Focus and recompose

[Woody Campbell]

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I thought I'd start a new thread on one of the points that came up in the endless, and endlessly interesting, "Very interesting answer from Leica . . . " thread. Thanks to Malcom Rains who furnished the following toward the end of the thread:

 

A quick check of the Photo.net archives revealed the work of Martin Tai who, in 2002, dicussed the mathematics of focussing & recomposing with the Leica M. He even produced a small Excel file, Leica M Recompose Focus Guide, that could be printed and kept in one's bag to aid the photographer.

 

The thread can be found here: Leica and Rangefinders Forum: off center focusing

 

Malcolm

 

Of course the problem with Tai's work is that it is pre-digital so the underlying assumption on CoC size is way too large. I've taken the liberty of adapting it for the M8. I used the Fuerstentum depth of field widget to calculate near focus distance assuming a focus distance of one meter for 28, 35 and 50 mm lenses and various f-stop values. I plugged these into the cosine rule formula to determine how many degrees one can rotate the camera and still stay within close focus depth of field. I then translated the degrees into the closest M8 frameline to obtain a practical rule of thumb.

 

Interestingly the Fuerstentum widget produces dof numbers that are general 3 stops more conservative than the numbers engraved on my Leica lenses. The pdf instructions on their website discuss the methodology behind their CoC assumption.

 

So . . . hold you M8 in landscape orientation and focus on an object one meter away. You can safely focus and recompose, rotating the camera up or down, within the following ranges:

 

28mm lens: at f2.0 and f2.8, within the 35mm framelines. f4.0 and beyond, within the full frame.

 

35mm lens: at f1.4 just inside the 35mm framelines. f2.0 and f2.8, within the 35mm framelines. f4.0 and beyond, within the full frame.

 

50mm lens: at f1.4, within the 90mm framelines. f2.0 and 2.8, within the 75mm framelines. f4.0 and beyond - within the full frame.

 

 

I tested these rules on a 28mm 2.8, 35mm 1.4 and 50mm 2.0, all the latest versions. They worked well on the 28 and 50. The 35 rule works as expected at f1.4 and f2.0 but at f2.8 "focus walk" - as descussed in the "Very interesting answer from Leica . . . . " thread - means that you start out up against the near focus point before you rotate, so recomposing leaves you out of focus.

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[msr]

Good work Woody! I have been "playing" all afternoon with this myself.

 

As you have demonstrated, the mathematics for 35mm film Leicas do not apply to the M8.

 

I have printed a PDF of Martin Tai's original film-based "Leica M & R Recompose Focus Guide" and so if someone can tell me where & how to upload it to one of those file sharing sites I will post it. Just remember that it is for pre-M8 Leicas.

 

Malcolm