you say that the plane extends up and down and left to right, you are correct, it fills the viewfinder - agreed - the plane follows the plane of the sensor - I think yo are alluding to that as well. I am then attempting to have readers visualize that as you change your aperture this plane gets wider from front to back to include more in the "Depth of Field" - the distance

What do you think? (and thanks for the post, I enjoy the discussion)

I fully understand that you want to keep it simple for beginners, that's my goal too which is why the terms you are using are confusing them.
Lets start with a "Plane" think of it as a Sheet of Glass in front of you That is 10 Feet wide and 6 Feet High but it has No depth to it. (Point/plane of focus) So when you tell people that the DOF get's "wider". It doesn't because then people image that the 10' gets "Wider" What happens is you have an area of "Depth" In front of and behind that "Plane" that is in acceptable focus. So DOF would get "Deeper or Shallower from the point of the Camera. I appreciate you letting me talk :)

Lance Nicoll's picture

makes sense - deeper better choice of word rather than wider, point well taken

Andrew Merefield's picture

Technically, I would say focal length does effect DoF but practically it can't be used to control DoF because as you change the lens you have to change the distance to achieve the same image. I would also like to say that, technically, sensor size doesn't effect DoF a XXmm lens focused on a subject at a certain aperture and distance will give you the same DoF regardless of what sensor is capturing the image, what changes is the angle of view. What we do to change the angle of view, i.e. different focal length lens or change the distance, is what changes the DoF.
I'd just like to say that I really care about how much of my image is acceptably sharp rather than what the background looks like.

Lance Nicoll's picture

I agree with that Andrew, generally my decisions are driven but what and how much I need in focus and not the other way around.

Andrew, the focal length and DoF related to each other. But you should think about them in an opposite order than in your example. You choose a lens with needed focal length to get an image you need. This way with shorter focal length you create depth in the image by defining planes (foreground, middle- and background), or, with longer focal length, get effect of a subject isolation.

Ramon Acosta's picture

You cheated with the last image, to obtain the same subject size with the 2 different lenses, (one wide one not) you had to place the camera at different distances.
But I agree with you, technically. What counts is what we perceive, and at a given fixed distance from an object, lets say a 50mm lens 3 feet away from a bird, if we change 1 or more of the 4 parameters (distance, aperture, focal length, or sensor size) the apparent depth of field is going to change. Again, the apparent DoF, it doesn't matter if you are able to measure it, and it is the same, it looks different.

This does look like an argument if not a debate.

Lance Nicoll's picture

Hey Ramon, thanks for the insight and post. I took those pics about a year ago for one of my classes as a demo, but I don't think I really moved when I took them, I way have swayed a bit to the right but you can see the same foreground element hanging there on the top right. If I remember correctly it was the same lens the 24 to 70 and I just zoomed and zoomed out.

Thanks for reading and posting !

Lance Nicoll's picture

actually the perspective shift of shooting at 24mm is what may be throwing you off, possibly.

Ramon Acosta's picture

The subject size can not be the same, from the same distance. Other wise zooming a lens would change only the background angle of view. That is why when film makers do the dolly zoom, they either move in while zooming back or viceversa.

Lance Nicoll's picture

yes, I know and appreciate your comments the subject size isn't the same though

great article, its not all about shooting and f1.2 all the time. nice way to explain something complicated and make it easier to understand.

Ramon Acosta's picture

Thanks for taking the time to reply to the comments.

Lance Nicoll's picture

Of Course, I appreciate you guys reading and being engaged in the conversation

Bethany Seagrave's picture

I'm curious.... how do you feel about the Lytro Camera then? Me, personally I'm on the fence.... it seems like awesomely powerful equipment... but it also feels like cheating and you are removing the organic quality of the actual image capture. If you wouldn't mind, I'd love your perspective. Thanks.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&...

Lance Nicoll's picture

I've shot with one before. I love the idea and the potential to create pictograph or animated stills with them, but the actual camera itself feels off when you take a picture, it work slowly and just doesn't feel like a camera. The quality is ok.

Bethany Seagrave's picture

so, would you say maybe with some refinement in coming years it may be competitive with the DSLRs? That would be interesting indeed.

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