Please Explain Focus vs Lens Speed

[Spleenrippa]

Okay guys, I feel like a complete idiot for asking, having used RFs for the past few years... But I see it referenced so often that I'm just going to put myself out there and say it:

I don't quite 'get' what you guys are saying in regards to the difficulty of focusing a fast/wide open lens on a rangefinder.

Primarily, I use a 40/1.4 Nokton on my R3A, and all I do is line up the pictures in the rangefinder patch- which is what you do for any lens 😛

Is the difficulty that people talk about just due to the thin DOF wide open and making sure you're focused on the right thing in the patch?

Just going to grab my dunce cap and sit in the corner now 😉

 

[furcafe]

Yes.

Same goes for a fast lens on any camera, including an SLR (though you have a bigger focus area).

Is the difficulty that people talk about just due to the thin DOF wide open and making sure you're focused on the right thing in the patch?

 

[Spleenrippa]

Yes.

Same goes for a fast lens on any camera, including an SLR (though you have a bigger focus area).

Thanks for confirming that for me. I feel much better now, haha!

 

[Luddite Frank]

Also, the longer the focal legnth of the lens, the "thinner" the DOF becomes; that's why 135 to 150 mm is generally regarded as the practical limit for RF focusing in 35 mm format.

 

[kossi008]

Is the difficulty that people talk about just due to the thin DOF wide open and making sure you're focused on the right thing in the patch?

The difficulty also comes from the accuracy with which you overlap the patches (human factor) and the accuray the rangefinder provides for the remaining focus error (camera factor).

By the latter, I mean that if you have aligned the patches visibly to the best of your ability on two different camears (say, a ZI vs. a Bessa R4x to make the point), this translates into different focus error margins depending on the effective base length of your rangefinder.

Concrete example: the King of Sloppy Focusing (myself) has greatly profited from switching from a Bessa R to a Zeiss Ikon. 😀