Why is the RF patch the size it is and where it is?

[jaay]

Advertisement (gone after registration)

I apologise if this has been answered many times before or if it's a daft question but why is the RF patch the size it is and why does it have to be centered?

For example why does the window not have multiple patches that would allow off centre focusing without recomposing (death at low DoF)? Or why is it simply not a lot bigger?

 

I've had a rangefinder for years but prior to the M8 it was contax g which, as you know, were AF anyway. This has confused me too as the g lenses were superb quality and just as small as Leica ones yet had AF in them, and although it was at times unreliable with a digital body it would be less of an issue as you could at least see on an LCD if it had got the focus right or not!

[Advertisement]

The Leica Q3 is available for pre-order at B&H Photo Video and Adorama

[dpattinson]

Basically if it wasn't in the middle of the screen it would lie outside the FOV of the longer lenses - making focusing rather less accurate due to the need to reposition for framing. Having it in the middle of the screen guarantees the smallest average repositioning from focusing to recomposing - which is handy for accurate focusing.

 

Leaving aside the mechanical/optical proacticalities, multiple RF patches would interfere with composing imho by introducing more clutter into the VF, also some of the multiple patches would lie outside the framelines for longer lenses.

 

It might be possible mechanically to shift the patch right to left - which would reduce focus error due to reframing, but it would complicate what is already the most expensive part of the camera (apart from maybe the CCD in the digitals) and would be of very limited utility imho.

[giordano]

The RF patch is the size it is because - one presumes - the M3 development team reckoned this was the best compromise between "too big" and "too small". Putting multiple patches in the viewfinder would require multiple rangefinder mechanisms, horrendously complex, extremely difficult to keep synchronised and impossible to fit in an M body.

 

Making the one patch moveable would be equally difficult.

[Guest mc_k]

Basically if it wasn't in the middle of the screen it would lie outside the FOV of the longer lenses ... Having it in the middle of the screen guarantees the smallest average repositioning from focusing to recomposing

 

I have wondered about an off-center patch, too. I think the centered patch and the reframing error is a big disadvantage compared to an AF camera with multiple focus "patches." Depends on your style, of course, but in most of my photos the subject is closer to an edge than the center. I don't see why the patch couldn't be off-center if one was using, say, 90mm or shorter lenses...the 135mm or longer lenses are maybe not so ideal for a rangefinder to begin with.

[jaay]

Interesting responses. I think a movable RF patch would be excellent.

[stunsworth]

I think a movable RF patch would be excellent.

 

An engineering nightmare, more likely an impossibility. It isn't just a case of moving a patch in the viewfinder.

[Guest mc_k]

Advertisement (gone after registration)

I think a movable RF patch would be excellent.

 

Not a movable patch--an offset patch. Is this impossible? I wouldn't have any idea. The patch already travels diagonally in the viewfinder to correct parallax error I think.

[adan]

Yeah, but offset where? I frame off-center, too, but sometimes left, and sometimes right.

 

When you look at the "patch", you are basically looking through a long tunnel all the way to, and then through, the smaller RF window below the shutter dial. Roughly behind the "red Leica dot" is a movable lens that is what actually shifts the image when focusing, and can be adjusted up and down for vertical alignment.

 

The hard edges of the "patch" are actually just a rectangular cutout in the black mask that also has cutout slits that you see as framelines. So it moves with the framelines for parallax, but can't move independently of the framelines. They are all the same piece of metal with holes in it.

 

As to size: ideally, the RF image should be a point. Using a larger "patch" is already a slight compromise to accuracy, because the left and right sides have subtly longer and short base-lengths than the center. I used a Russian 6x6 RF with a really large RF patch (and a short true baselength) and when the images were aligned on one side, they were not aligned on the other side (and vice versa). I had to be really careful to ignore the edges and focus using only the exact center of the patch, or things were back- or front-focused.

 

So the size as it is is a compromise to be useful but not inaccurate, and to not come too close to the frameline cutouts (or else the frameline "stencil" would fall apart).

[Guest mc_k]

Yeah, but offset where? I frame off-center, too, but sometimes left, and sometimes right.

 

Offset to one side...which I could get away with because I mostly shoot people in portrait orientation. But I see your point of course.

 

I started out on a rangefinder, so I never really thought much about how the camera dictates composition in a lot of circumstances.

[Guest mc_k]

P.S. thanks for the detailed description. Are there any good books about the innards of rangefinder cameras?

[luigi bertolotti]

It is so from M3 intro.. and was a big advance over old Leicas ; surely the size is a compromise between usability and precision (and the need to couple 135 mm) but I think it's still a very good compromise; focusing in off-centered parts of the frame, when necessary, is quick and easy, much more than the terrible complication of a movable or multi-patch RF; until the basic principle of Leica M remains like it is, I think is better not to have something different.

[Guest mc_k]

I think is better not to have something different.

of course...I think the main point was to know why things are the way they are. I use the camera because of its simplicity and its history, too, and wouldn't want to change that aspect.

[SJP]

size -because it has to be much smaller than the full frame to provide a selected point of attention

 

where - because the RF mechanism is fully mechanical/optical and cannot be rearranged at will considering the extreme accuracy of the mechanism

 

Granted - it takes some getting used to but you have a lot more control than with AF, or at least more intuitive control.