P3D a bit of clarification

[Tom Deering <tmd@xxxxxxxxxxx>]

In his retraction, George Themelis wrote:
>The 1/30 rule is derived assuming that the furthest object is at
>infinity.

An incomplete explanation.  Besides requiring infinity, the 1/30 
"rule" works only if your camera has a certain lens spacing, and has 
lens of a certain focal length. Change any of these, and the 1/30 
"rule" is meaningless.

>If it is not at infinity, then there are other "rules".

Turns out, there would be in infinite number of rules, to account for 
all of the possible combinations of far point, interaxial distance 
and focal length.

>For example, using the f8 marks in a Realist DOF scale will give
>you the depth range to aim for.

Maybe okay for 5-perf, but not full frame 35mm or other formats. And 
what if you don't shoot with a Realist?  What if your Realist has no 
DOF scale? Personally, I think that a rule with too many exceptions 
is not a very good rule.  It's starting to sound like "Sunny 16".

And what if you are shooting with a wider lens spacing?  What if you 
are shooting macros?  What if you are shooting in a larger or smaller 
format?  What if you want to use a longer or shorter lens?  We would 
need a "rule" for each one.

However, there is a mathematical formula that takes all of these 
variables into account.  It's heavy math, but there are spreadsheets 
that make it easy.

One thing you could do is to run some numbers through a spreadsheet. 
Jot them on a cheat sheet that you carry in your camera bag.  THEN 
you've got real geometry, not voodoo.

Certainly, you could develop a feel for the geometry involved by 
shooting a ton of film.  I know a fellow who said he shot eighty 
rolls of film in a year.  Me, I can't afford to learn by hit and miss 
that way.

Cordially,

Tom Deering

PS: I've just added some super-simple spreadsheets to 
http://www.deering.org that anyone can use.