There has always been confusion as to the wisdom of changing shutter speeds on a cocked shutter. After spending a good many hours of re-working and studying an actual shutter, I can unequivocally say that it is not at all recommended. To clear up confusion as to the "why" of it is to first dispel worry of stripping a gear (in this case the Graphex shutter). There is no danger of instant destruction of that sort. You won't "break off"anything.
However, you will soon ruin any accuracy the shutter has, or had. The cocking of the shutter puts pressure upon the speed cam, which is made of a fairly soft brass. In this case the drive gear segment has a peg that rides on the speed setting cam. Turning the cam (changing speeds) causes wear in a short period of time, which increases the travel of the drive gear segment, which in turn causes speeds to grow slower and slower. The time frame for this wear is surprisingly short--on the order of mere weeks for an old-time newspaper photog with his Speed Graphic. Months at most.
When changing speeds with the shutter uncocked, the peg does not even touch the cam at all, so no wear. All this said, these iris shutters have such an infinitesimally small adjustment range that mere weather, hot or cold alone, can cause expansion or contraction issues as much as 3/4 of a stop. A shutter serviceman who slightly over-tightens the faceplate after a service job can add to the problem by another 1/2 stop. Additively, I can easily see that errors of a whole stop or more can occur when factoring in wear from changing speeds with the shutter cocked.
One f/stop is a lot. Anyone who spends half his darkroom time just making test strips can relate to the annoyance.
Don't change speeds on a cocked shutter.
However, you will soon ruin any accuracy the shutter has, or had. The cocking of the shutter puts pressure upon the speed cam, which is made of a fairly soft brass. In this case the drive gear segment has a peg that rides on the speed setting cam. Turning the cam (changing speeds) causes wear in a short period of time, which increases the travel of the drive gear segment, which in turn causes speeds to grow slower and slower. The time frame for this wear is surprisingly short--on the order of mere weeks for an old-time newspaper photog with his Speed Graphic. Months at most.
When changing speeds with the shutter uncocked, the peg does not even touch the cam at all, so no wear. All this said, these iris shutters have such an infinitesimally small adjustment range that mere weather, hot or cold alone, can cause expansion or contraction issues as much as 3/4 of a stop. A shutter serviceman who slightly over-tightens the faceplate after a service job can add to the problem by another 1/2 stop. Additively, I can easily see that errors of a whole stop or more can occur when factoring in wear from changing speeds with the shutter cocked.
One f/stop is a lot. Anyone who spends half his darkroom time just making test strips can relate to the annoyance.
Don't change speeds on a cocked shutter.