Several people made comments that they liked the photos that we took during our Environmental Portrait workshop with wide open apertures to create blurry backgrounds. Hélène Dujardin and many other food photographers also like playing with blurry backgrounds as she demonstrates in the book Plate to Pixel. Photographers like blurring backgrounds because the eye of the viewer is drawn to the part of the picture that is in sharp focus.

A blurry background is created in a photograph by having a shallow depth of field, in other words, a narrow range that is in focus. For a portrait, it may be just the tip of the nose to the back of the ear, with everything behind the ear out of focus. In a food shot, perhaps the turkey is in sharp focus at the front end of a long table. The rest of the table is suggested in blurred lines, but it’s the turkey that captures the attention.

There are many things that go into creating a shallow depth of field. The Wikipedia article on depth of field is pretty good. Today, I want to talk about the size of the sensor in the camera because it’s taken me forever to understand this. This is not the same as the mega-pixels that camera manufacturers are always trying to market.

We have an unusually high number and variety of cameras in the house right now because we’re in transition. So, we decided to take the opportunity to do a little experiment and compare how well the cameras are able to blur the backgrounds. Here is the set-up showing, from above, the scene that we shot. The idea is to create a photo with the apples in focus in the foreground and the cover of The Joy of Cooking blurred in the background. With all four cameras, we set an aperture of 2.8. We used a 35mm lens on the large full-frame camera and the equivalent focal length on the smaller cameras.

First up, the camera with the smallest sensor, described as 1/1.7″ type, is my Canon S100 PowerShot. The sensor size description of 1/1.7″ is a bizarre measurement that has to do with the size of vacuum tube sensors in old video cameras. For a small camera, that’s a fairly large sensor. The Apple iPhone 5S has a smaller sensor of 1/3″ type and an old generic cell phone might typically have an even smaller 1/6″ type.

The Canon S100 is a high quality small camera for point-and-shoot pictures. It also lets you change some settings to do more sophisticated things like shoot in aperture priority. As you can see, though, even with a wide aperture, the background doesn’t blur.

The camera with the next larger sensor, 1″ type (still using vacuum tube sensor sizes here), is the one that Rick took to France (and the one that I am considering taking over as my small camera). It’s a Sony Cyber Shot DSC-RX100. In this photo, the text on the cover of the book begins to soften and blur.

The third camera is the new small camera that Rick recently acquired, a FujiFilm X100S. The size of the sensor in this camera is APS-C (Advanced Photo System – Classic), equivalent to a film size that was originally used in a film cartridge for small point-and-shoot cameras. Now, we’re starting to see some real blur on the cover of The Joy of Cooking.

And, finally, we have a camera with a full-frame sensor — the sensor size is roughly equivalent to the frame size of 35mm film. If you remember 35mm slides, you have a good idea of the size. This camera is Rick’s big camera (and I just got a used one of the same model for my big camera), a Canon 5D Mark II. This gives the most blurry background.

Rick made a logarithmic graph of the Depth of Field (DOF) versus the sensor size to show how smaller sensor sizes have larger depths of field when other conditions stay the same. For this graph, the subject is about .75 meter, or about thirty inches, from the camera — appropriate for a food photograph. The graph would be different if the camera were many feet away from the subject as for portraits. However, the graph would remain linear in the middle part, so cameras of the types we tested would behave in similar ways compared to each other.

The format factor, in this graph, is a diagonal measurement of the sensor relative to a full-frame, 35mm sensor. When manufacturers report this number, it’s typically called a crop factor.

The 1.00 in the middle represents the full-frame sensor, equivalent to 35mm film. The depth of field is shown as about .15 meter, or about 6 inches — roughly the depth of a bowl of apples.

To the left of the 1.00 are the large format cameras, big bulky things like 4×5 or 8×10 cameras that are used in studios or for landscape photography. Early photographers only had large format available, so if you imagine the guy standing under the hood behind a big boxy camera, you’re getting the right idea.

To the right of the 1.00 are all the cameras with the smaller sensors. Rick’s  FujiFilm X100S is at 1.53 along the bottom axis, so about halfway through the wide column to the right of the 1.00. There, the depth of field goes up by 53% to .23 meters, about 9 inches.

The Sony Cyber Shot DSC-RX100 is at the 2.73 format factor where the depth of field is about 0.4 meters, roughly 16 inches. No wonder the book didn’t blur very much if 16 inches was in focus and the focus starts to noticeably drop off only beyond that point.

My little Canon S100 PowerShot has a format factor of 4.55 where the depth of field is approximately 0.8 meters, about 31 inches. An Apple iPhone 5S would have a format factor of 7.21, where the depth of field is almost 2 meters, over 6 feet. It would be hard to set up a shot that would give a blurry background using an iPhone. Clear to the right of the graph, you’re seeing what a cheaper cell phone does — the depth of field is quickly approaching infinity, meaning everything is in focus.

This just touches one basic aspect of creating blurry backgrounds, but an important one. If you don’t have a camera with a large enough sensor, you won’t be able to create a shallow depth of field to get the effect of the blurry background. In this case, the camera really does make a difference.

This post was long with a lot of numbers. I’m still learning all this myself, but I’m happy to explore further. Let me know if you have questions.

This is my post for Weekend Cooking. Let me know if food photography feels like a relevant topic for this! At Beth Fish Reads, today, we get a peek into what’s going on in her kitchen via one of her popular Kitchen Journal posts. Check the links for book reviews and recipes on other blogs.

I’m also linking this post to Saturday Snapshot, currently hosted at West Metro Mommy Reads. Her post today celebrates the word of year – selfie! Over a dozen other bloggers have also posted photos today for your viewing pleasure.