I've just bought a Yashica Mat 124G and I have a question about it. When I look at the taking lens via the chamber, the iris is not closed, there is an opening depending on the aperture selected. Is this normal? Shouldnt this rear iris stay closed until the shutter is pressed?
No, the shutter leaves are in front of the aperture leaves and are normally closed. The iris can stay at its setting, there is no need for both to be closed.
