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Message: <blockquote data-quote="WayneF" data-source="post: 429429" data-attributes="member: 12496">Well, pretty close, better idea than before.  Yes, as slit gets smaller, exposure decreases.  Except it is not about pixels.  It is about mm of sensor height, and velocity over mm of travel.  I suppose 4016 pixels is a D750, which is 24mm tall.  The D750 and D8x0 actually sync at 1/320 second, making curtain travel time be (at least) 24mm / 1/320 second = 7.68 m per second.  You can compute pixel microns, but it is unorthodox.   Yes, the pixels are exposed when they are under the slit, for the duration as the slit passes over them.But sensors vary arbitrarily in number of pixels, where FX is always 24 mm tall (more universal meaning). D750 and D8x0 have very different pixel counts, but have the same FX 24mm and same 1/320 sec sync speed. The way below describes both.EDIT:  Jsee pointed out that the D750 does not sync at 1/320 sec.So the following numbers instead apply to D800 and D700, which do.For 1/500 second, second curtain starts closing 1/500 second after first curtain starts opening. Technically, nominal 1/500 second has to be 1/512 second, but that's only about 2%, I'm ignoring that.Crystal controlled, very accurate.  One curtain speed for every case, very consistent. A leaf shutter is no good past 1/500 second, if even that.So focal plane is best possible shutter accuracy, even at very fast speeds, but flash sync can be an issue. But sync is still very adequate in most situations except in bright sun.Slit width = shutter duration / curtain velocity = 1/500 sec  / 7.68m/sec = 7.68/500 = 15.36 mm, or 64% frame height (slit).Twice as fast starts sooner, half that time, for half this slit width.For 1/4000 second:Slit width = shutter duration / curtain velocity = 1/4000  / 7.68m/sec = 7.68/4000 = 1.92 mm, or 8% frame height. Which is 500/4000 = 12.5% of 1/500 slit  (12.5% of 15.36mm is 1.92mm slit) You can confirm the 7.68 m/sec number by measuring the pixels in the non-dark band of flash at 1/500 or 1/4000 second.  It could be a bit faster? All we know is that it does sync at 1/320 second. My notion is this:  You're obviously pretty sharp, and clearly from a technical background, but which definitely was not photography. <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile    :)" loading="lazy" data-shortname=":)" />   New at forums, and you tend to explain in detail to others about how things work, for things that you are seeing for the first time.  Because you're Jsee, you know?  You're used to explaining, but that has downsides when you make things up.  It is common, we all tend to do that (to imagine how it might work), but we can learn to really pay close attention to if we actually know or not. We do know the difference. Tip:  Try asking: How did I learn what I am about to say?  How sure am I?  Did I know it yesterday? Hope that helps. </blockquote>

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