Weighing In on Sensor Size

February 23, 2013, 2:34 pm

Wow. My post on Thursday about the relationship between sensor size and soft-focus background certainly lived up to its opening premise: “photographers online are a cantankerous lot. Photography, in the end, is something of a black art, and everybody's got an opinion.”

By Saturday, about 100 photographers had flooded the comments with posts ranging from “You're an idiot” to “You're exactly right.”

(How contentious are photographers? They can't even agree if the blurry background effect is desirable. One poster calls it “lazy” and a “visual crutch.” Never mind that it's been a sought-after hallmark of professional portraiture and product photography for decades.)

But if you set aside the vitriol, you can see a lot of people trying very hard to make sense of a complex topic. Here are some follow-ups to my post, based on the comments:

* “Bokeh” does not mean “blurry background,” as I implied (and as many articles online have characterized it). Instead, it describes the character or quality of the blur. I shouldn't have used them interchangeably.

* On the other hand, “depth of field” does not mean blurry background, either, as some of the commenters have said. A shallow depth of field gives you a blurry background, but you wouldn't say, “Ooh, look at that lovely depth of field!”

* My central point was that cameras with small sensors make it hard to get blurry backgrounds. But many readers pointed out that even a phone's camera (like that of the iPhone) can produce blurry backgrounds if you hold it very close to the subject.

That's true: small-sensor cameras can sometimes blur the background when shooting macro, or close-up, shots. I'm sure commenters will explain why!

But can a small sensor camera blur the background when shooting a standard portrait, where the subject is, let's say, six feet away? Very rarely, says I. Yet it would be very easy with a large sensor camera.

There are myriad mathematical and physics-based explanations of the relationship of aperture, distance, focal length, sensor size and blur. Most of them make fairly opaque reading.

But I liked the explanation of Roberto, a 50-year career photographer from Mexico:

“The facts are really quite simple. The more a lens acts as a wide-angle lens (giving a wider field of view), the greater will be its depth of field, assuming that aperture remains constant,” he wrote. “Conversely, the more the lens acts as a telephoto lens (narrower field of view), the shallower the depth of field will be (given the same aperture).

”

He added: “The reason a small sensor digital camera will not easily blur backgrounds is that they do not have any telephoto lenses which also have wide open apertures. The zoom lenses they come with, by the time they get to telephoto length, are also typically stopped down to f/5.6 or so. If one of these small cameras were equipped with a telephoto lens which could open to f/1.8 or f/1.4, you would see plenty of background blur.
”

So I accept the critiques of readers who found fault in the way I reached my conclusion. But I'm happy to report that the conclusion itself - that large sensor cameras are better able to create background blur than small sensors - seems to be correct.