Autor teksta Simon Stafford
The formula to calciulate the hyperfocal distance is as follows-:
H = (F x F)/ (f x d)
Where-:
H = Hyperfocal distance (mm)
F = Focal length (mm)
f = lens aperture value
d = diameter of the circle of least confusion (CoC)
The CoC factor is determined by the ability of our eyes to resolve detail. It is calculated on the basis of our eye being able to tell the difference between a circle and a solid dot at a specific viewing distance. It is from this that we either percieve an area in a two dimensional image as being in or out of focus. Remember that critical focus is only achieved at the specific focused distance of the lens. If we are able to resolved detail in front of and behind this plane, then that area of the subject is said to be within the depth-of-field zone. As the magnification of the original image is increased the percieved sharpness of detail in the DoF zone decreases.
Back in the 1920's engineers working for Leitz, designing lenses for the early Leica cameras, decided on a CoC factor of 0.03. Later, Zeiss calculated that the CoC factor should be 1/1760th of the diagonal dimension of the image sensing area. So for a 24 x 36mm 35mm film frame this equates to a CoC factor of 0.025. Applying this to the smaller 15.6 x 23.7mm DX format sensor used in Nikon digital SLR cameras (D1 series and D100) the CoC factor is 0.016.
Nikon continue to calculate their DoF tables using the older, less conservative CoC factor of 0.03. I, and many other photographers, prefer to use the Zeiss method. If we take all three CoC factors for the circumstances you describe above (20mm at f/16) the hyperfocal distance is as follows-:
CoC @ 0.030 = 0.8m
CoC @ 0.025 = 1.0m
CoC @ 0.016 = 1.6m
You can see from the calculations that there is a significant difference between the hyperfocal distance depending on whether you are shooting on film (as determined by the Nikon method of calculation) and the same lens focal length and aperture value on the smaller DX format sensor.
To be on the safe side I always shoot at one aperture value smaller than the one I use to calculate the hyperfocal distance. For example I will calculate/look up the hyperfocal distance for an aperture of f/11, set the focus ring at this distance, but shoot at f/16.
As you have discovered stopping the lens down to the working aperture, when you are shooting at very small apertures, results in the viewfinder image becoming too dark to be usable. My tip for using the depth of field preview button on any camera is to stop down the lens to no more than f/8, because can still see enough to make a visual assessment.
