How to Take Pictures With Blurred Background (With Applications in Microscopy)

This instructable is a quick guide on how to take pictures with a blurred background. This is not a new concept. In fact, professional photographers do this all the time. However, it took me a while to figure this out, and I thought maybe writing this instructable will expose a few more people to this idea.

Taking pictures with blurred background is not only pretty, but it's also really helpful when you want to focus your audience's attention to a certain region of the screen, I try to use that when I take still images as well as videos. If that wasn't motivating enough, this method is super useful not only for everyday photography, but also for microscopy, and you'd be surprised how many researchers I met who did not know how to exploit this phenomena.

I hope you enjoy reading, and if you have any questions or comments - I'll be happy to hear it all!

Supplies

You'll need a camera :) To have optimal control, you'll need one with variable aperture (most dedicated cameras have this feature, most smartphones do not). Manual focus is also highly recommended.

Step 1: How to Take Images With Blurred Background

Let get right to the point! To get the background blur we're looking for, the aperture must be as wide as possible (low 'f-number' = f-#).

First, zoom-out as much as possible. This allows you to open the aperture to its widest configuration. If you want to get a closeup with a blurred background, sometimes you're better off zooming out and getting closer to the object. However, you can usually zoom in and still have a blurred background, you just have to find the right compromise between the two.

Once the zoom is set, switch the camera to manual mode or AV, whichever allows you to set the aperture value. Lower f-# (wide aperture) leads to brighter images and blurred background. Photographers call the distance from the camera that is in focus the 'depth of field' (DOF). So blurred images = very limited DOF. Finally, change the f-# to its minimum so that the aperture is open as wide as possible.

The first image shows two almost-identical shots, taken with two different aperture settings. At the stop, I set the f-# to its maximal value (36, nearly closed aperture). You can see the figure (robot) in focus as well as the van behind it. On the bottom shot, I set the f-# to it's minimal value (which was 5.6 for this lens). This time, the robot is still in focus, but the van is not. On other lenses the f-# can be much lower than 5.6, and typical not-too-expensive lenses can get to 1.8-3.5.

The GIF shows you a video I took with a wide-open aperture. I shifted the focus along the metal rod, and the red target shows the region that is currently in focus :)

Step 2: What Is It Useful For

Other than being aesthetically pleasant, controlling the aperture is a really useful too. In every-day photography such as portraits, videos and project documentation, having a limited region of focus helps your audience see what's important. For example, if you're shooting an interview in a busy street, you want the viewer to focus on the person who's talking and not on the things happening around it (see first GIF)

Another use for this method is in microscopy - controlling the DOF is a really nice tool. For example, if you want to only view cells moving along some plane and not at other places. The GIF I attached is of swimming algae (each alga is about 10 microns in length). On the left, you see that the algae are quickly moving out of the focal plane and move out of sight, while on the the right the stay in focus the entire time. For instance, if I wanted to track the their swimming behavior it would be wiser to have the aperture closed and so an increased DOF.

Step 3: Things to Look Out For

Note that having a very wide aperture sometimes makes it hard to take pictures! You can have extremely limited DOF, which makes it hard to shot! For example, when I take portraits of people with a very low f#, I make sure to take a few - because very often I find out that my subjects were out of focus.

Also, if you have a really limited DOF, some of your object can be in focus while other parts are not. This is nice when you want it to happen, but can be annoying in others. If this is the case, and you still want a blurred background, try increasing the f# until you hit a sweet-spot where the background is blurred but your object is not. This is a skill you'll develop with time :)

The Depth-of-Field and Light-Intensity Trade-off

One thing we didn't discuss is the DOF vs light-intensity trade-off. Closing the aperture allows you to have a larger DOF, so a larger region of the image will be in focus, but you're not letting as much light in. I think that this is kind of a magical trade-off, and you always find these type of trade-offs in physics. You can have a large aperture, with lots of light coming into your camera, but at the cost of a very limited DOF (which is actually a nice thing sometimes). Or, if you want a wide DOF so that much of the picture is in focus (for example, taking a picture of an entire room, or a picture of people sitting around a table), you'd have to close the aperture, which means you're not getting as much light in! This means that you may need longer exposure times, which may make the image blurry or prevent you from filming fast-moving objects.

Step 4: The Physics Behind Depth-of-Field

Initially, I thought I'll explain what are the physical origins of the depth-of-field concept, but I realized this will be too long to squeeze into small detour within an instructable, so I found a great explanation online. Honestly, I couldn't have done better. The author did a really great job in her explanations, and if you appreciate knowing the 'why' and not just the 'how', I highly recommend it! :)

Step 5: That's All You Need to Know to Get Started!

That's it, at this point you have all you need to start taking pictures with blurred background. I hope this instructable was helpful to you! If you still need help, feel free to ask!