Depth of Field – In Depth
Everything you wanted to know about Depth of Field and some things you didn’t but you will be glad you did
So you may think you understand Depth of Field (DOF) but do you? Let’s try to confirm what you do know and maybe show you a couple things you didn’t.
What is Depth of Field?
Let’s start with a definition: Depth of field is the total distance that is in “acceptable focus” from our actual point of focus. Let’s clarify that definition further because we artistic minded hate reading definitions.
When you focus on an object, you have a “point of focus” that is the only part that is 100% sharp and in focus, then you have a “field of acceptable focus” in front of and behind that point. That area is the total Depth of Field
So what’s “Acceptable focus”? Well I could go on and on about “Circle of Confusion” and “Airy disks” but then that Charlie Brown teacher voice would start… Wah, wah, wah, wah…
So let’s make it easy, pick up that magazine on your coffee table and hold it at arms length, if it looks in focus, then it is in “Acceptable focus”. It doesn’t mean razor sharp at 100% on your LCD or looking at a print under a Loupe (Those glass things you see jewelers use when looking at diamonds), Just if that magazine looks in focus at that distance (believe it or not that is how the standard actually is measured, a 6 x 9 print at normal viewing distance)
OK, so now we get what DOF is, now what controls it?
Three things: Aperture, Focal Length (Your Lens) and Distance to subject. But as we’ll find out, sometimes one or more of them may not matter at all (This is the part that will blow your mind! Buwahaha)
What Controls Depth of Field
- Aperture; the wider the aperture (lower number) the more shallow the DOF. The tighter the aperture (higher number) the deeper the DOF
- Focal Length: The wider angle the lens, the deeper the DOF, The more telephoto you go, the shallower the DOF
- Distance to Subject; The closer you are to the subject, the shallower the DOF, The farther away you are, the deeper the DOF
(Side note: There actually is one more thing that affects DOF: Sensor/Film size. Even accounting for lens crop factors, A Point and Shoot has more DOF than a Crop Sensor which has more DOF than a Full Frame, which has more DOF than a Medium Format, which has More DOF than Large Format cameras)
Let’s look at what the things that control DOF look like in practice.
I shot some of my old cameras at different settings and position so we can see how it actually looks. The front camera and the cameras behind (spaced 1 foot between each) remain stationary. Only the camera I’m shooting with MAY move in some circumstances.
Here all the images are shot with my shooting camera in one location, The Focal Length is 80mm and I varied the Aperture from f/4 to f/22
TDOF (total depth of field) 2.5 inches
TDOF 3.48 inches
TDOF 5 inches
TDOF 7 inches
TDOF 10 inches
TDOF 1’ 3”
As Aperture increased, so did Depth of Field
In this next set, I kept the Focal Length the same, 93mm, the aperture at f/8 but just changed my distance to the subject. Notice the difference in focus on the green camera to the right. It has significantly less focus (DOF) as I move closer to the subject
Moving closer to my subject decreased the Depth of Field
In this last set, my distance to the subject remained the same, 42”. My aperture remained at f/8 but I changed from a telephoto lens to a wide angle thus increasing the DOF
TDOF 2 inches
TDOF 4’ 2”
The shorter the focal length, the more Depth of Field
Back to our story
So basically, if you are shooting with a 200mm, lens 4 feet from your subject at f/2.8 you are going to have a VERY shallow DOF. ¼ inch to be exact…yes I said ¼ Inch.
By the same token, with a 24mm lens, 30 feet from your subject at f/16 you will have for all practical purposes, infinite DOF
You’ll probably want something in between those.
Also bear one thing in mind as was shown in the first diagram. NOT ALL THE DOF IS BEHIND YOUR SUBJECT. For the most part it splits about 50/50 but can go all the way up to about 30/70, (70% behind). So If you have a subject 8 feet away and a DOF of 2 Feet, That’s 1 foot in front of the subject and 1 foot behind, so, a person standing 2 feet behind will NOT be in acceptable focus.
No, what you do is find a good DOF calculator like this one: http://www.dofmaster.com/dofjs.html or better yet, do like I do and download an Ap for your smart-phone. There are tons of them out there. Put in the Camera you have, Your Lens, Your Aperture and Your distance to subject, Viola, done, DOF calculated.
You really should take a little time and think about what you normally shoot and put a couple of those scenarios into a DOF calculators just so you get an idea, wow I need a lot more or I need a lot less so let me change this or that. But maybe we should discuss well, just how to use DOF
How do I use DOF?
For the most part, we use DOF to either Isolate a subject, or instead, Place a subject into the background of our image.
If you are a Portrait shooter or maybe you shoot still life subjects, a lot of times the background behind those subjects may not be so interesting. Plus from a composition point of view we want to Isolate Our Subject. That draws the viewer’s eye to our subject and nothing else in that photo.
135mm, f/4, Distance to subject 19’, TDOF 1’ 5”
For this instance we want a shallow DOF, so we will use a more wide open aperture (Lower Number) Or we may just put on a longer lens and get closer to our subject (say Headshots) Or a combination of them all (just be careful your DOF doesn’t get too small when you use all 3, we want to isolate our subject from the background, not one eye from the other)
But let’s say you are a Landscape shooter and if you are a good one you know to include a foreground subject in your image. (If you don’t have any and only have a background in the distance, DOF doesn’t matter I’ll explain this in a bit). In this instance most of the time we want a Deep DOF because in our composition. We have a foreground subject that leads us to the background which also has interest to it.
10mm, f/16, Distance to subject 11 feet, TDOF Infinite
So for a landscape, you would use a more closed down aperture (higher number) and you also probably shoot wider angle, to give you maximum DOF.
These are some of the basic reasons we use varying Depth of Field in photography.
While on the subject of Landscapes. Let me talk about one other thing related to Depth of Field; Hyper-Focal Distance. For every Focal length and Aperture, there is a Hyper Focal Distance. Focusing at that distance will assure that you have the Maximum Depth of Field possible. In fact it will be in the field of acceptable from every point ½ the distance to the hyper-focal distance all the way to infinity.
So for example, you have a 24mm Lens at f/16. The hyper-focal distance is 4 feet If you focus at that point you will have a depth of field from 2 feet (half the distance) all the way out to infinity! (your DOF calculators will also give you the Hyper-Focal distance)
One note of caution though when using hyper-focal distance; some people confuse this with what will give them the sharpest image. It may very well do that. BUT, if you have a clear and important foreground subject in your image and it is in front of the hyper-focal distance. You should, in most cases focus on THAT for the sharpest image or I should say sharpest subject. Or you could place that subject AT the Hyperfocal distance for the best of both worlds; however that doesn’t always work out from a compositional standpoint
A couple final notes on DOF and Landscapes.
If everything in your scene is past the Hyper-focal distance and you are focusing past the Hyper-focal distance or Infinity. Aperture no longer matters. The DOF will be infinite with all apertures.
Looking at these three images and 200% crops shot at f/4, f/16 and f/29 show this to be true and they also show one last thing
The last thing to remember is there is no free lunch in photography. Just upping the aperture to maximum, while it may yield the greatest DOF may not yield the sharpest image, the image will begin to soften because of something called “Diffraction”. Which is caused by… COC… airy disc… pixel size… science …math…wah, wah, wah…charlie brown teacher. Simply put there is a cutoff to the benefits of a smaller aperture and DOF.
For a full frame camera, you should not go past f/16. For a crop sensor, f/11, for the sharpest image. Sometimes you have to trade one for the other to get the image you want. More DOF/ softer image etc. Are the effects of Diffraction huge? Not really but they are there as we can see in these 200% crops, f/4 is sharp, f/16 diffraction softness begins, f/29 it’s visible
The Hidden Truths about DOF
So as we just saw and what I alluded to earlier. The 3 things that affect DOF don’t always. We just saw that in some cases Aperture doesn’t affect DOF. Now for the last big secret; Sometimes Aperture is the ONLY thing that affects DOF.
We know that the three things that affect Depth of Field are Aperture, Focal Length and Distance to Subject. But in real life, the truth is Focal Length and Distance to Subject don’t matter…for one simple reason. We move.
When we frame our subject equally, (same magnification), as we change focal lengths we also move an equal and opposite distance from our subject to keep them framed the same. If we were shooting a head and shoulders portrait with a 50mm lens from 10 feet away. Then we changed to a 100mm lens we would move to 20 feet away to frame our subject the same. So how does this affect DOF?
In this series of images I have recreated a very real thing we do in the real world. I have framed them all the same, all were shot at f/5.6 but, as I changed focal lengths, I moved back to keep the front camera framed the same (equal magnification). Let’s see what happens
Except for the front camera being the same size in each image and in focus, they all look very different. However they all share one more thing. EVERY IMAGE HAS THE SAME DEPTH OF FIELD – 4’ 2”
You may say “How can that be?” the background seems so much more out of focus in the image with the 200mm Lens than it does with the 17mm Lens. This is true; it does look different. However, It’s not due to DOF, it’s something else: Perspective Compression
Perspective Compression is another tool photographers can use to change the look or feel of an image or help in the composition. As we change focal length we change perspective or what is visible in the image. We can see far more of the background in the 17mm image than we can in 200mm image.
The background also appears closer to our subject. Look at the distance between the front camera and the green camera in the images. It appears much further away in the 17mm image than it does in the 200mm image. But none of the cameras have been moved.
Because of perspective compression, they appear to our eye to now be closer together. Also the background now seems to be closer to us and THAT is the reason that the DOF APPEARS shallower on the 200mm image. The background is just as out of focus in both images but because we have compressed and made it look closer we can see better how out of focus the background actually is.
Think back to our 8 x 10 picture we were holding at arms length. If it was 20 feet away we would have a hard time telling if it was in focus or not. That’s all we have done here.
Let’s see this up close in this example; one shot at 24mm, one shot at 200mm
And now let’s zoom to the Lime Tree in the background, equally cropped
Same Depth of Field even though the 200mm appears to the eye to have a shallower DOF.
So in the real world and how we frame our subjects, the only thing that affects Depth of Field is…Aperture
Final words about Depth of Field
Depth of Field gives us artistic choices. That’s what photography is all about; making artistic choices that fit our vision. So I’m asking you to NOT get stuck in thinking there is only one way to see or do things.
I have taken Landscapes at f/2.8 because that was my choice
I’ve shot Portraits at f/11 because that was my choice
Take what you now know and understand about Depth of Field and use it for your vision and don’t get stuck in someone else’s because at the seminar they said “I always shoot wide open”. If you truly know DOF, you will know that may not be the best decision and to be stuck there because it is your mantra doesn’t quite make sense.
When you try to shoot 8 bridesmaids in a row going away from you and you try to shoot them with a 50mm @ f/ 1.8 with the first bridesmaid 6 feet from you (DOF 5 ½ inches) that 8th bridesmaid will not be so happy, in fact neither will the second.
So know and understand DOF and use it wisely. There are many times I shoot wide open, but I also know the times when it can be done better and suit my artistic vision. Make it suit yours whatever that may be.