What is Martin Tai's one third focal length rule of thumb ?

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What is martin tai's one third rule for landscape ?

-- martin tai (martin.tai@capcanada.com), January 10, 1999


F/3 Rule of Thumb for Landscape Photography

For 35mm landscape photography, where distant objects such as moutains, buildings are the main object of interest in composition, focus the lens at infinty is the way to go. That greatly simplies depth of field in travel photography.

But how to choose aperture in the middle of busy travel schedules ? You are not going to pull out a Depth of field chart are you ?

I need a very very simple "rule of thumb"

By analysis of the travel photo habit of myself and other poeple, I find that most of the time, people take pictures hand held camera at eye level.

A Minox 35mm (or a SLR )held at eye level is ahout 1.5-1.6 meter off ground, holding the camera level, the closeset point visible in the viewfinder is the point of ground cut off by the bottom line of of viewfinder.

Now suppose I want that point to be the hyperfocal point , so what is the fstop ?

I invented "Martin Tai's F/3 rule of thumb for landscape", as I found out, that the required aperture = one third of focal length of 35mm camera !

Once I set the camera's aperture at F/3, focus the lens at infinity, then depth of field rules tells us that everything from hyperfocal point to infinity will be sharp---- that is, everything I see in the viewfiner will be sharp !

For Minox GT-E, I set it at 35/3 = f11 , focus at infinity and shoot.

For my Leica Vario-Elmar 28-70 zoom lens which has no DOF scale as most zoom lenses are, I too set fstop = one third of focal length, and focus at infinity. for example if I use 28mm short end, I choose 28/3 = F9 if I select F=45mm I select aperture at 45/3= F16.....

-- martin tai (martin.tai@capcanada.com), January 10, 1999.

Proof of F/3 Rule

The mathematic proof of "Martin Tai one third focal length rule of thumb" was posted to usenet .

   () ---                                 ()
    |  -                                   |
    |     ---                             /| \
  h |           ----                       |
    |                ----                 | \
   / \---------------------------------------------------------------------                      -
                         D                                 infinity

When a 35mm camera is held vertical and point at distant horizon, the closest point on the ground cut of by the bottom line of viewfiner is


in which

Let D= hyperfocal distance H=F*F/(fstop*coc)



       F*h/18 = F*F/(fstop*coc );

from which

          fstop = 18*F/(h*coc )



         fstop =~  F/3. 


Lu Yu Pick Tea in the Mountains

-- martin tai (martin.tai@capcanada.com), January 10, 1999.

Does the F/3 rule of thumb applies when camera is held horintally ? Of course it does, as the closest cut off point on the viewfinder is further out. It can be proved that the required fstop to make "what you see in viewfinder is what is sharp in picture" , the fstop =~ F/4. But to make thing simplest, F/3 rule of thumb should cover both situations.

It is not necessary of to hold camera absolutely level for F/3 rule of thumb to work;

-- martin tai (martin.tai@capcanada.com), January 10, 1999.

Hey, what about MInox ultraminature cameras ? Well, Minox 8x11 cameras has fixed aperture, you have no choice. But still does F3.5 achieves "viewfinder sharp" when you stand up ? Yes, it can be easily proven, that substite 5.5mm for 18mm and substite Minox's coc = 1/60mm, then the required "viewfinder sharp" fstop =~ F/5 Minox lens' focal length = 15mm, hence viewfinder sharp fstop =15/5 = F3. So you see, F3.5 is more then enough for you Minox A/B/LX/TLX for travel photography, just set lens at infinity and shoot, what you see in viewfinder will be sharp.

-- martin tai (martin.tai@capcanada.com), January 10, 1999.

Any exceptions ? There are always exceptions for any rule of thumb. If you squat down, F/3 rule does not work :)

-- martin tai (martin.tai@capcanada.com), January 10, 1999.

Martin, Your f/3 rule is really interresting, but how does it change in my case, where the eye level is 1,9 m? Any difference?

-- Wolfgang Fischer (wolfischer@hotmail.com), May 12, 1999.

Wolfgang, for camera height >1.6M, the F/3 rule should apply. But you do have and interesting question-- what is the range of applicability for F/3 rule ? Let me do you calculation then post the reply here.

-- martin tai (martin.tai@capcanada.com), May 12, 1999.

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