Depth of Field

When reading about photography, most of the articles are focused on either gears or the “exposure triangle” to get clear and sharp photos. Plenty talks about “razor-sharp” images or “scene compression”. Many spoke about wide-aperture for a creamy background (bokeh). So, I’ve dreaded my lens and thought there was something wrong because my photos were out-of-focus.

I upgraded to a 50mm lens with settings for ISO 100, f/2.8 and 1/125 sec. The portrait came out unsatisfactory. Either it was sharp around the eyes (or just one eye), nose, ear, forehead – seldom do I get good sharpness for the whole face.

The image on left was taken at f/2.80 and it misfocused on the ear and blurry around the eyes. The image on the right was taken at f/3.50 and focussed on the eye and the ear almost quite clear, showing the facial features quite clearly.

I have been following the exposure triangle: setting the ISO, aperture and shutter speed. But something was still missing. So, I go back to my root and put on the mathematician hat. There is definitely a missing equation (or less talked about).

Depth-of-field. This is another factor that affects the sharpness around the focus area. It varies based on the distance between the face and the camera. To put it simply, in my understanding, depth-of-field is a buffer area on the focus object perpendicular to the lens. Toggling the aperture changes (compressing or widening) the depth of the buffer area. For a given aperture, the depth-of-field is also different based on the distance of the object to the lens.

If the aperture is fixed, putting the object farther away makes the depth-of-field wider.

Two examples of the effect of “depth-of-field” and the distance between the object and the camera. Assuming the aperture is fixed, and the focus is on the eye. On top, the buffer region between A and B is where the objects are going to look sharp perpendicular to the lens. Keeping the aperture similar and moving the object farther away from the camera, the buffer region, C and D expanded. The distance from A-to-B or C-to-D is directly related to the aperture size and the distance between the object and the camera. Narrower the aperture, the wider the region is. (Note: The buffer regions are not exactly scaled).

Using this understanding, I revisit the technique used to collect facial images for my experiment. The settings of the parameters are laid out below.

Parameters

ISO: 100
Speed: 1/125 sec.
Light source: Flash + bouncer
Distance: 1.5 meters
Focus: Wide (AF-C) + Eye AF
Camera: Sony A7R iii on a tripod
Lens: Sony FE 50mm F1.8

Using a DOF calculator, if I want the subject’s face to be overall sharp, the aperture has to be f/4.0 or narrower.

DOF Calculator downloaded from Google Play. For the selected settings, the buffer for depth-of-field around the focus area is 20.98cm (9 cm to the front and 11 on the back from the focus spot).

The distance was quite close because my study room on the campus is quite limited in terms of space.

Result

The overall facial features are clearly sharp from the top-point of the nose and back to the ear at f/6.30

Take-away

  • ISO-Aperture-speed is not the only factor for sharpness.
  • Knowing the distance between the object and the camera is also important as it affect the depth of field around the focus region on the object.
  • If the object has a depth perpendicular to the lens and is relatively close to the camera, use a narrow aperture so that the front and the back of the object is in focus.
I found a good explanation on depth-of-field by Kellan Reck.

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