Focusing system of rangefinders

[Compaq]

I've been reading on the interwebz about how these works, but I'm a little confused. I don't want to bother people, but I have a few questions. First, here's how I've understood it:

Obviously we see the scene through the viewfinder. There's a second window that gathers light. This light is reflected first on a mirror or prism, which is coupled to the lens, and then reflected again to the viewfinder. Turning the focusing ring makes the prism move, which again is what we observe in the viewfinder when we're focusing.

But, I've that there are three light gathering devices on a rangefinder. The viewfinder, the other one I mentioned (which I don't know what is called) and the lens. How is the lens coupled to the rangefinding device??

 

[pvdhaar]

But, I've that there are three light gathering devices on a rangefinder. The viewfinder, the other one I mentioned (which I don't know what is called) and the lens. How is the lens coupled to the rangefinding device??

The other one is the rangefinder window.

The lens is mechanically coupled to the RF mechanism. A roller on an arm inside the camera is pushed against the rear of the lens. When focusing, the lens moves in/out, and the roller/arm follows. The movement of the arm is then transferred to the prism/mirror assembly.

 

[Richard G]

Viewfinder, finder frame illumination window and rangefinder window. There is no optical connection of the lens.

 

[Compaq]

I wish this illustration was bigger. That pretty much explains what I wanted to know. I think much in pictures, and illustrations really help me learn!

 

[btgc]

Is this a bit larger? 🙂

 

[Compaq]

Thanks!! 😀

1: viewfinder, what we look through. We don't see that huge mirror, though?
2: light gathering thing. I suspect the size of this will greatly affect the brightness of the viewfinder?
3: rangefinder window. Reflects the small patch which we focus by. The prism turns as we adjust the focusing ring on the lens.

The bigger the distance from 1 and 3, the more accurate will the focus be, yes? Any more major points I should be aware of?

 

[ColSebastianMoran]

Compaq, you are close.

Blue arrow at #1 is the light coming in. Creates our basic view of the scene through the viewfinder.

Green arrow #3 is the light coming in through the (usually) smaller rangefinder window. provides the rangefinder patch in the view. The prism b rotates as the lens is focused (mechanical connection). When everything is setup right, the patches match in the viewfinder when the subject is in focus.

But, what is light path #2? It's the illumination for the bright-lines (item e) that we see in the viewfinder.

Brightness of the viewfinder depends on the quality path #1. Visibility of the patch depends on path #3.

The whole thing is remarkably precise. Perhaps delicate. Needs adjustment from time to time.

 

[btgc]

On many cheaper cameras instead of prism (b) there's only mirror, but function is same.

Window 2 illuminates moving frame w/ framelines, automatically providing parallax correction. Some older or newer but cheaper cameras have fixed framelines and additional parallax correction marks, those cameras may not have this middle window. And middle window doesn't affect brightness of viewfinder, it's for framelines only.

 

[Compaq]

Thanks again, guys :hug:

On this pic, the viewfinder is easily identified as the big window above the PC socket. Left of that comes the rangefinder window, yes? What's that big thing around it?
Left of that again is what I expect is illuminating the EV range in the viewfinder?
BUT, the thing all the way to the right. Is that which illuminates the framelines?

 

[batterytypehah!]

Easy to find out, just cover the windows one at a time as you look through the VF and you will see what each of them is for...

 

[Compaq]

LOL, that's a clever solution. Should have thought of that myself! 🙂

 

[farlymac]

Thanks again, guys :hug:

On this pic, the viewfinder is easily identified as the big window above the PC socket. Left of that comes the rangefinder window, yes? What's that big thing around it?
Left of that again is what I expect is illuminating the EV range in the viewfinder?
BUT, the thing all the way to the right. Is that which illuminates the framelines?

The framelines ilumination is the corregated frosted glass surrounding the rangefinder window. To the right of the viewfinder is the CdS cell window. You are correct on the other windows. In later cameras, the CdS cell was moved to inside the lens barrel, either at the top, or 90 degrees from the top (3 or 9 o'clock position), with at least one manufacturer placing it at the bottom.

PF

 

[Compaq]

Ahh, okay!

I just want to know how this all works. I've wanted a rangefinder for some time, but I didn't know how they work. My friends say "dude, awesome camera" and stuff, and I want to be able to explain how it works... you know, be all nerdy! Nerdy's good! 🙂

 

[farlymac]

Well, if that is your camera, then you can download a manual for it at http://www.butkus.org/chinon/ , which will also give you some tips on it's usage. Nothing wrong with being nerdy about your cameras, unless it bores the crap out of somebody. But then they are the ones missing out on all the fun.

PF

 

[Compaq]

I like to think I'm a nerd with social intelligence 😉

The Manual is an invaluable tool indeed!

 

[Compaq]

So, how exactly is the distance calculated, here? Is that hard? If someone knows, don't be afraid to post it, I have some background in maths from the uni.

 

[Phil_F_NM]

It's a mechanical calculator that does the triangulation for you.
You're looking through 1) and the physical distance between 1) and 3) are the rangefinder baselength. When you focus the camera, you are angling the mirror or prism at 3) to a point in space where it is apparent that the view matches the view through the viewfinder 1).

Phil Forrest

 

[btgc]

Once you tear down your first RF and put it back you'll be able to tell friends how it works 🙂

 

[Compaq]

I like that notion, btgc 😛

 

[wolves3012]

The bigger the distance from 1 and 3, the more accurate will the focus be, yes? Any more major points I should be aware of?

Not quite. It also depends on the magnification of the RF system and whether there's any mechanical play in it. All other things being equal though, yes.

Assuming there is zero mechanical error, then a figure called the effective baselength can be got by multiplying the actual distance 1-3 and the magnification. In other words, if you have the distance as, say, 25mm but a magnification of x2, you'll get the same effective baselength as having the distance 1-3 as 50mm but a x1 magnification. Hence, you can increase focus accuracy by magnifying the view (provided the mechanical system is good enough). Up to a point, there are some advantages to shortening the baselength and using magnification - such as reduced parallax.