Blurry Backgrounds, Big Sensors and Bokeh

February 21, 2013, 3:37 pm

In my New York Times column on Thursday, I reviewed the Sony RX1: the first compact digital camera with a full-frame (that is, HUGE) sensor inside.

As I wrote, “these sensors are as big as an old piece of 35mm film (1.7 inches). They deliver unparalleled low-light quality, richness of color, detail and soft-focused backgrounds.” Later, I pointed out, “This camera creates beautiful blurry backgrounds. That soft-focus background effect is a hallmark of professional photography â€" and of big-aperture, big-sensor cameras. Until now, few pocket cameras could defcus the background at all.”

Now, photographers online are a cantankerous lot. Photography, in the end, is something of a black art, and everybody’s got an opinion. So it didn’t take long for a few e-mails to arrive along these lines:

“I don’t know why you keep insisting that soft focus backgrounds are dependent on ‘professional cameras’ and/or sensor sizes. The amount of soft focus in a photo is entirely dependent on the lens aperture and focal length. A bigger sensor may have better image quality and better grain upon very close examination, however, the depth of field and area that is in soft focus will be exactly the same. It’s PHYSICS.”

Well.

I’m confident that I’m right. After reviewing hundreds of cameras, I can say with certainty that those with big sensors do the best at blurring the background.

You’ll also hear that defocusing effect called “bokeh” or “shallow depth of field.” (The “field” is the area ! that’s in focus, as measured in its distance from the camera. A shallow depth of field means that only a thin slice of the scene â€" back to front â€" is in focus, and everything closer and farther is pleasingly blurred.)

Pocket cameras, which generally have tiny sensors, generally keep everything in focus. They have a huge depth of field. You usually can’t get the bokeh effect at all.

To answer this fellow by e-mail, I decided to find out why a big sensor contributes to blurry backgrounds.

If you Google “sensor size bokeh,” you can find many confirmations that sensor size is, indeed, a factor in blurring the background. Here’s an example:

Background blur depends on your depth of field. Depth of field depends on several factors:
1.Lens focal length (35mm, 200mm, 50mm)
2.Lens aperture (f1.8, f5.6, f8)
3.Sensor size (APS-C, 35mm, medium format, large format)
4.Subject distance
With (1), the longer the focallength, the thinner the DOF. With (2), the larger the aperture (smaller number) the thinner the DOF With (3), the larger the sensor, the thinner the DOF. With (4), the closer the subject, the thinner the DOF.
Example: If you have a 200mm lens, at say, f2.8, on a 35mm full frame sensor, and the subject is near you (2-3m), you can blur the background quite a lot.
Inversely, if you have a 35mm lens, at f8, on a cropped dslr (APS-C), and the subject is 6m from you, the background won’t really be blurred out.

So it’s true that sensor size is a factor â€" but I was still looking for the reason.

After an evening of reading dense articles and even watching some technical YouTube videos, I have  my answer.

It’s true that a larger sensor does not, by itself, create the blur effect. And yet it’s also true that a camera with a larger sensor does create the blur effect.

I know, right Huh

I found the ! clearest ! explanation here, at Cambridge in Colour, (and it’s still not all that clear). It boils down to this: a smaller sensor requires a wider lens. And a wider lens requires a smaller aperture (the opening created when the shutter opens) to capture a scene of the same width. And a smaller aperture means bigger depth of field â€" less blur.

Or, as the article puts it: “As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject.” (Focal length means how much you’re zoomed in.)

The same article, by the way, makes two other points pretty well. First, bokeh isn’t always desirable. It’s fantastic for portraitsâ€"but for landscapes, you usually want everything in focus. maller sensors, like the ones in all small, inexpensive cameras, may be terrible at blurring the background, but they’re great at keeping everything in focus. On cameras with huge sensors, it may be difficult to create large depth of field (and keep everything in focus).

The article also explains why big-sensor cameras are so much more expensive than small ones (the Sony RX1 compact, for example, is $2,800). It has to do with “how manufacturers make their digital sensors. Each sensor is cut from a larger sheet of silicon material called a wafer, which may contain thousands of individual chips. Each wafer is extremely expensive (thousands of dollars), therefore fewer chips per wafer result in a much higher cost per chip.”

In addition, every time you stamp out a big sensor, you’ve increased the odds of including a defect that was present in the wafer. Therefore, “the percentage of usable sensors goes down with increasing sensor area.” The bottom line: “A sensor with twice the ! area will! cost more than twice as much.”

Anyway. To conclude today’s class: Clearly, sensor size does affect your ability to create the blurry-background effect. Not on its own, but because of its relationship to the lens. The smaller the sensor, the wider the lens, and the harder it is to offer a large apertureâ€"which is the key to the blurry-background effect.

Or, as my correspondent might put it: “It’s PHYSICS.”