When I was a beginner photographer, I distinctly remember going out one weekend to shoot an outdoor event. I thought I had the whole aperture thing figured out. In order to get a sharp subject and a blurry background, all I needed to do was select the widest available aperture (smallest f-number), or use Portrait Mode. I selected Aperture Priority and I shot the whole event at f/4.
When I looked at my photographs after the event, I was really disappointed. Despite those large apertures, the images all featured backgrounds that were either just as sharp as the subject, or only very slightly blurred. I got the focus right, I got the exposure right, but for some reason I wasn't able to get that background blur that I wanted. And it was actually sometime before I figured out why. Today I'm going to share that secret with you, so you don't have to figure it out for yourself the way I did.
pleasure (28) by Flickr user rayzee12
First let's talk a little bit about aperture and what’s actually going on when you get that blurry background in an image.
The term “aperture” refers to diaphragm inside your lens. The diaphragm is made from overlapping blades that can be adjusted to vary the size of the opening. This diaphragm is what allows light to reach your image sensor, which in turn creates the image. Think of the diaphragm as your camera’s pupil — your own pupil contracts or dilates depending on how much light there is, which is why you can see well in both bright light and in dim light. The diaphragm inside your camera’s lens works the same way — when it’s larger, it lets in more light, when it’s smaller, it lets in less light.
But what is it, exactly, that causes those larger apertures to produce blurry backgrounds, while the smaller apertures produce sharp backgrounds? The answer has to do with the direction of light rays.
This is a simplification... but light doesn't come from a single direction — it’s all over the place, coming from multiple angles and directions all at the same time. It's the job of the lens to direct those rays to be focused on the sensor. However, because of the nature of lenses not all light rays can be sharply focused - the further away your subject is from your camera's focus point, the less the rays will converge on your sensor, and thus the subject will be blurry on the image. When you make the aperture smaller, you’re limiting the rays that reach the camera’s sensor so that only those that converge sharply on the sensor get through. The result is an image that is sharp from foreground to background.
So that is all well and good, but the problem is that aperture is not the only factor that goes into determining how much depth of field you get in any one particular image. The other important factor is distance. When you place your subject very close to your background, for example, you are essentially placing subject and background on the same focal plane. So it doesn’t matter how wide your aperture is, your background is never going to fall out of focus because it’s just too close to your subject. In order to get good blur on the background you need to move the subject forward, possibly a considerable distance away from the background.
Another potential solution to this problem is to change the distance between your camera and your subject. As a general rule, the closer you get to your subject, the more blurry your background is going to be. And the more distance you place between your camera and your subject, the less blur you're going to get in that background. This is true whether you are optically closer to your subject via your zoom lens, or whether you are physically closer to him, that is, you’ve zoomed with your feet.
Right eye by Flickr user dubstard
You can see an extreme demonstration of this principle when you shoot subjects at macro ranges. When you shoot a flower up close, for example, you are not going to get clarity from foreground and background even if you're using a very small aperture such as f/22. The reason for this is because when you are at macro ranges, depth of field can be measured in millimeters. So no matter how hard you try, you're always going to get some blur in the background, and you may even get blur on the more distant parts of the subject itself—even when you're using a very small aperture.
Now you may have heard that you can get less depth of field with a longer zoom lens. That is true in the sense that the zoom lens will allow you get optically closer to your subject, as described above. What is not true is that you get decreased depth of field compared to an image shot from a closer physical position. In other words, if you shoot two images in which your subject is the same size in the frame, and in one version you zoomed with your lens and in the other you zoomed with your feet, you will not get more depth of field in the wider angle version than you do with the zoomed version. What you do get is a zoomed-in look at that background blur, which can create the illusion of shallower depth of field. So while you can't technically get shallower depth of field with a zoom lens, you can make the blur look blurrier.
So how does all of this to translate it into those coveted blurry backgrounds?
Let’s go back to the event I shot as a beginner all those years ago. The reason I ended up with so many photographs that had crystal-clear backgrounds despite that larger aperture was that the arena where the event was taking place just wasn't very large. My subjects were too close to the background, and too far away from where I was standing with my camera. All that distance between me and my subjects meant that even when they moved a little further away from the background it wasn't enough to cause the background to fall out of focus. In order to accomplish that, I would have needed to zoom in a lot closer, or be physically closer to them. Or, they would have had to move a considerable distance away from the background.
Combine those factors with that larger aperture that I already knew about, and I would've gotten the blur that I was looking for.
Now what if you're in a similar situation, and you would really like to get some blur on that background, but you're just a little bit too far away from your subject? Well unfortunately I don't have an easy answer for you. Wider angle shots just aren’t compatible with blurry background. There is a popular technique some photographers use called "bokeh panorama", where the photographer shoots a series of zoomed in shots and stitches them together in post processing (like a panorama) to create a wide-view image with a blurred background. But it’s a post processing technique, it’s not something you can accomplish in-camera.
Sasha - Bokeh Panorama by Flickr user Julian Schroeder
In short, you either need to have a very powerful zoom that you can use to get optically closer to subject, or you need to somehow figure out how you're going to get physically closer. If you can control how far away your subject is from the background, that’s also an option you have. Ask her to walk forward a few steps, and that may be enough to get you the blur you were looking for. If not, don’t worry—you’re not a miracle worker. Even an experienced photographer can’t achieve a blurry background if the conditions aren’t right.
Achieving background blur is a little more involved than just setting the aperture as wide as it will go. Don’t worry, if you’re not able to achieve blurred backgrounds with your f/3.5 lens, it’s not because there’s something wrong with your lens. Take a long look at that photo and ask yourself about the distance between yourself and your subject, or your subject and background. Chances are, that’s the culprit in that too-sharp background.
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