Blurry Backgrounds

If I want a sharp foreground subject with a blurred background – you have heard me say it many times, there are several ways.

The reason this subject is always confusing is that it is very complicated. “Sharp focus” and “depth of field” are subjects for mathematicians (check the Wikipedia entry, if you wish). Hyperfocal distance, lens geometry, approximations, cropping, aperture, magnification, f-numbers, image format size, sensor size – all these have an effect. The main factors that affect DOF are:

  • Sensor size
  • Proximity to subject
  • Zoom
  • Aperture number
  • The ratio of subject distance to focal length
  • Cropping

Several of these factors are complicated and need not be taken into account all the time – but several can help you in practice. Chief among them: it is not just aperture that affects depth of field – it is also the distance to the subject. As Wikipedia puts it:

For a given format size, at moderate subject distances, DOF is approximately determined by the subject magnification and the lens f-number.

In practice, this means that to get less depth of field (i.e. a blurrier background), you need to either:

  1. Select a lower f-number, or…
  2. You need to magnify more. And you can magnify more by zooming in, or by getting closer.

So to get an image like this, with the person behind the object blurred out, you do not necessarily need a fast lens or a full-frame camera:

In fact that was taken with a Canon 7D with a 35mm lens (equivalent to a 50mm lens) set to f/5.6. This is an aperture that every lens can achieve. But I was close to the object!

That said, of course a lower f-number in the same situation gives you more blur. Here’s f/2.8, which good zooms can achieve:

And here’s f/1.4, for which you need a prime lens:

So the lesson, I suppose, is that if you want blurred backgrounds but you cannot right now afford that full frame camera and the low f-number lenses that you should really invest in, at least get close.


Wide means deep

“Wide angle means deep depth of field”. Meaning, a wide angle lens makes everything sharp, from close to far.

That’s true, but as this image of my friend and colleague Joseph Marranca shows,  it is not quite all there is to be said:

Even with a 16mm lens you can create selective depth of field, by:

  1. Using the lens open (this was at f/4)
  2. Getting close to the subject that is closest.

You see, it is the ratio between close and far that counts. If the far subject is twice as far as the closest subject, then both will be sharp. But if the far subject is, say, twenty times as far as the closest subject, then it’s a different story: you can get the far subject blurred.

And getting it 20 times farther can be done in two ways: move it farther, or move the closest subject closer.  Or get closer to it. And that is why, and how, this works.

Crop sensors and depth of field

So a crop camera has wider depth of field?

Depends how you look at it. The way I look at it: “only because you will use it differently. If you use it to take the same picture, you’ll get the same depth of field”. But of course for that “same picture” you’ll need a different lens.

OK, this is complicated. So let me just show you by example.

Picture one: full frame 1Ds MkIII camera, 50mm lens at f/1.4.


(You can click to see the large version).

Picture two: crop camera, a Canon 7D, with a 35mm lens also set to f/1.4. That 35mm lens on a crop camera is about the same as a 50mm lens on a full frame camera; i.e. enables me to be at the same point and yet get roughly the same picture. Now I see this:


As you see, this gives you roughly the same depth of field. If there is any difference, it is minor. SO:

A 35mm lens on a crop camera gives you roughly the same picture as a 50mm lens on a full frame camera, and roughly the same depth of field.

A few notes, by the way:

  1. These results are consistent; not just “one randomly picked picture”.
  2. The DOF (depth of field, i.e. how much is sharp) is extremely narrow. Great care is needed when shooting at f/1.4!.
  3. I was aiming a single focus spot at the dot (near the 10) in all pictures (On the 7D, I was using the extra small focus spot).
  4. I have noticed that the Canon cameras (or is it the lenses?) focus too closely (they “front-focus”) when open at f/1.4. By f/2.8, this effect is either gone or too small to see clearly. In the last picture, I added an adjustment of +15.  This does not change the depth of field; it is just to make both pictures about the same.

Point 2 explains a few things to me, by the way. That’s part of the Canon-effect, more on which in the next few days; it’s why I have to adjust extra when wide open.