FX-3 SUPER 2000 M42 adapter darkens the viewfinder

Thank you for replying first of all, I wasn't aware of this way of working of these lenses.

Is there a reason why though? I mean, even regular C/Y stop down manually but they don't produce that black ring.

What is that produces that black ring while going to smaller aperture I guess is what I'm asking?


Thank you again
Enrico

First... the M42 part of lens connections.

An FX-3 and any other C/Y bayonet mount camera makes use of the levers/rods/stop-pieces on the rear of the lens to control several things independantly. There's the spring-loaded aperture setting lever to actuate the iris into it's set position when the shot is triggered - then it's released once the shot is exposed. Then there's a bar/bracket/block (depending on the lens design) that tells the camera what the maximum aperture of the lens is when it's inserted into every Yashica body. Then there's a 3rd lever that corresponds to the current aperture as-set by the f/stop ring when it's turned. That lever will vary position, but stay put while it transmits it's position to the camera for exposure calculations. The spring loaded lever that auto-returns after firing is also attached to any DoF preview button that many body's have. It activates the currently chosen aperture when pressed, and springs the blades back open when released.

The M42 lenses only have one connection, and it doesn't equal any of the functions in use by a C/Y lens mounted on a C/Y body. The M42's len's spring-loaded pin is used to activate the iris blades when the shutter is triggered, and on some M42 camera bodies, there's a small 'paddle' inside the mirror box at the bottom that presses in on the pin to offer the user a DoF preview when it's preview button is pushed.

The semi-automatic functions on a M42 lens can't be physically connected to the mechanisms inside an automatic C/Y body, so there's no way to control the aperture between the two - either before, during, or after, an exposure is made.

How it makes the 'circle' dark...

The micro-prism ring around the focus dot in the ground glass sitting directly above the mirror,  is actually made up of several dozen pyramidic prisms molded into that ring. The light is refracted and reflected differently when the light level decreases, and the ring starts to appear darker - and then black - when you reach the end of the aperture range and can't be stopped down any further. It's like an indicator of how much depth is dialed in when you preview the DoF. It was never intended to be an 'always-on' condition.

M42 lenses on M42 cameras will only darken the circle when you preview the DoF. The same applies to C/Y lenses on C/Y bodies. However... putting an M42 lens on a C/Y body locks the preview into it's 'on' position unless there's an M/A switch to disengage that pin from being pushed in by the adapter ring.

Unfortunately there's no workaround if your M42 lens has no M/A switch. The same thing will happen when it's adapted to any camera that uses a bayonet mount - but will accept M42 lenses when attached to mount adapters.

Thank you very much for the incredible amount of information you shared with me, it's really helping me out figure out what's happening here (as it's the first time it happened to me).

So let's say that I have a M42 body with an M42 lens. That pin gets pressed down in DOF preview and (as the aperture gets smaller) the viewfinder gets darker. When it's not pressed instead, I am changing the aperture but it's only going to close down to the correct value once I press the shutter. In the meantime (while I compose), it's wide open.

Now. I have the adapter so that the M42 works on my Yashica. That button is permanently pressed down by the adapter itself. Let's say that the adaptor wasn't pressing it down permanently. The lens would be constantly wide open during composition. Would the aperture go to the correct value once the shutter was pressed?

In other words.. If I'm able to let the pin not be pressed down, I'd be previewing wide open all the time, but once I shoot, the blades should protrude according to the aperture setting chosen on the ring.

Would that be a correct assumption?

Adapting M42 to Y/C doesn't really work as the required flange to film distance change is negative. The Y/C distance is 45.5mm while the M42 is 45.46mm. This, in theory, means you won't be able to focus on infinity though the difference is probably not going to be noticed and will be accommodated by the depth of field. But there is another potential problem. Because the adapter sits fully inside the C/Y mount and doesn't sit on the flange face, if the back of the lens is narrow enough to fit inside the C/Y mount also then the mount springs have nothing to pull against and the lens won't be firmly mounted.