I've bought a film camera (Konica Z-up 110) and i'm testing it out before committing to putting a film in it. It works fine overall. The only issue is, when the flash is switched on, the shutter will not open fully. Is this normal? Is it to prevent overexposure? Because the shutter does fully open when the flash is switched off.
I'm guessing the z-up 110 is using a leaf shutter; which is essentially using aperture control for both the exposure amount (size of opening) and exposure duration (time open). And I would guess it is reducing the opening size to prevent overexposure; because it doesn't have fine control over the flash output.
A leaf shutter can be the aperture blades of the lens itself, or it can be a variable opening behind the lens... it is probably the latter in this case. This type of shutter mechanism is fairly common in smaller compact cameras.
Based on observing this video at youTube, it seems that your camera has a leaf shutter, sometimes called an iris shutter, based in the lens. Such shutters are much different from two-curtain focal plane shutters that sit directly in front of the film plane and must travel much further to open and close. This allows leaf shutters to be much "faster" than typical focal plane shutters.
A leaf shutter opens from the center out and then closes back to the center. In many cases they also function as an adjustable aperture by controlling exactly how wide they open. This appears to be the case with your camera.
So the fact that the aperture is not opening fully means that the camera has selected a narrower f-stop to prevent the flash from overexposing the shot.
Obviously, a mechanical shutter is not going to beat the speed of light, and so doing a second curtain sync which may appear appear as not being fully open clearly makes little sense (and should spawn concern).
However, and this is my guess, if the camera has control over the flash, a modern flash isn't just like a bulb going off -- where one opens the shutter full, does a quick burst, and closes it again. A modern flash can be kept on for the duration of the shutter to go by. This still may seem near instantaneous.
As such, the camera might be making a decision about how much light it needs, holding the light on longer, and passing the shutter by accordingly.
(I'm curious if the exposure on film came out okay, or whether part of the image was obscured in darkness.)