Yes, I actually have experienced a locking cable release unexpectedly unlock. Luckily it didn't happen while I was messing around in the back of the camera. It happened during a long nighttime exposure while making "open flash" multiple exposures. Somehow the locking nut slipped, ruining a carefully planned exposure. Later I wrote an article for Shutterbug Magazine about open-flash technique and included some of my photographs:
"Special Effects From Open Flash"
Shutterbug, 1989
https://www.halfhill.com/openflash.html
But the immediate question is whether this thread's original questioner is more likely to have handy a roll of film or a locking cable release and a ground glass cut exactly to size to fit inside the film channel of a 35mm camera. I say the roll of film is more likely; others disagree. Or else they think it's easier to buy a locking cable release and some ground glass, cut the glass to size, bevel the sharp edges, clean off the powdery residue, and then perform the suggested focus-checking experiment. I can shoot and develop a roll of film in as much time.
Now to the hazards of focal-plane shutters. Forty years ago in Cleveland, I discovered a marvelous little shop filled with odd merchandise and operated by an elderly sole proprietor. It wasn't really a camera shop, but he was a surviving member of that near-extinct species, the general fix-it man. Among other odd things, he sold me long-discontinued but new-in-box vacuum tubes for my 1940s Crosley AM/SW radio. I also bought a powerful muffin fan salvaged from a scrapped mainframe computer. (Still have it.)
What could he fix? How about a mysterious shutter-curtain leak in my Canon TX? A stuck between-the-lens shutter in my Konica Auto-S2? A slipping film advance that overlapped frames in my Canon QL17? (He never fixed my Ricoh 500G, though, because that cheap little camera worked flawlessly for decades until a collision destroyed the viewfinder.) I wish I could remember that man's name. I wish even more that I had taken his picture in his little shop of wonders.
He had nothing nice to say about focal-plane shutters. Lens shutters are beyond the photographer's reach, but an FP shutter is exposed whenever film is loaded. In dim light, or in a hurry, even an experienced photographer can punch a finger through the fragile shutter curtains. I'm certain he would recoil at the idea of a typical amateur photographer checking focus with a ground glass. His bone yard told the story. Doctors say "Never stick anything in your ear except your elbow." My fix-it man would say "Never stick anything in the back of your camera except film."
It doesn't take a moron to make an expensive mistake. Repair people have been feeding their families on that truism since 1839, I suspect.
Let's unpack some of the above comments.
I've personally cut and ground glasses for focus checking. Takes less than half an hour with some basic equipment and cerium oxide. Glass is pretty water resistant stuff. Getting it clean after takes at least five seconds under a tap. Whenever I need to use a glass: it's waiting for me along with my others (more on that later).
On a cable unlocking itself.
I notice that when describing your experiences of that, you don't say that your camera shutter was toasted when the cable released. It appears I
have to state the obvious. So I will. A cable closing the curtains will cause
no damage to the camera when using a ground glass, because the glass is not going to impede the curtain travel. In order for any curtain damage to occur you would have to insert the glass into the rear of the camera body and then close the curtains. Even then (if the curtains are fabric) you're unlikely to actually hurt those. What might be more problematic is if the ribbons slack off and jump off their pulleys or roller (depending on the design). This could be anything from a minor inconvenience to a major disassembly to rectify (again, depending on the design).
I suppose the take away is that, yes, there is an element of risk involved in using a ground glass. This is true. But there's a risk involved in getting out of bed in the morning. Where I object to the thrust of your arguments is in the level of risk involved, which you have
vastly overstated. If you aren't confident of being able to gently place a small glass over your camera film rails and tape it in situ, temporarily, without lunching your shutter, then-- frankly--you should probably dispose of it, post haste. Because you're at least as likely to stick a thumb through a curtain the very next time you try to load a film into it. Speaking plainly (again), I find your arguments, preposterous.
If I have time after dinner this evening I may just set a ground glass over the rails of an Exakta SLR I need to check. I've got around a dozen glasses here of various sizes for 35mm & 120 film camera gates and a selection of loupes for examining said glasses. I am not the only member of this site to possess similar equipment. I use it
frequently. I suppose I run the risk of ruining the Exaktas curtains when I do. But seeing as I have just fitted new ones to it, if that happens, I'll just curse a little (or a lot) and install another set. Does this make me an "expert"? "nonexpert" by your definitions? I don't know (and care less). I do know that
I can check focus with a glass without breaking a camera in the process. Incidentally, my wife trusts me to operate, solo, sharp kitchen knives, the coffee machine, and, (on a good day), even the toaster.
If you reckon the likelihood of sticking your pinkies (or anything else) into a shutter
is as big a deal as you have suggested: Quit photography. Now. You're about to destroy your camera.
And stay out of kitchens.
