Originally posted by Steelski Is it possible that Pentax just ordered a boatload of 1/180th sync shutters.
Or that their sized shutter is actually nonstandard. being 20mm makes it larger than a Nikon/Canon APSC shutter, therefore noone makes one at that size and speed combination.
I computed the exact shutter opening height from my image now. It is 20.9mm.
The exact sensor height is 15.7mm, which is ~19.7mm with SR margins. Probably, the shutter opening height is another 1mm larger to avoid any vignetting and allow for some mounting tolerance.
This makes 17mm for a "standard" APSC shutter, 20.7mm for a Pentax APSC shutter, 25mm for a FF shutter and 27mm for an SR FF shutter.
However, I don't see why Pentax couldn't have used the Sony A700 shutter (1/200X 1/8000s) if from a 3rd party. BTW, the 250/200 diff. is exactly the 16.7/20.7 diff. (25%).
I also looked at the 2nd curtain stack of 3 blades sitting in the top of the shutter in the cut model image. It is 6pixels thick (in the "O" size image) which translates to 0.08mm or 1/12mm thickness per blade. Could be thinner if the cut made an imprecise cut.
Each blade is 9.5mm high, totalling to 28.5mm for a 20.9mm opening. That's 1.3mm for each of the 6 overlaps making the blades cover 23.4mm. Assuming the same horizontally makes the blades 31.4mm wide.
3 blades of 9.5mm x 31.4mm x 0.08mm is 23.9mm^3. At 7.9 mg/mm^3 (high quality steel) this is 189 mg. Or because not all blades move with equal speed, an effective weight of (1/3+2/3+3/3)/3=2/3 of this which is 126mg or
1/8 g. Note that the Sony A700 shutter blades are made of carbon fiber though. In the video, you can see the curtain blades flatter. So, carbon fiber can be a better material (lighter) but this isn't necessarily so (less stiff).
Of course, the whole shutter has more moving parts than just the blades and therefore, its total effective weight is larger than this.
I mention this because we looked at this number in our shutter blur study.