Rangefinder equivalent base length for critical focus

A

AlexMax

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Hi Gents,

this is my first post here. After one decade of SLR photography, as a Hobby, I have decided to use some of the RFs i inherited from my uncle, an Army and Intelligence photog.

These include several M bodies, 2 Canon RFs, one Topcon Beseler, and several lenses.

The biggest RF magnification factor that I had in the M line up, was the .72

So I bought myself an M7 .85x, and an M6 TTL 0.85x, plus a leica magnification eyepiece 1.4x.

I plan to use these with razor thin DOF ( ultra fast lenses at close ups ), with a Nokton 35mm F 1.2, a Nokton 50mm F 1.1, an Avenon 21mm F2,8, and a Zeiss Sonnar F 1.5. I also have a Jupiter 9 85mm f2.0, and I am fancying one Canon 35mm F 1.5.

So, my question here is :

Is there any mathematical relation between the RF equivalent base length, the DOF, and the largest upper bound on focusing error ?

I just want to check wether critical focusing at close ups is even attainable with my setup...

Any Ideas ??

Best regards

Alex
 
Alex,

while unable to answer your question (math and me never really hit it off), I'd still like to say "welcome to the forum, post some pictures!"
 
I suppose you might have found http://www.cameraquest.com/leica.htm already? Otherwise, it is a good resource.

Now, one thing to consider is that exact focusing may not be a strong point of RFs compared to SLRs, in particular not for thin DOF lenses where good SLRs provide good visual feedback. For wide angle lenses the picture is different.

Additionally, the RF mechanism relies on mechanical linkages from some part of a removable lens to a prism (or mirror?) in the camera - a more complex and sensitive arrangement than ensuring that the matte screen is at the same (optical) distance from the lens as the film is. Try out the cameras and lenses to learn what the limits are for your equipment in your hands.

/Anders
 
Erwin Puts in his "Leica Compendium" lists the minimum RF base x magnification needed in order to focus accurately the lenses, however in the list of your lenses, there is nothing that appears particularly challenging to a 0.72x Leica M, not to speak about the 0.85x. Your focusing error and body sway will have a much bigger impact in practice. What is more important, is actually to verify if a given lens/camera combination focuses precisely, and if not, adjust. Most often, the bodies need calibration, but sometimes the lenses too. In critical cases. lens and body should be calibrated together. Keep in mind, that Nokton 50/1.1 and Sonnar 50/1.5 shift the focus as you stop down, so check them only wide open. It might be, that your Sonnar is actually optimized for f 2.8, in which case you might want to get it re-calibrated by Zeiss for f1.5.
You could benefit from checking the effective DOF of your lenses here:http://www.dofmaster.com/dofjs.html
 
Thanks for your hints :)

@ AndersG : I have already checked that site, lots of useful info btw...

Additionally, the RF mechanism relies on mechanical linkages from some part of a removable lens to a prism (or mirror?) in the camera - a more complex and sensitive arrangement than ensuring that the matte screen is at the same (optical) distance from the lens as the film is. Try out the cameras and lenses to learn what the limits are for your equipment in your hands
Click to expand...

AFAIK matte screens are not as precise as RFs for a properly selected effective base length. Let´s not forget that RFs can be deadly precise, making errors of some hundredths percent in the calculated range.

It was a RF coupled to a Fire Director table that allowed DKM Bismarck to score a "kill" on the second shot against HMS Hood, at a distance 24200 m... so, i guess this is more than just "beginner's Luck" ( I´d say it is Zeiss Optics onboard a warship.. )

@mfogiel :

Thanks for the hints and the link :)

Best regards,

Alex
 
Of course, as a general rule, longer base lengths are better; but you also need to think about about precision in manufacture (most especially consistency or "slop"); wear (again affecting tolerances); focus shifts with aperture; individual variations in eyesight; and the care taken in focusing.

Cheers,

R.
 
AlexMax said:
AFAIK matte screens are not as precise as RFs for a properly selected effective base length.
Click to expand...

Only in that a matte screen cannot have a base length bigger than the aperture, while rangefinders can be arbitrarily dimensioned.

AlexMax said:
It was a RF coupled to a Fire Director table that allowed DKM Bismarck to score a "kill" on the second shot against HMS Hood, at a distance 24200 m...
Click to expand...

A RF several meters in width (or more common in ballistics, height).
 
Way back it was found 90 mm was the break point for accurate focus. Below 90, use RF, above SLR.

In any case, if you want close ups, a RF is the wrong tool.
 
I've found that with 75mm at f/2 and at .9x (.72 + 1.25x maginifier) at min. focusing distance, errors in focus are readily apparent with focus shift and framing being nowhere near accurate according to the framelines. The only option I've found to be usable (in digital, film may be more forgiving) is Live View on the M 240 or any of the various adaptable bodies. A Sony with a helicoid-type adapter will also allow for macro work. On the M's the minimum focusing distance of the lenses will affect just how close you get.

I've succumbed to the Siren of limited DOF more then once, but have learned better. Unless it's for an artistic interpretation, it rarely led to satisfying photos. If the former is your pursuit, baselines and accuracy are not all that important.
 
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