fujifilm - Why I am getting different values for depth of field from calculators vs in-camera DoF preview?

When using depth of field and hyperfocal calculators, I get very different result versus the in-camera DoF preview of my Fujifilm X-M1.

For example:

With DOFMaster (or any other online calculator), for a Fujifilm X-M1, with a focal length of 16mm and f-stop = 8, I am supposed to get hyperfocal distance at 1.62 meters with DoF from 0.81 meters to infinity.

If I use those settings on the camera, in-camera DoF preview shows that DoF is about from 1.2 to 2.5 meters.

What am I missing?

Answer

First, a word about what depth-of-field is and is not:

In a way, depth-of-field is an illusion. There is only one plane of focus. Everything in front of or behind the point of focus is out of focus to one degree or another. What we call DoF is the area where things look, to our eyes, like they are in focus. This is based on the ability of the human eye to resolve certain minute differences at a particular distance. If the slightly out-of-focus blur is smaller than our eye's capability to resolve the detail then it appears to be in focus. When you magnify a portion of an image by making it larger or moving closer to it you allow your eye to see details that before were too close together to be seen by your eyes as separate pieces of the image.

Since things are gradually blurrier the further they are from the point of focus, as you gradually magnify the image the perceived depth of field gets narrower as the near and far points where your eyes can resolve fine details moves closer to the focus plane.

Since depth-of-field is dependent upon viewing size and distance as well as the visual acuity of the viewer it is hard for a camera to indicate depth-of-field if it doesn't know what the display size of the photo will be. Any in-camera DoF measurement is going to be based upon an assumption regarding the eventual viewing conditions of the photograph.

Assuming the standard 8x10 viewed at 10 inches by a person with 20/20 vision is probably a little too broad in the current digital environment. But most of the online calculators still assume this standard viewing size and distance. If you expect to view images at a 1:1 pixel size on a computer monitor, the DoF for the same exact image will be much narrower. After all, viewing a 22MP or so image on a monitor with 96ppi pitch is like viewing a part of a 60x40 inch print!