[MF3D.FORUM:340] Re: 1/30 Yes!

["Oleg Vorobyoff" <olegv@xxxxxxxxxxxxx>]

I'm so sorry.  I should have included an example calculation.  I used a 0.0024m
on-film deviation since we are talking MF here:

base offset = 0.0024*(3048*0.914/(3048-0.914))*(1/0.07-1/1.5)
                 = 0.0299m or 1.18 inches

I agree, Harold, that using a simple heuristic would be better than running
calculations, especially under pressure in the field.   But my point is that the
1/30 rule is something more than a heuristic - it can be quite precise, as I
think my examples demonstrated.

One point pertinent to MF.  It was a bit of a shock to check the depth of field
scale on my 75mm lens.  I would not focus on 1.5m as in the example.  That might
work with a 35mm lens on a 35mm camera.  Your 3m focus point would be more
suitable for MF.  But you would still have some softness in the foreground and
background.  That is stopped all the way down to f22!  Looks like I may want to
take most of my MF stereos with a roll film holder in the view camera.  The
change in focus point, incidentally, has a negligible effect on the calculation,
yielding 1.21 inches instead of 1.18.  The 1/30 rule gives 1.20.

Oleg Vorobyoff


Harold Baize wrote:
>However, when I applied the formula to your second
>example I found a lens separation of only 0.603 inch.
>
>>From your second example (low landscape with clouds)
>using the General Solution formula:
>
>base offset = d*(far*near/far-near)*(1/focal - 1/focus)
>
>d is set at 0.0012m
>far is  10,000 ft (3048m)
>near is 3 ft (0.914m)
>lets try a focus at 3 meters
>focal length is 70mm (0.07m)


...and

>What is my point.. I really want to make the
>point, that the 1:30 "rule" is not a
>rule, it is a heuristic. A simple and quick
>way to find a solution that works in most situations
>well enough to make more precise calculation
>unnecessary. If one needs to use a calculator and
>a metric conversion utility to find basically the
>same result (or make a mistake and be way off),
>then the argument is strong for using the simple
>heuristic.