DEPTH OF FIELD AND HYPERFOCAL DISTANCE
A question was raised at the recent discussion group held at the rugby club as to the calculation of depth of field when taking photos.
In the days of pre digital, and when cameras seemed easier to use, most lenses had a depth of field scale engraved round the edge. These featured various ‘f’ numbers – with the smaller stops being set widest apart. Having determined the aperture to be used to take the shot one would estimate the furthest distance required to be in focus. Then, by rotating the focussing ring , one could bring that distance to line up with the aperture being used. By checking the corresponding matching aperture value one could read off the nearest point of sharp focus. If there appeared to be an adequate depth of field one could take the shot -but without touching the focussing ring. If not, then one had to select a smaller aperture and repeat the above.
It all sounds complicated but it was very simple in practice. By manipulating the aperture and focussing ring the lens was actually being set on what was known as the hyperfocal distance(HFD) Note From now on you can ignore the HFD as it can get very complicated.
The HFD is dependent on the following:-
Effective focal length of lens (Can be variable with a zoom lens)
The furthest and nearest points of sharp focus required.
The image resolution needed ( ignore unless very big images are required.
Reference to the classic books on the technical aspects of photography will give pages of formulae and detailed charts but, unless you’re doing very serious or technical work, IGNORE IT ALL.
While most modern digital ‘cameras’ such a compacts, I-Phone or Pads, Bridge, and even the simpler DSLRs, are OK for the ‘point and shoot’ brigade by having auto focus, few if any have aperture control or selective focussing. They are also missing flash and cable release sockets, but that’s a different issue.
However all is NOT lost as, from a practical point of view, depth of field is only generally relevant for landscapes or street scenes, and here one has usually selected a ‘wide angle’ setting i.e. about 30 – 35mm. and, unless the light is particularly poor, the aperture will be automatically set to about f11. With this combination of focal length/aperture everything will be sharp from infinity back to 6’0” (infinity to 8’0” if using f8) BUT the focus must be set to the HFD of 10’0” or (15’0”). To do this, guess something that distance away, aim the camera at it, depress the shutter button slightly to lock the focus, and then take the shot required. You might need to up the film speed to fool the camera into selecting a smaller aperture
Basically the smaller the aperture, and the shorter the focal length of lens being used, the greater the depth of field. Also, as a rough guide, the depth of field achieved will extend out twice the distance from the point of focus (HFD) as that back towards the camera.
For those few members who wish to take object d’art, depth of field is all important. I used to put a card of fine print at the nearest point of focus required, then focus on that point, and note the distance shown on the lens(a) ( Don’t use a tape measure). Do the same for the furthest point of focus(b). Take 1/3rd of (b – a), add to (a), and focus on this distance.