Why only 1/250 shutter curtain speed

[Vich]

Here (http://www.rpphoto.com/howto/view.asp?articleID=1026), and another here (http://photonotes.org/articles/eos-flash/index2.html), are nice articles explaining some things about flash use and how to get around the 1/250 (or on Rebel, 1/200) actual shutter speed.

It explains how there are 2 curtains that slide vertically, leaving a slit for the actual exposure.

So that makes me wonder:

1. If its just a slit, and electronic flashes have actual speeds of about 1/20,000th (actually 1/1000th to 1/50,000th depending on intensity), then how's that work? Does the curtain slide differently when flash is used so that it's not really a slit, but curtin 1, flash while entire film exposed, then curtain 2. Of course, it has to, but these explanations say there's always a slit between the curtains. That doesn't add up.

2. How was this handled on film? My last film camera did 1/2000th. There was no fancy computerized pixel control. Did the shutter actually move that fast?


My guess: that 1/250th is the static curtain speed.

That "shutter speed" actually means "time every pixel is exposed", even if the pixels on the top are exposed at a different time as the pixels at the bottom. So fast moving objects moving at the wrong angle, captured at 1/8000th "shutter speed", may appear "bent", but not blurry.

IF so, then specifying a speed below 1/250th determines how long is the delay before curtain 2 and everything above 1/250th is how narrow the slit between them. At 1/250th, curtain 2 begins exactly when curtain 1 finishes, so each spot on the film (sensor) is exposed exactly 1/250th.

This is where George tells us how Nikon's CCD allows superior flash sync because the CCD actually controls "shutter speed" by turning off light collection. And his point will not be an invalid one.

The first article above explains how "fast sync" actually strobes the flash many times for the entire 1/250th of a second. I'm guessing that's really only practical for fill flash situations because if the flash is the sole source of light, and the slit is fairly narrow (as with say, 1/2000th, the slit would be only 1/8th the height of the sensor because 2000 / 250 = 8), then you may get banding. For fill flash, the flash is only a small part of the light source so any banding wouldn't be noticed.

Just thinking aloud. Did I guess right? Did I make any sense?

[coldrain]

Hmm... superior flash sync due to CCD again huh? That sounds like Ken whatshisname. He went on and on about that you need 1/500th of a sec flash sync as professional. Funny stuff... that would only qualify the D70(s) as professional. Not even the Nikon Dx2. Nor D200.

And why does the D70 offer that, when the D200 or Canons do not? Because the D70 lacks one very important feature.... Mirror lock up.

It is the mirror that triggers the shutter in the D70, and therefor it can not offer mirror lock-up. Why this enables faster flash sync and shorter exposure time, I do not know, ask the Nikon engineers.
But since they put the dearly missed mirror lock-up on the D200, it is again "as slow" as DSLRs from other manufacturers.

[cwphoto]

Hmm... superior flash sync due to CCD again huh? That sounds like Ken whatshisname. He went on and on about that you need 1/500th of a sec flash sync as professional. Funny stuff... that would only qualify the D70(s) as professional. Not even the Nikon Dx2. Nor D200.

And why does the D70 offer that, when the D200 or Canons do not? Because the D70 lacks one very important feature.... Mirror lock up.

It is the mirror that triggers the shutter in the D70, and therefor it can not offer mirror lock-up. Why this enables faster flash sync and shorter exposure time, I do not know, ask the Nikon engineers.
But since they put the dearly missed mirror lock-up on the D200, it is again "as slow" as DSLRs from other manufacturers.

The original EOS-1D had x-sync up to 1/500s too. Also went to 1/16,000s at the top.:cool:

[cwphoto]

Here (http://www.rpphoto.com/howto/view.asp?articleID=1026), and another here (http://photonotes.org/articles/eos-flash/index2.html), are nice articles explaining some things about flash use and how to get around the 1/250 (or on Rebel, 1/200) actual shutter speed.

It explains how there are 2 curtains that slide vertically, leaving a slit for the actual exposure.

So that makes me wonder:

1. If its just a slit, and electronic flashes have actual speeds of about 1/20,000th (actually 1/1000th to 1/50,000th depending on intensity), then how's that work? Does the curtain slide differently when flash is used so that it's not really a slit, but curtin 1, flash while entire film exposed, then curtain 2. Of course, it has to, but these explanations say there's always a slit between the curtains. That doesn't add up.

2. How was this handled on film? My last film camera did 1/2000th. There was no fancy computerized pixel control. Did the shutter actually move that fast?


My guess: that 1/250th is the static curtain speed.

That "shutter speed" actually means "time every pixel is exposed", even if the pixels on the top are exposed at a different time as the pixels at the bottom. So fast moving objects moving at the wrong angle, captured at 1/8000th "shutter speed", may appear "bent", but not blurry.

IF so, then specifying a speed below 1/250th determines how long is the delay before curtain 2 and everything above 1/250th is how narrow the slit between them. At 1/250th, curtain 2 begins exactly when curtain 1 finishes, so each spot on the film (sensor) is exposed exactly 1/250th.

This is where George tells us how Nikon's CCD allows superior flash sync because the CCD actually controls "shutter speed" by turning off light collection. And his point will not be an invalid one.

The first article above explains how "fast sync" actually strobes the flash many times for the entire 1/250th of a second. I'm guessing that's really only practical for fill flash situations because if the flash is the sole source of light, and the slit is fairly narrow (as with say, 1/2000th, the slit would be only 1/8th the height of the sensor because 2000 / 250 = 8), then you may get banding. For fill flash, the flash is only a small part of the light source so any banding wouldn't be noticed.

Just thinking aloud. Did I guess right? Did I make any sense?

You got it Vich. The x-sync speed is the maximum shutter speed at which the shutter is fully open at any one time - allowing the flash to illuminate the whole frame. In the old days, shooting above the x-sync with flash could result in only part of the image being exposed as the rear (or second) shutter curtain had already begun to close prior to the first curtain opening fully.

[noyjimi]

My guess: that 1/250th is the static curtain speed.
That "shutter speed" actually means "time every pixel is exposed", even if the pixels on the top are exposed at a different time as the pixels at the bottom. So fast moving objects moving at the wrong angle, captured at 1/8000th "shutter speed", may appear "bent", but not blurry.


Does this apply to the Nikon D70/D50/D1 though? If those indeed use an electronic shutter (or hybrid at high speeds), then it's possible that the mechanical shutter is just a blind in that scenario in which case the sensor "pixels" would be "illuminated" simultaneously when the electronic switch turns on. Anyone with these bodies know the answer?

[referring to stroboscopic mode] I'm guessing that's really only practical for fill flash situations because if the flash is the sole source of light, and the slit is fairly narrow (as with say, 1/2000th, the slit would be only 1/8th the height of the sensor because 2000 / 250 = 8), then you may get banding. You mean if the multiple strobes don't maintain consistent intensity throughout the duration?

[Rhys]

Ah. I did some experiments with my Nikon FM to see just how a focal plane shutter works. They are all pretty much the same.

The first curtain opens and the second one begins to close after a specified time.

That is essentially how they work.

The faster the shutter can open and the faster the second shutter closes dictates the top flash synch rate.

To achieve the top shutter speeds the second curtain begins to close before the first curtain has finished opening so we have in effect a postbox-like slit moving across the frame. This slit gets narrower for the faster shutter speeds.

As a flash has a duration of typically 1/15,000 - 1/30,000 second, the shutter must be fully open (first curtain open, second curtain not closing) in order to illuminate the whole frame.

The top sych I've heard of so far has been 1/250th on an SLR.

Many medium format cameras employ a leaf shutter which may have a tendency to underexpose one edge of the frame very slightly. Hence they generally have a top shutter speed of 1/500th but flash synchs at all speeds.

[cwphoto]

The top sych I've heard of so far has been 1/250th on an SLR.

Many medium format cameras employ a leaf shutter which may have a tendency to underexpose one edge of the frame very slightly. Hence they generally have a top shutter speed of 1/500th but flash synchs at all speeds.

Some 35mm DSLRs are 1/500s as mentioned above.

Also the leaf shutter is in the lens - it looks just like an iris diaphragm. Some of my Rollei lenses (as used on my medium format SLR) sync up to 1/1000s which was really great for fill flash while maintaining low DoF.

[Vich]

Does this apply to the Nikon D70/D50/D1 though? If those indeed use an electronic shutter (or hybrid at high speeds), then it's possible that the mechanical shutter is just a blind in that scenario in which case the sensor "pixels" would be "illuminated" simultaneously when the electronic switch turns on. Anyone with these bodies know the answer??
I've read that CCD has very good on/off qualities and the writing, I believe, said the exposure shutter is electronically executed by the CCD.

So, the slow curtains open, the fast pixels snap on/off in 1/500th, the curtains shut.

I'll look later for a reference to confirm that, or give more details.


You mean if the multiple strobes don't maintain consistent intensity throughout the duration?
I mean,

Strobe 1
curtains move a little
strobe 2
curtains move a little
strobe 3
curtains move a little
.
.
.

I don't know that strobe intensity is adjustable on electronic flashes. I've read that flash intensity is managed by flash duration, not brightness.

Also consider, if it strobes too fast, then the EV lighting will be too great.

So then, it has to spread the strobes out more.

If the shutter is set at say 1/4000th, then the slit will be 1/16th the shutter height (because the shutter moves across the shorter dimension, top to bottom).

Rather than try jumping to assumptions and overtaxing my feeble remaing brain cells, I'll just ask if anyone knows the answer about how fast sync works.

My gut feel is that it won't work well when the flash is the primary source of light, but it's fine if the flash is just 20% of the total light received, provided you've set the EV correctly.

[Rhys]

Some 35mm DSLRs are 1/500s as mentioned above.

Also the leaf shutter is in the lens - it looks just like an iris diaphragm. Some of my Rollei lenses (as used on my medium format SLR) sync up to 1/1000s which was really great for fill flash while maintaining low DoF.

Are you sure of that? The leaf shutters I have seen have been leaf shaped and have risen vry quickly then fallen very quickly. Top speed of around 1/500th. Typically the disposables use leaf shutters.

The diaphragm shutter is something I have seen on a large format camera.

[cwphoto]

Are you sure of that? The leaf shutters I have seen have been leaf shaped and have risen vry quickly then fallen very quickly. Top speed of around 1/500th. Typically the disposables use leaf shutters.

The diaphragm shutter is something I have seen on a large format camera.

Pretty sure Rhys.;) I have nine such lenses.:p

1/1000s for Rollei PQS lenses (the fastest in the business).