How to Select a Lens for Your Camera Part 1: Field of View

Field of View

Field of view is the area that the camera sees. If you were to draw a line from the center of the camera lens to each of the edges of your photo, you would have the outline of a cone. That cone is your field of view.

To understand the impact of sensor size on field of view, look at the full frame image below that was taken with an 85mm lens at f/1.2:

Now, if we were to set up a GH2 (or similar camera with a 4/3-inch sensor) with the same lens and aperture setting as the full frame camera, its field of view would be in the area defined by the red box:

So, the GH2 image would look like this:

If we were used to taking photos with a 35mm camera, this image is framed much tighter than we would expect from this lens. The cause of this narrowing effect is what we call crop factor.

Crop Factor

Since the 4/3” sensor is about 18mm wide, and the full frame sensor is 36mm wide, the M43 sensor is capturing a field of view that is ½ that of the full frame sensor. Therefore, our 85mm lens visually becomes the equivalent of 170mm on the GH2.  This is important to remember. The narrower the field of view, the higher the focal length. The GH2, with it's sensor size half as wide as a 35mm camera, ends up with an image equivalent that is twice as long. Therefore, if we want to capture the same field of view on the GH2 as we did on the 5D MkII, we would need to use a lens that is half the focal length of a lens on a 35mm camera, which leaves us with the focal length of 42.5mm.

The key takeaway here is to know the crop factor of your camera, which is easy to find online. Here is one such calculator.

To summarize, to match the field of view of a 35mm camera, such as the 5D MkII, you must divide the focal length by 2. That will be the desired focal length in Micro 4/3. See the table below to illustrate this idea:

35mm to GH2 Lens Conversion Table

If on a 35mm camera you use…	…on a GH2, you should use
25mm	12mm
35mm	17.5mm
50mm	25mm
85mm	42.5mm
100mm	50mm

Now that we understand the basic mechanics of Field of View, and how to select a lens that matches the FoV characteristics of our 35mm reference lens, it's time to move on to Depth of Field.

Activity
Go online and research your camera. Find the sensor size and use the calculator located here. Write down your crop factor value. Also jot down the lens that would match the angle of view of a 50mm lens on a 35mm camera. This will aid you in your future lens selection exercises:

Reference focal length (Fr)
Crop factor (c)
Target focal length (Ft)
Fr / c = Ft

Next, we will explore the impact of Depth of Field.

How to Select a Lens for Your Camera Part 2: Understanding f/stop, Depth of Field, and Aperture