Blurred background, if I hadn't cropped this image, you'd see the foreground was blurred too.

Almost perfect Bokeh, you can see the foreground is also starting to blur.

Bokeh - can you do it?
the visual quality of the out-of-focus areas of a photographic image, especially as rendered by a particular lens.

We all have heard of it, though perhaps we just called it "blur" because we didn't know the real name or, if we did, we weren't sure how to pronounce it, so we called it "blur." I'm guilty of both.
It's that area of an image that we intentionally (or accidentally) introduced an "out-of-focus" area into our image for effect. It makes our subject stand out from the rest of the frame, and it does it artistically.
Bokeh is a difficult concept to master and a technical one.  It involves several aspects of photography, such as lens focal length, circle of confusion, sensor size, and distance from the subject.
In the past, I've tried to explain it, and I even tried to teach it. I failed miserably, leaving my students more baffled than when I started.  Recently I read a related article by famous wildlife photographer Jared Lloyd of The Journal of Wildlife Photography.  His report was the most detailed description of bokeh and its application I've read yet.  I decided to share his knowledge with you. I've also included the text of his article and a great example.
Many of us believed that bokeh was simply an aperture function. Aperture is the opening of the lens, and as we understood it, a larger opening results in better bokeh.  Aperture is designated in photography as an "f#," and it's a reverse relationship.  In other words, the smaller the f#, the larger the aperture opening. Subsequently, we thought, the smaller the f#, the better the bokeh. Then we found out that this involved something else, especially after we made dozens of images we were confident would display beautiful bokeh but didn't.  We also didn't know why.
Nikon describes aperture this way:
The lower the f/stop
The larger the lens opening
The less depth of field
The blurrier the background

The higher the f/stop
The smaller the lens opening
The greater the depth of field
The sharper the background

It isn't quite that simple, though.
Jared tells us that the next most important aspect to consider is the distance from the lens to the subject besides aperture. Bokeh is a function of the Depth of Field of a particular lens, set by you, the shooter, regarding f-stops and distance to the subject.  You can even create decent bokeh with a smaller aperture. Each lens has a different DoF at different f-stop settings and distances from the subject. There is a painful mathematical formula for determining DoF.  I wouldn't say I like math, so, fortunately, there are handy-dandy Photo Apps to help figure this out.  These Apps have all the lens and camera specs needed to calculate DoF for the best possible Bokeh effect.  Your lens will come with a spec sheet that will allow you to do the math yourself if you like pain.  The app I use for this and a slew of other tips and tricks, Is PhotoPills.  You can get a free version from Google Play or the Apple store, or you can pay for the pro version.  The trick is to do the math before you go on your shoot.  If you take the time to call up the app on your phone, apply all the parameters, and get a "good" camera/lens setting, you'll probably have missed the shot.
To define it, DoF is that area in your image that appears in focus.  Depending on the lens, aperture set, and distance to the subject, there will be an area in front of and behind the focal point that will appear to be in focus.  DoF is smaller as you get closer and larger as you get farther away from the subject with any lens at any f-stop.  Thus, you can increase or decrease DoF depending on the lens, f#, and distance to the subject.
In short, the closer to your subject you are with a large aperture set, the DoF will be relatively short, and you'll create a Bokeh to please the most discerning eye.
Now, get out there and bokeh me.

The text of Jared's article:
Questions and Actions: The Secret to Better Bokeh

“How do you create such soft foregrounds and backgrounds? I understand that using a smaller aperture is necessary, but even at f/4 I never seem to get the effect that others do.”
This is a fantastic question and may be one of the least understood concepts when it comes to composition. But it’s also a complicated one as well. There are several different variables that go into this, and aperture is only one part of the equation. Other things like lens focal length, the circle of confusion, sensor size (full frame vs crop vs micro 4/3rds) also factor into all of this. 
But for the sake of answering this question within the context of a Q&A article, I am going to distill things down to the one part that is just as important as aperture: distance. 
When we talk about what’s in focus and what’s out of focus in our photography, we are really talking about Depth of Field. 
This is one of the most important things to know about photography once you have learned how to use your camera. Depth of Field (DOF from here on out) is that point in which the technical meets the artistic. And thus coming to understand DOF is the segue into the visual art of wildlife photography. 
First, a definition. . . 
Depth of field: the distance between the nearest and furthest objects that appear to be acceptably in focus. 
Emphasis should be on appear and acceptably. 
Technically, your camera can only focus on one point. However, DOF dictates what else also APPEARS to be in focus as well. 
Back to distance. 
When we talk about creating soft and creamy foregrounds and backgrounds, both the distance of the camera to the subject AND the subject to the background are extremely important. 
In a nutshell, the closer your lens is to the subject, the shallower the depth of field will be for a given aperture. 
This is why when we are photographing animals, once they begin to take up 1/3 of the frame or so, it’s a good rule of thumb to stop down in order to get their head and shoulders in focus. Of course, the larger the animal, the more you need to stop down. With something like a horse or a bison, I will stop down to f/8 anytime I am doing a frame filling portrait for this reason. 
But this question isn’t about how to capture animals in focus. It’s the opposite of that. It’s about how to make everything else as out of focus as possible. 
When we talk about DOF, we are talking about the area in the composition that will be in focus. This isn’t just the animal though. Think of it as a bubble, with your subject in the middle of that. More depth of field means a bigger bubble. Less depth of field means a smaller bubble. 
Your subject isn’t quite in the middle of that bubble, however. Your bear or wolf or bird is actually somewhere around the first 1/3rd of that bubble (this is by no means close to being exact and is just a good generalization to keep in mind). This is why in landscape photography we talk about focusing about 1/3rd of the way into the composition (don’t do this for wildlife). 
And this also means that you are always going to have more in focus BEHIND your subject than in front. 
There are some pretty nasty looking mathematical formulas used to calculate DOF. I don’t use them as there are plenty of DOF calculators out there. But honestly, I don’t use those either because it’s not practical in wildlife photography. However, a cursory glance at some DOF statistics is extremely enlightening when it comes to beginning to understand this stuff. 
So, let’s say we have a 400mm f/2.8 lens that we are going to use wide open – meaning at 2.8. 
If the subject is 15 feet from us, the DOF with this lens at this aperture is going to 0.07 feet. 
This is VERY small. This is literally less than 1 single inch that will be in focus. Have you ever photographed an animal close up only to realize that the nose or part of the beak was in focus but not the eyes? This is why. 
Now let’s do this again with a greater distance. This time we are going to use the same setup (400mm at f/2.8) but at 100 yards. This may sound like a lot to some of you. Who photographs animals at 100 yards away? But this is EXTREMELY common. It’s something I see all the time in the field when other photographers are around. And it’s usually done with the idea that they will then “crop” later. 
At 100 yards (300 feet), at f/2.8, the DOF is going to be about 30 feet. 
Big difference. From less than 1 inch at 15 feet in distance, to 30 feet of DOF when at 100 yards away. 
To say that everything within a 30 foot bubble around something like a coyote or an eagle is going to appear in focus, means that you are going to have a VERY difficult time trying to “isolate” your subject. And, of course, this doesn’t mean as soon as something falls outside of that 30ft bubble it’s going to be super soft and creamy looking. There is a slow and steady fall off. 
Now, a 400 f/2.8 isn’t a very common lens. Much more common these days is something like a Sigma 150-600. With this lens, at 600mm, the maximum aperture is f/6.3. And when we run the math shooting at 600mm and f/6.3 from 100 yards away, we find that the DOF is almost identical to the 400 f/2.8. It’s still about 30 feet of DOF and then with that slow fall off of focus. 
This is why the mentality of “I can always crop later,” doesn’t work for me. And this is why “fast lenses” are still relevant. 
Using a DOF calculator in the field isn’t practical for wildlife photography. Nor is it necessary. If you want lots of bokeh in your photos, if you want those soft and creamy backgrounds and foregrounds you see in some other’s images, then you need to get closer to your subjects and not depend on cropping in. Get it right, or as close to right, in the camera.
Yes, f/stops matter. But the other major part of the equation is distance. The closer you are to your subject, the shallower the depth of field will be for the given aperture. And the shallower the DOF, the more bokeh you will have before and after your subject. The more bokeh, the more isolated the subject will be from their environment and the more “magical” the photo will appear.

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