I have always had a love-hate with my Leica APO-Summicron 90mm f/2.0 ASPH lens. I love the rendering of it but I realized I was always back-focusing it. What I mean by that is that I would focus on my subject but the thing in focus after I press the shutter was something behind what I had been focused on.
If you’re coming from an SLR or EVIL camera (Electronic Viewfinder Interchangeable Lens) camera this might seem strange since you’re looking through the same lens that you’ll be taking the picture with. The thing with a rangefinder camera is that you’re not looking through the same lens. In the picture above the main image I’m looking at is from the window on the top right in the picture above. As I focus the lens, the lens mechanically communicates with the camera about the focus distance and an image from the little window on the top left of the camera is used to focus. Don’t worry, I’ll make a full post about how a rangefinder works soon.
So the real key is that the camera has to be in sync with the lens about how far something is. The fact that it’s mechanical is kind of secondary.
Here’s the view inside. The lens has a surface (later in the post) that moves the roller on top. That roller is linked through a set of mechanical linkages to a mirror that moves an image around.
As it turns out mine was a scosh off. It really only was visible with the 90mm lens since the depth of field was so shallow.
Inside that roller is a screw:
This is a slightly eccentric screw such that as you turn it the roller’s position is slightly moved fore and aft. The next screw in the system controls how much the focus moves the mirror. That’s been fixed with some shellac and thankfully I didn’t have to touch it.
A 2mm hex turning around 5 degrees counterclockwise was all it took to solve things! Now when something lines up in the rangefinder that thing is in focus!
If you take a look at the cutout in the lens mount you see a piece of metal. The first shot has the lens focused at infinity and the second has it at around 3 feet (1m). What you’re seeing is that surface has moved out. That’s the surface that the roller that I adjusted rolls in. It’s through that linkage that the lens communicates mechanically with the camera. All of the rangefinders do something similar.
How much does it move?
It moves as much as a simple 50mm lens would move. Any other focal length has to have some conversion built in through some screws.
I’ll dive into that more later on though. I thought it was pretty neat though.
Before you think I figured this all out myself, nope. I got it mostly from the l-camera-forum.