Here is an inside view of a rangefinder, with a sketch to help explain how it works.
Johns picture shows the effect of a split image but is a lot clearer than the image viewed through this one. As can be seen the semi silvered mirror has a yellow tint and that appears in one half of the split image.

With the cover removed it turns out to be very basic.

Don

Hi Don,

Thanks for the inside view. They are, as you say, very basic. They are also very flimsy and suceptible to damage by knocks. I am sure you realise that my illustration was not taken through an actual range finder. The view through a real one can be difficult to see properly, that is, until you get used to it, as I am sure you already know. How lucky we are today to have auto focusing and micro prisms if you are not a fan of auto.

John

They used to make giant rangefinders on battleships to help them with elevation etc needed to hit the enemy. Guess they were somewhat more robust than photographic ones.

I can see a rangefinder is used as an instrument to ascertain distance from it to the subject. I guessed at focus aid, I believe to be correct but what's the difference, if any between rangefinder and focus aid?

Gosh I'm scraping the bottom of the barrel of my memory and knowledge but in essence I don't think there is a difference. One tells you the distance to set the lens at for correct focus and the rangefinder worked on the same principle of bringing two images into one the distance being read off a scale and then entered into the necessary equations with all the other parameters to give bearing and elevation to the guns. The optics were set feet apart to give good trig readings rather than inches so merely a matter of scale. Range finders are probably lasers now as in modern Leica range finding binoculars.

Stephen,
None really. I guess that 'range finder is more specific because it finds the range, whereas a magnifying glass could be used as a focus aid but not as a range finder. The term 'focus aid,' is often used to describe a specially mounted magnifiying mirror used with enlargers during wet printing.

John

John,

We will be showing our age on this thread. I have 2 of what you describe in the loft. :D

Don

Unlike you Don I only have one. That's why my right arm is 3 inches longer than my left arm.

John

I have a Voitlanger Vitessa at home with an F2 lens, unfortunately the iris vanes are a bit damaged. It was my first 35mm camera and I loved the built-in rangefinder. Yes, happy days.:)

Hello ADJH,

In my early encounters with photography I had a Voigtlander Vito B. It didn't have a range finder but it did have a Color Skopar f2.8 50 mm lens which was renowned for the quality of its images.

John