Understanding
The Math
Most of us can skip this part and just refer to the chart for the
point on which to focus our lens. But for those wanting to calculate
other formats or recalculate using a different value for the circle
of confusion we've provided the formulas.
(L x L)
H = 
(F x D)
Where: 

H = 
Hyperfocal distance (in
millimeters) 

L = 
Lens focal length (i.e., 35mm,
100mm) 

F = 
Lens aperture fstop 

D = 
Diameter of circle of least
confusion (in millimeters) 


Digital SLRs 
D = 0.02 


35mm format 
D = 0.03 


6x6cm format 
D = 0.06 


4x5in format 
D = 0.15 
Why use a different value for the circle of confusion? The
constant used here is based on what is considered to be acceptable
sharpness in an 8x10 print at a normal viewing distance. The problem
is that "normal" and "acceptable" open the door for interpretation.
The constant we used has been around for a while and is widely
accepted. But it's been argued that this constant doesn't take into
account the tremendous improvements in lens design and film
sharpness that we've seen over the last few years. Search around the
Web for a while and you'll find numerous values to substitute for
the circle of confusion constant, along with very technical sounding
arguments for each that can even make it sound like the author knows
what they're talking about.
In reality the markings on your lens barrel aren't all that
accurate and if you're shooting with a newer lens you don't even
have the lens markings going for you. That means you're estimating
where the hyperfocal point is. So unless you're using and electronic
rangefinder the slight differences in hyperfocal distance you'll
get by recalculating with a different constant for the circle of
confusion just won't make any difference for real photographers in the real world. Also
useful is being able to determine your near focus distance:
H x D
NF = 
H + (D  L)
Where: 

NF = 
Near focus limit (millimeters) 

H = 
Hyperfocal distance (in millimeters from
equation above) 

D = 
Lens focus distance (in millimeters) 

L = 
Lens focal length (ex. 35mm, 50mm, etc.) 
