Sensor sizes and DOF

One factor that affects depth of field is the sensor size. Simply put: the smaller the sensor, the more extended your depth of field in any given image.

This is an approximation and simplification (it also depends on angle of view, point of view, etc)  but it is good for us as photographers.

Clearly, this means that if we want blurry backgrounds, we want large sensors. So what are the choices?

  1. Lower-end (and many higher-end!) point-and-shoot cameras usually have very small sensors. These do not make it easy to get blurry backgrounds!
  2. Then there are “almost-APS-C” sensors such as the “Micro four thirds” format – these are almost as big as a crop camera’s sensor. Micro four third cameras are twice as small as a negative. This is the trend in small cameras.
  3. The next step up is the APS-C crop sensor – 1.6 times smaller than a negative for Canon; 1.5 times for a Nikon. Most DSLRs have this size sensor. Some small cameras now also do (like my Fuji X100).
  4. Next, there is a Canon-only size that is 1.3 smaller than a negative – this is the 1D’s format.
  5. And finally, there is the full-frame sensor – it is exactly the size of a 35mm negative.

The bigger the better – also because a larger sensor gives you lower noice and hence higher ISO capability, and a larger, brighter viewfinder.

And this is why we are seeing today’s wonderful move to larger sensors. So my advice: when buying your new camera, do ask how large the sensor is, and go for the largest one you can afford.


Size matters (2)

As I said earlier, in sensors, bigger sizes are better.  Let me expand on that a little.

A bigger sensor means five essential things:

  1. Lower “noise” (and hence, greater ability to shoot in low light).
  2. Greater ability to use selective focus (“blur the background”).
  3. Better use of the maximum resolving power of the lenses’ glass.
  4. On an SLR, a bigger, brighter viewfinder.
  5. Greater ability to use small apertures (high F-numbers”), which otherwise lead to distortion due to the bandwidth of light – so this is physics, and will not be overcome. This is why lenses have a “sharpness sweet spot” – say, at around f/8 on a full-frame camera with a typical lens.

And less fundamentally, but very importantly in practical terms, for those of you choosing between full-frame and crop sensor SLRs, a larger sensor also means that the same wide angle lenses are wider on the full-frame camera.

So what sensor sizes are there?

  • A typical point-and-shoot: 6 x 4 mm
  • Four Thirds (as used in Four Thirds and Micro Four Thirds cameras): 18 x 13.5 mm
  • APS-C, the sensor used on a typical crop SLR, like a Canon 60D or a Nikon D90: around 22 x 15 mm
  • APS-H, a Canon format, as in my 1D Mark IV:  28.7 x 19.1 mm
  • And “full frame”, as in a negative: 36 x 24mm

And every step smaller means less ability to close down the aperture and stay sharp, less ability to blur the background, and more noise.

The benefits of small sensors are the small size, weight and cost; the ability to use small and hence cheap lenses; the fact that they make lenses appear longer (which is good if you are a sports shooter, but bad if you are a wide-angle shooter); and the ability to get close, so point-and-shoots are usually very good macro cameras.


I cleaned my sensors today. On the 1Ds Mark III and the 1D Mark IV. This took more than an hour.

So I thought this might be a good opportunity to talk about sensor dust.

Unlike a negative, which gets “replaced” for every shot, the sensor on your camera can gather dust over time. This then shows up under certain circumstances on your images.

When? When do you notice it?

To understand this, image a small piece of dust just above the sensor. If the lens has a wide open aperture, this piece of dust will not cast much of a shadow, because light from the left might cast a shadow on the right, but light from the right lightens that shadow. The wider the lens aperture, the less defined the shadow cast by the dust.

Now imagine a narrow aperture, a pinhole. Each piece if dust casts a nasty shadow. I.e. it is visible. That’s what dust spots are: shadows from dust specks.

Now think along with me. Say I want to shoot this, as I did during the Henry’s Creative Urban Photography session I taught in Oakville on Sunday.

Lion and Water

Lion and Water

Evidently I need a long shutter speed to blur that water: in this case I selected a quarter of a second.

For which I need a small aperture. f/22 or f/32. A tiny opening in the lens.

So then, when you shoot at small apertures (large “F”-numbers), and especially in plain areas like the sky, dust shows up. Which got me here.

To clean dust, you need:

  1. A freshly charged battery.
  2. A spare camera in case you break the sensor (which I have never heard of, but I am sure it happens).
  3. A rubber air blower, your first port of call.
  4. A rotating brush from Visible Dust.
  5. Also from Visible Dust, pads in the size of your sensor (1.0, 1.3 or 1.6), and the appropriate liquid.
  6. A healthy dose of patience, and a calm demeanor.

First, measure. Switch the lens to a wide angle, and switch to manual focus. Focus close, then while gently moving the camera, shoot a distant white wall, using an 8 second exposure at f/32 at 100 ISO. Adjust as needed to get white, but not blown out. Now check by zooming in and you see dust and smears.

Now clean. Make sure you have a full battery. Now use the manual sensor cleaning function on your camera to open the shutter. Then remove the lens. Carefully blow first, using the bulb blower. Now close the camera, turn it off and on, and repeat the test.

Then if it’s not yet fixed, repeat using the brush, which you first rotate for a few seconds first. You may have better results. But you will equally probably make it worse instead of better.

In that case, repeat again using a pad, after you drop 3-4 drops of liquid onto it. Again, you will make things worse before you make them better.

This is where the patience comes in: after using up three or four pads you will despair. Smears, dust: it gets worse and worse. Every time you remove one dust speck, you add two. Will you ever get it done? Is your camera toast?

And yet… after an hour you get to a point when suddenly, there’s no significant dust. That is when you stop.

So after more than an hour, I now have two as-new cameras.

Two more things:

My camera does this by itself! Yes, but it does not clean off all the dust with its ultrasonic shaker.

Is this not risky? Yes it is. Do this at your own risk. (That said, I have been cleaning sensors for a decade without any mishaps.) My advice: do this when needed, but do not obsess. If you never shoot at f/16 or beyond, don’t worry. If you do not see the dust, do not worry. But if you do – get it done.