Why lens mounts break off when lenses are dropped

Reflector

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NaTrD.jpg
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Saw this image and I suddenly remember those old stories about how some lenses would break clean in half midway down the lens after being dropped mounted. This brought some memories up about various forum discussions going back 10-20 year with some random postings about how lenses aren't durable anymore because "they broke in half."

I think it'd be easier to recover a lens that had the mount shear off or break down in a controlled manner over this.

Posting just because I remembered the discussions about the 100-400 and the mount breaking off... I think I saw something similar on mu-43 (but less horrific looking) a while back with an Olympus body and a Panasonic lens with one of those really unlucky drops where it pulled the mount from the camera.
 

PakkyT

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One of the problem with lenses these days is that even the ones which have metal mounts, the screws that hold those metal mount into the lens are often threaded into the plastic body or into inserts in the plastic body. So what happens is the screws either strip out the plastic threads or yang out the inserts from the plastic. In either case, it is MUCH harder to fix since it involved not the mount but instead the main body of the lens.

A broken lens mount would be much easier to replace. So concerns of wear on the lens mount aside, one solution would be to use only plastic mounts on lenses and design them so that the breakaway force to shear off the mount is a bit less than the force that would strip out or yank the inserts in the body. Probably not a popular solution since everyone seems to be of the opinion that quality lenses must be metal mounts and plastic mounts are inferior. As I hinted at, for wear a plastic mount may have issues with many mounts and unmounts wearing the plastic mount, but for damage control is seems like an idea to consider. Heck, for pro level lenses, make the mount plastic, making it have a connector for the contacts on the other side, three screws, and make replacement mounts readily available. If you feel your mount is getting loose from wear, order a new one. Three screws, unplug, plug in new mount, three screws back on, and you are back in business.
 

Reflector

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One of the problem with lenses these days is that even the ones which have metal mounts, the screws that hold those metal mount into the lens are often threaded into the plastic body or into inserts in the plastic body. So what happens is the screws either strip out the plastic threads or yang out the inserts from the plastic. In either case, it is MUCH harder to fix since it involved not the mount but instead the main body of the lens.
That might be very intentional in regards to the engineering since that rear section can be swapped out rather than having a more involved breakage that splits the lens in half towards the middle. I can only assume that there could be better design in regards to it acting as an easily replaced "mechanical fuse" by having the rear section be more easily replaced than having it be a significant structural component (alignment/expenses) of the middle of the lens barrel like the below two examples. Most of these lenses can be disassembled from the rear end and I assume yanking off the rear part of the barrel is easier than undoing the entire stack to get to the middle.

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b_rubenstein

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The parallelism tolerance between the sensor plane and the camera body lens mount is in the micron range. When a heavy lens is mounted, it doesn't take much of an impact to bend the area of the camera's lens body casting out of range. Most camera mfg's won't repair a camera with damaged castings. A plastic lens mount is a very effective controlled break point that can keep the camera body and/or the lens from being damaged.
 

JDS

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I agree- this whole panic that the mount might break is missing the point. With a big hit, something has to break or bend, and having a plastic mount break is much better than probably any other option. Although, an Olympus engineer might be pretty thrilled with the above photo. If they use self-threading screws into the plastic, they might be able to swap screws and reattach the mount above in a few minutes. Or, maybe the lens is toast, who knows?
 

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