May 17 2007
Depth of Field How-to
What makes the background (and part of the subject) so fuzzy in this picture?
In this, the second in my sporadic photography how-to series, I’m discussing depth of field. This is useful for those using point-and-shoot cameras as well as SLR (single lens reflex) models. Almost every digital camera sold allows some degree of control over settings, and getting a handle on depth of field is a quick, easy way to make your photos stand out on purpose instead of accidentally. (If you don’t care a thing about this, then don’t worry, there are several pictures to see.)
First, a simple definition. Depth of field refers to how much of the picture is in focus. The amount is determined by how wide open the lens is. You know when you go get your eyes checked and they dilate them? Your vision is out of focus until your pupils close back down to normal. Your camera lens works the same way as your pupil — the wider open the aperture is, the more light it lets in, but also the fuzzier it sees things. It isn’t a direct comparison, but it helps relate it to something a lot of people understand.
Now, on to examples — the best way I know to explain this concept.
Figure 1
Aperture: f/18
Shutter speed: 1/90 second
Focal length: 105mm*
Figure 2
Aperture: f/4
Shutter speed: 1/1500 second
Focal length: 105mm
In Figure 1, the lens shutter is closed down fairly tight. In Figure 2, it is as wide open as it can get. It can be confusing because it’s a relative fraction of the focal length of the lens. Just remember how in fractions, 1/18 is a smaller number than 1/4, and you’ll start to get the hang of it.
Notice how much of the image is in focus in Figure 1 versus Figure 2. That’s because of the difference in depth of field. Also notice that at f/18 (smaller opening), the camera had to leave the shutter open longer to gather enough light than it did at f/4. (These are the only two pictures I took specifically for this how-to. The tracks run behind my office’s parking lot.)
The desire for tack sharpness throughout is why you see so many landscape photographers using tripods even on a sunny day. To have everything look sharply in focus even on large prints, they need maximum depth of field. For that, they have to close the shutter as tight as it will go and, as a consequence, leave the shutter open longer. A tripod helps take best advantage of depth of field’s power by eliminating imperceptible camera shake.
Going the other direction — intentionally blurring parts of the photo besides the subject — often is desirable. Of course, we have examples.
Figure 3
Aperture: f/3.5
Shutter speed: 1/20 second
Focal length: 75mm
Figure 4
Aperture: f/1.8
Shutter speed: 1/80 second
Focal length: 75mm
In both, the background is distracting, and it isn’t a particularly well-composed shot. But in certain situations, the photographer cannot get in a position to eliminate clutter. “Blowing out” the background can help. Even the subtle difference between f/3.5 and 3/1.8 is noticeable and makes Tow Mater stand out more in Figure 4 than in Figure 3. Note that in Figure 4 we start to lose detail on the truck, too.
Here’s an example of how depth of field can help make the subject stand out by blurring things in the front and the back, yet still keep those things sharp enough to be recognizable.
Figure 5
Aperture: f/4.5
Shutter speed: 1/60 second (with flash)
Focal length: 52mm
In this shot, the little girl’s face is in slightly sharper focus than anything else, but we can still tell who everybody is. This is from our early Monday evening visit to the neighbors across the street. Admittedly, I wasn’t thinking about DOF when taking this picture. I merely discovered it as a good visual aid for this project.
Figure 6
Aperture: f/5.6
Shutter speed: 1/250 second
Focal length: 315mm
Finally, to demonstrate a way to make the viewer look through an insurmountable obstacle, or merely one whose avoidance would frighten away the subject, Figure 6 is my recent picture of a bird grabbing scraps from a local Black Eyed Pea’s patio. The wrought iron fence is blurred to the point of serving as a composition device rather than a distraction (I hope).
One major difference between point-and-shoot cameras and SLR’s is that many of the latter provide a depth of field preview button. When pressed, it “stops down” the aperture to show the photographer (in the viewfinder) the finished product in regards to focus. It will appear dark when using smaller apertures (remember, that means bigger numbers).
There is much more to be said on this topic, but that should start anybody completely new to this. I won’t even begin to discuss Bokeh, a Japanese word with no English equivalent that refers to the quality of the blurriness caused by short depth of field. Ken Rockwell posted a great description and examples at KenRockwell.com.
* All focal lengths listed are the result of multiplying the actual lens setting by 1.5, due to the digital camera’s sensor being smaller than a 35mm film frame. Lenses made specifically for digital cameras do not require such complicated math and can be taken at face value.
The first in this how-to series is a flash how-to.
That was a very cool tutorial, Mark!
**resist SLR temptation, resist SLR temptation**
I am imagining the DOF example can also be explained by thinking of a pinhole in a box and observing the light displayed on the other side of the box? The smaller the hole, the sharper the circle of light. The larger the hole, the blurrier the edges. But it’s the exact same image (light) coming through.
Neat!
First off, I have to be up right now. What the hell are you and Simon doing at this time of night on a weekday?
These are great explanations Mark. The photos really demonstrate the concept. I look forward to more in the future.
Simon - You are exactly right. In fact, many model train aficionados modify cameras to be pinhole cameras, often cranking the aperture down from f/32 to f/70. This ensures everything in their tiny little towns is in focus.
Pinhole cameras started photography.
Blitz - I had another post written for Thursday and planned to just work on this post a little bit and then finish it tomorrow for Friday’s pic of the week post.
But, instead, I’m an idiot up until an ungodly hour because I got caught up and finished it tonight.
Excellent post Mark…. I knew what DOF was from my previous SLR cameras, but your explanation is wonderful.
A very cool post, Mark. Controlling DOF can in cases save a poor photo. There has been times I’ve even changed the DOF of a photo on the computer by blurring around the subject. Touchy, but doable.
Blitz, I was up doing some of the set-up for our garage sale, which starts this weekend. I have decided to part with dozens (hundreds, millions!) of childhood books and had to ensure they received the proper treatment and handling before being sold to someone who will invariably put creases in the spine.
So I took a couple pictures of them to preserve the memory, thereby making this tangential comment loosely related to Mark’s post.
OK, so I’d like to schedule my first photography lesson for some time after lunch this weekend. I’d love to know how to make these adjustments with our cameras, if possible. Also, you threw me with the DOF reference after figure 5. My father refers to himself as DOF. The only button we’re ever concerned with on a camera is the PhD button, you know the “push here dummy” button. Perhaps your next lesson can be about something I’ve come across while working with optics, “The Circle of Least Confusion.” It just sounds cool.
I’ll admit that I haven’t played with this concept before although I was vaguely aware of it. Thanks for the tips. I’ll play with this setting on my camera tonight.
I love the bird photo. Quite cool. Oh, and the tracks are greta to. But I have a soft spot for rail photos.
Si - Come on, man. With all that loot you’ll be hauling in from the sale of your millions of books…surely you can swing a new camera so you can play along with Mark’s tutorials.
Uh, isn’t point and shoot….point and shoot? If it involves more than turning the dial to that goofy face setting, or the “A” for automatic before pushing the button…forgetaboutit. I’m lucky if I’m able to get the subject matter centered, so I don’t think I’ll get too wrapped up with depth of field in my photographs. I think it’s great that you know about that type of thing, and I LOVE looking at your pictures. That’s enough for me.
I understand your example of DOF, because it applies directly to our camera systems for minimally invasive surgery. I have had several surgeons comment on how much they like the depth of field (which in our world refers to how well everything in the image is in focus) of our camera system. What they don’t always realize is that it is the quality of our lenses (Telescopes in the surgical world) that makes by far the biggest difference. They keep things of various depths in focus better than everyone else.
Oh well…I better get back to work. I rarely get office days, and when I do I tend to get distracted from time to time between phone calls.
Dear Moksha,
Re: garage sale fiscal windfall.
There’s also the matter of monies owed my wife’s parents, a line of credit, a fence we want to re-build this summer, basement renos to start, … etc.
Any mention of a cool digital toy for Si would get laughed out of consideration.
:(
I’ll be that kid who peers through the fence at the cool toys in the yard next door, unable to get them for myself just yet.
Like Ta-Mater…without the Ta…..
Hehehe. I love these shots and all the info…thanks!
:) Have a good tomorrow!
BTW, I snapped that trash bin shot pretty quick….so I just threw my scarf over my camera to protect it (sorta) from the rain.
Hey Mark, Loved the tutorial. I especially appreciated the comments above about the model train enthusiasts since I am one. I have gotten satisfactory results from the F32 setting on my camera. There is now software that can blend shots taken at higher aperature settings where you have the camera focused beginning in the front moving to the rear. It takes those and blends them into a picture that is completely focused from front to rear. Kind of like knitting a panoramic together. Pretty amazing.
Mike
Crazy day today, folks. I’m back now, though, and I’ll reply in case anybody checks this post again.
Dave - I’m surprised I haven’t seen some of your photos (besides car shows, that is). I know you’ve done a lot of traveling.
Curt - I’ve seen cases where the photographer tweaked it to blur the background. It would have saved plenty of my photos, but boy is it time-consuming. I’m up late enough as it is!
Simon - Loved the pictures of your books before the big garage sale.
Josh - It seems Ben and I are free Friday night, if you have anything in mind.
Hey, I’ve read about circle of least confusion before, and it definitely ties directly into depth of field (but not your DOF — he’s in the circle of most confusion. *bump-crash* No, really, you’re far too kind.)
Moksha - When you said the tracks are “greta” I thought maybe you knew some fancy word for “great” or “special,” but turns out it’s probably just a typo. Should have known, coming from Mr. Fatfingers.
Charles - Sure, if you want it to be, but most point and shoot digital cameras have very good optics and open up wider than all but one of my lenses. So, even if you’re not playing for depth of field, you can take more pictures without flash if you learn how to shoot in aperture priority mode and open it up wide.
Cool about the DOF in surgical cameras. “Um, excuse me, but do you sell one of them fancy cameras what I can shove up my bunghole? I thank I lost somethin’ up ‘ere last weekend when me and my prison buddy was horsin’ around.”
Anna - Aha! So you are quite well-versed in the Cars lingo. It’s just such a great all-around family movie. Not quite as tame as Curious George, but one of the few that I feel truly deserves a G rating these days.
I’m sure your camera can ward off a drop or two of London rain. Just saying that makes me want to go to Europe.
Mike - I visited your site shortly after posting this, and was reminded that you’re a train guy. That feature is something I had heard of, but never tried. Sounds cool.
I don’t plan on checking this post again. Which is fortunate…given the horrible insult my fingers were given. I would have been offended…had I returned to read it.
Just read this. Thanks for the post. I’ve been fooling around with this very concept. Perhaps I’ll post some of my pictures, not that they are great.