Today, while cleaning the aperture blades of a Carl Zeiss Jena 25mm f/4 M42 lens, I removed the front optical assembly and took out the aperture blades for cleaning. After carefully reinstalling the blades in their correct positions, I attempted to refit the aperture control cylinder, only to discover that it would not go back into place because it was being blocked by a small metal tab that actuates the aperture mechanism. It became apparent that the M42 mount at the rear of the lens had to be removed first so that this component could be detached before the aperture control cylinder could be reinstalled.
Since I was unable to find any videos or photographs online showing how to remove the M42 mount on this particular lens, I decided to document the entire disassembly process myself. I hope this record will serve as a useful reference for anyone attempting a similar repair in the future.
今天為了清理Carl Zeiss Jena 25mm f4 M42 的光圈葉片，我拆了前鏡組，取出光圈葉片清理後，重新擺好葉片，再裝回葉片控制圓筒，發現裝不回去，因為有一個控制光圈葉片的金屬突起阻擋。看起來必須先拆鏡頭M42接環，才能將該零件移除才能安裝。由於網路上找不到相關的Ｍ42接環拆除影片或照片可參考，所以我就拍下整個過程，做個記錄與分享。
Carl Zeiss Jena 25mm f4 M42 Zebra Version
This is the lens base after the front optical assembly and aperture blades have been removed. Once the aperture blades are correctly repositioned, the aperture-control cylinder can be reinstalled. However, at approximately the 8 o’clock position, there is a small metal protrusion that engages and rotates the aperture-control cylinder. This protrusion prevents the cylinder from being inserted back into place, so it must be removed first before reassembly can proceed.
這是前鏡組和光圈葉片取出後的底座，重新擺好光圈葉片，就可以放入葉片控制的圓筒零件，但在8點鐘位置有一個轉動葉片控制圓筒的金屬突起導致無法放入圓筒，必須先移除才行。
The rear of the lens features the common M42 screw mount, and at first glance there appear to be no visible screws for disassembly. In fact, the black base beneath the mount is an aluminum outer housing. To remove it, firmly hold the lens body and rotate the black housing counterclockwise. The housing will then unscrew and can be lifted off, revealing the internal mechanism underneath.
鏡頭底部是很常見的M42接環，外觀看不到任何可拆卸的螺絲。其實接環下方的黑色底座是一個金屬鋁外殼，先握緊鏡身，再將黑色的外殼以反時針方向旋轉，即可取下。
After removing the black outer housing, you will see a ring-shaped component. This part connects the aperture actuator pin on the M42 mount to the depth-of-field preview switch on the lens body, forming the mechanism that controls whether the aperture remains fully open or stops down. Once the single spring attached to this mechanism is carefully removed, the ring-shaped component can be lifted out, allowing access to the parts beneath it.
After removing the black outer housing, you will see a ring-shaped component. This part connects the aperture actuator pin on the M42 mount to the depth-of-field preview switch on the lens body, forming the mechanism that controls whether the aperture remains fully open or stops down. Once the single spring attached to this mechanism is carefully removed, the ring-shaped component can be lifted out, allowing access to the parts beneath it.
取下黑色外殼後，可以看到一個環形的零件，它連接了接環上的光圈頂針和鏡身的景深預觀鈕，是用來控制光圈「開/縮」的機構，將機構上唯一的彈簧拿下來後，即可移除這個零件。
After removing the ring-shaped component, you will see four screws located beneath the mount. Once these screws are removed, the M42 mount can be detached easily, providing access to the internal aperture mechanism and other components inside the lens.
取下環形零件後，就可以看到接換下方有四個螺絲，將這些螺絲旋起後，M42接環就可以順利拿下來。
Next, the metal linkage that directly controls the opening and closing of the aperture blades becomes visible. Attached to it is another spring, whose purpose is to keep the aperture blades positioned at the f-stop selected on the aperture ring. This spring provides the necessary tension to ensure that the aperture mechanism follows the setting of the aperture ring accurately and remains fixed at the chosen aperture value during operation.
接下來，真正控制光圈葉片「開/縮」的金屬連桿出現了，上面有另一只彈簧，是用來將光圈葉片固定在光圈環所選定的光圈值。
One end of the linkage extends into the lens barrel and connects directly to the aperture blade control mechanism. The other end features an L-shaped metal tab, which interfaces with the ring-shaped component removed earlier. Together, these parts form the complete aperture actuation system. Through the interaction of the M42 aperture pin and the depth-of-field preview switch, the mechanism ultimately controls the opening and closing of the aperture blades, allowing the lens to switch between full-aperture viewing and the selected working aperture.
連桿的一端是深入鏡筒內的光圈葉片控制機構，連桿另一端L形鐵片則是與先前拆下的環形零件連動，最終透過光圈頂針和景深預觀鈕完成「開/縮」光圈的動作。
By removing these two screws, the component that controls the opening and closing of the aperture blades can be detached. Once this part is out of the way, the aperture blade assembly and its control cylinder inside the lens barrel can be reinstalled without obstruction, making reassembly much easier.
拆下這兩顆螺絲，可以把控制光圈葉片「開/縮」的零件取下來，這樣鏡筒內的光圈葉片零件就可以順利安裝了。
I referred to mikeno62's video, Carl Zeiss Jena Flektogon 4/20 for Exakta, as a guide during the disassembly/reassembly process.
我參考了 mikeno62 的影片 Carl Zeiss Jena Flektogon 4/20 for Exakta
https://youtu.be/XkzOlicu1bo?si=cEWQ1F8tx5XDYN_p
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