How to Select a Lens for Your Camera Part 3: Depth of Field and Cameras with Small Sensors

To loop back around to our early observations in Part 1 of this series, cameras with small sensors, even professional models that create beautiful images, have a difficult time of creating shallow Depth of Field. To illustrate, let's take a look at a professional camera.

This is the JVC GY-HM170. It is a camcorder that has a flexible lens system like our SLRs, so it qualifies for this exercise. The big difference is that it has a 1/3-inch sensor, rather than the 4/3-inch or full frame sensors that we've previously explored. The 1/3-inch sensor has a 4.8x crop factor when compared to 35mm, whereas the Micro 4/3 had a 2x crop factor. To put this into perspective, look at the drawing below:

Let's return to our discussion about field of view. Remember how we can increase our distance to increase the field of view (viewing angle)? And, do you remember what happens to depth of field the farther away from the subject the camera is moved? It compresses. And what was our observation about the image from small sensor cameras?

Compressed depth of field.

This is the wrinkle of sensor size on depth of field. Like the Angle of View, the Depth of Field is affected by the size of the sensor. 

In the case of this JVC camera, the computation looks something like this:

50mm lens (reference)

Aperture f/1.4

Crop factor 4.8

50mm / 4.8 = 10.4mm. There are 10mm and 11mm lenses out there that will fit the 1/3 inch sensor, so the correct Field of View isn't hard to achieve. However, opening the aperture from f/1.4 by nearly 5 stops isn't possible. The lowest possible f-number is 0.7, and I don't think I've seen more than 3 of these floating around. From a practical standpoint, the fastest available lens in this format will be f/0.95. This is one stop faster/more open than f/1.4.  Therefore, from a practical standpoint, the lowest f-number in 35mm that can be reproduced by a 1/3 inch camera is f/5.6. So, f/0.95 on a 1/3 inch camera is approximately the same as f/5.6 on the 35mm camera. This can be calculated by simply counting five stops down from f/0.95:

0.95    1.4     2     2.8     4     5.6     8     11     16     22

An f/stop calculator can be found here.

Let's use the reference 50mm lens on a 35mm camera combination to visualize how shallow a depth of field the 1/3 inch camera is capable of, regardless of field of view. Since we are able to achieve f/0.95 on a 1/3-inch camera, we need to count down 5 stops on that 50mm lens. F/5.6. There isn't a lot of focal separation there. Since we know that the shallowest focal area occurs when the subject is closest to the camera, let's look at another chart to see what we can achieve:

1/3" Sensor 10mm Lens 1.5-foot Subject Distance

The chart indicates that we have focal separation of 1.8 inches. That's basically 3x the depth of our first experiment with the GH2. So, shallow Depth of Field is achievable with the small sensor camera, but the requirements to achieve the effect are much more demanding.

With these concepts and tools, you should be able to determine which lens to use to achieve the look you want, or even to determine whether or not the look is possible.

Key references that should help you with your calculations:
Crop Factor Calculator
Depth of Field Calculator
F/stop Calculator

I welcome your comments and questions.