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10-15-2010, 07:02 AM - 2 Likes   #1
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About the 1/250 x-sync flash rumor...

Well, I have to admit I was maybe wrong saying that the 1/180 limit was physical (shutter) and faster x-sync was impossible, and those waiting for a miraculous firmware update were daydreamers...

After careful examination of the P-TTL patents (Patent #6,718,135 for those interested), I think I've found out why P-TTL is limited to 1/180...

EDIT: in fact, this patent describes an early implementation of P-TTL, and it was slightly modified since, so what you're about to read is false!

In fact, in wireless mode, the main flash (built-in or external) issues during the exposure a preliminary low-power flash burst prior to its main burst to sync the slave flashes. There is then a 3ms wait between this low-power synch burst and the main flash...

And in all modes, there is at least a 1ms wait between the main flash and trailing curtain to let the flash reach its max power.

Sequence is :
  • Leading curtain
  • wait for half the Tv (this one I'm not sure about, maybe they mean Tv to be the X-sync duration.)
  • if wireless: (EDIT: This is the part that was modified)
    • low-power synch burst
    • 3ms wait
  • Main burst
  • 1ms wait (to allow the main flash burst to fully occur)
  • Trailing curtain (to occur when Tv is reached)

So in wireless, there is already 4ms spent in waiting... It translates to 1/250 (+ the Curtain travel time) being the fastest speed at which we can do the waiting!!!

From this we can conclude that Curtain travel time is less than 1.5ms, or 1/640, by the way : as time difference between 1/250 and the 1/180 x-sync is 1.5ms, and we can hope they haven't squandered precious time elsewhere...

So, theoretically, the fastest x-synch that you could attain with an instantaneous flash burst should be 1/640 with the current shutters... Allow for some time for longer (and realistic) flash burst (Canon 550ex has a max flash duration of 1.2ms, for instance), and you end up with roughly 1/360 being the fastest x-sync you can have with modern flashes... If you keep the 1ms wait mentioned in the patent, you end up with a 1/400 shutter speed.

EDIT: the following sentence was proved false in the subsequent dialog... (The error was coming from the wireless mode needing those 3ms, as it was proven to be false and the sync burst is done right before the exposure)

SO IT SEEMS IN FACT TRUE THAT 1/250 (and even 1/360) CAN BE OBTAINED THOUGH A FIRMWARE UPDATE! (as long as you don't use wireless, though).

And it should be compatible with all existing Pentax DSLRs and Flashes!!!

All this is caused mainly by the wireless waiting time...
It's a pity they limited the whole flash system this way.

I guess they set the x-sync the same for wireless and standard because:
1) it would have confused people otherwise, or
2) wireless would have got flamed for being slower than standard flash...

Could someone verify all this?
(and next time I'll do the math before opening my big mouth!!!)

So maybe we will see two x-sync speed from now on : standard (1/250 or maybe even 1/360) and wireless (1/180)...


Last edited by dlacouture; 10-15-2010 at 07:11 PM.
10-15-2010, 08:58 AM   #2
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Please see: Metz ? Useful T.1 Flash Duration Data - Pentax P-TTL Flash Comparison

If flash duration is 5ms or longer, you're not going to get 1/250 sync for full-power flash.

I would conclude that possible sync speeds will vary by flash unit and amount of power required.

In another thought, if the early flash is only for sync and includes a wait time, there's no reason to wait for the leading curtain to start. In fact, it's makes more sense (to me) that the sync is done prior to the opening of the leading shutter - there's no effect on exposure from the sync flash. The shutter, then, can begin to open during the 3 ms wait time.

In any case, I think you put your finger on the issue when noting the 1.2ms max flash duration of the Canon.
10-15-2010, 11:50 AM   #3
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Apparently, you haven't looked at how a focal plane shutter works.
10-15-2010, 12:26 PM   #4
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QuoteOriginally posted by Wheatfield Quote
Apparently, you haven't looked at how a focal plane shutter works.
I know exactly what you'll say, I've said the same thing before today...

But I think a misconception we have is that at 1/180 (5.55ms), the 2nd curtain starts moving as soon as the first one stops...

I think it's false! In fact there should be at least a 1ms pause so the flash can register fully on the picture... Yep, if you stop to think about it, it's logical... 2nd curtain should not move as long as a standard flash is still firing.

Now, if you take those patents, you'll see that Pentax will wait for at least 4ms between shutter movements in the case of wireless, even at 1/180... And this translate to a shutter travel time of 1/180-4ms=1.555ms, hence the other times I found...


Last edited by dlacouture; 10-15-2010 at 01:26 PM.
10-15-2010, 05:23 PM   #5
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please look at my shutter blur paper, linked from my blog. It contains a 1000fps video of the shutter curtain travelling (html version of the paper only). May change your reasoning...
10-15-2010, 06:58 PM   #6
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QuoteOriginally posted by falconeye Quote
please look at my shutter blur paper, linked from my blog. It contains a 1000fps video of the shutter curtain travelling (html version of the paper only). May change your reasoning...
In fact I've already done that after posting my replies, and while it shows me some of my errors, the results really bothers me...

First thing is that by timing the movement using the frames, you found a first curtain travel time of about 6ms, which is quite good regarding the 1/180 (or 5.555ms) sync speed if you allow for instantaneous flash occurring at x-sync... But looking at the 2nd curtain, you can see it fully moving in less than 5 frames!!! First and second curtain not the same speed??? There is a problem here...

Now, if you take 1/180 to be the absolute shutter travel time, where is the time allowed for a full flash burst???? Because then, it would mean that the top border of the sensor (bottom of the picture) would be gradually darker than the rest of the frame... (uh, got an idea I have to test here...)

(some time later...)

ARF!!!! I've made tests with both the Kx and the K7, firing my sigma 530 super at full power, AND THERE IS A DARK BAND ON THE BOTTOM OF THE PIC!!! Not always the same size, though, so there is a little leeway in the Sigma sync with the flash trigger.

What a dumb ass I make...

On one or two occasions, though, the dark band was gone on the K7, allowing for the flash to be fully recorded, so I'd inclined to think that its actual curtain travel sync is slightly less than 1/180. Maybe, say, 1/200... Because otherwise, we would always have the dark band at the bottom, and this is not the case.

Well, this proves one thing (and I stand corrected) : The only way Pentax could improve its X-sync is by issuing a new shutter...

Hope this will put the matter at rest...

On another point : There is only one flash, even in wireless, during the exposure.
I've made a simple test : move the camera quickly in front of a mirror, while taking a long exposure of the camera with a 2nd body...
The long exposure show 6 distinct bursts in two groups : one moderate and 2 small, then some time later 2 small and the main flash.
At the same time, the self-photographing camera only shows one burst. So even the 2 small bursts occurring right before the main burst are not recorded, even when the built-in flash is located in the bottom of the picture (first part of the sensor to be uncovered).

I'll try to exactly time the burst using an oscillo, but they must be occurring right before the first curtain starts moving.

So the patent mentioned above must be an early implementation of wireless in P-TTL (which is rather surprising, as the whole article is rather spot-on on the whole Flash system).
10-16-2010, 06:55 AM   #7
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This may have been debated already, can't remember, but could Pentax be faced with the same limitation as Sony (does Sony still has that limitation?) where sync is/was limited to 1/180 when using SR (well Sony's same for it) but 1/250 when disabling SR.

For what I remember, SR always work but keeps the sensor in its intended place rather than moving when SR is disabled (in fact it always work). Sony's implementation may work differently and allow 1/250 sync because their 'SR' does not need to start when disabled ?

Just some thoughts, I might be writing BS, really. Just some things I remembered and those things could be crap.
10-16-2010, 07:36 AM - 1 Like   #8
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QuoteOriginally posted by dlacouture Quote
Well, this proves one thing (and I stand corrected) : The only way Pentax could improve its X-sync is by issuing a new shutter...

Hope this will put the matter at rest...
I thought it would make you think it over
Thank's for having done the exact math.

As for total curtain travel time and difference between 1st and 2nd curtain ...

The curtains are accelerated and decelerated when moving. This doesn't explain the difference. But it makes it difficult to assess timing from the number of frames one sees the curtain travel. Because the don't start at the same moment of time within a video frame. Therefore, a difference of 1 frame is not significant. Say, the travel takes 5.5 video frames (ms) for both curtains.

Moreover, the shutter travels a longer distance than covered by the sensor, at least larger by the SR leafway. The camera could take the sensor position into account and sync the flash accordingly. Anyway, because the travel is so slow upon start (and stop although to a lesser extent), I guess it gains about 1ms this way.

Flashes can be slow. It may need longer than 1ms to fire at full power. E.g., Canon Speedlite 550 EX specifies 1.2ms or less, Nikon SB800 like attached chart and some Metz 8ms at full power. I couldn't find the Abbrenndauer (engl. ? -- burn time?) for the Sigma or Pentax flashes.

Looks like one better uses a 3rd party flash at 1/160s when needing full power ... And the fastest flash at minimal power would be ~ 1/220s.


Last edited by falconeye; 06-23-2012 at 03:08 AM.
10-16-2010, 08:15 AM   #9
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Falk, on a different note,

could this shutter work with a larger sensor?

QuoteQuote:
. . .
Moreover, the shutter travels a longer distance than covered by the sensor, at least larger by the SR leafway. The camera could take the sensor position into account and sync the flash accordingly. Anyway, because the travel is so slow upon start (and stop although to a lesser extent), I guess it gains about 1ms this way
. . .
Edit: dlacoutur, + rep points for your efforts on the math AND your objectivity to get the correct answer on this topic.
10-16-2010, 08:33 AM   #10
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So Pentax has miraculously, after a decade of technological, material science, and manufacturing advances, figured out how to make a slower sync shutter than they were capable of building for the PZ1p. Which is an amazing breakthrough considering that the new shutter has to cover less ground. At this rate we'll have 1/60 sec sync in the K1, and then won't all the other kids be jealous.
10-16-2010, 08:45 AM   #11
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QuoteOriginally posted by junyo Quote
So Pentax has miraculously, after a decade of technological, material science, and manufacturing advances, figured out how to make a slower sync shutter than they were capable of building for the PZ1p. Which is an amazing breakthrough considering that the new shutter has to cover less ground. At this rate we'll have 1/60 sec sync in the K1, and then won't all the other kids be jealous.

Apparently you missed the part about the SR possibly being part of the issue.
10-16-2010, 12:23 PM   #12
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QuoteOriginally posted by Blue Quote
Apparently you missed the part about the SR possibly being part of the issue.
Unless you're saying that the SR moves an APS-C sensor in a larger area that a full 35mm frame (because, again, we know that they've built a shutter that can sync an entire 35mm frame @ 1/250), then SR is irrelevent.

The size of the sensor mount and SR mechanics are irrelevant because they don't have to sync, only the surface area of the sensor does. The difficulty of getting a reading off a potentially moving film plane is irrelevant being you don't need that with PTTL.

So for your point to be valid, it would mean that Pentax:
A) Has a camera body with more than enough room for a FF sensor, but
B) Is putting an APSC sensor in that body
C) And this combination means they have to use a slower sync speed shutter

If that were the case, and mind you I'm not saying it's not, that would officially make them the dumbest f'ers on the planet. If for no other reason than for not explaining to users that SR versus sync speed was a deliberate choice and so they can stop waiting for it.
10-16-2010, 12:50 PM   #13
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I think that the decrease in sync time must be surely because DSLR shutters need to withstand a lot more actuations than their old film counterparts...

I mean, engineers surely know that you'll be happily snapping left and right now that you're free from the film tyranny.... And I'd bet they improved their shutters accordingly (sturdier, thus heavier and slower)...
I'm sure the lowest DSLR out there is rated for ten times the actuations a film SLR was given for...

EDIT: Thank you, Blue!
10-16-2010, 01:25 PM   #14
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QuoteOriginally posted by junyo Quote
Unless you're saying that the SR moves an APS-C sensor in a larger area that a full 35mm frame (because, again, we know that they've built a shutter that can sync an entire 35mm frame @ 1/250), then SR is irrelevent.

The size of the sensor mount and SR mechanics are irrelevant because they don't have to sync, only the surface area of the sensor does. The difficulty of getting a reading off a potentially moving film plane is irrelevant being you don't need that with PTTL.

So for your point to be valid, it would mean that Pentax:
A) Has a camera body with more than enough room for a FF sensor, but
B) Is putting an APSC sensor in that body
C) And this combination means they have to use a slower sync speed shutter

If that were the case, and mind you I'm not saying it's not, that would officially make them the dumbest f'ers on the planet. If for no other reason than for not explaining to users that SR versus sync speed was a deliberate choice and so they can stop waiting for it.
No Pentax film body had shake reduction so what is your point?
I know the sizes of sensor and film so I don't need to see that 4 year or older diagram. The question I had asked Falk had to do with whether or not the shutter was big enough for a ff sensor.

For the record, the *istD was originally built for a ff sensor. The *istD, however predates SR. I don't if the current shutter is large enough for ff or not without SR or with SR, which is why I asked Falk.

Last edited by Blue; 10-16-2010 at 01:33 PM.
10-16-2010, 03:33 PM   #15
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QuoteOriginally posted by Blue Quote
Falk, on a different note,

could this shutter work with a larger sensor?
If you look, e.g., at my K-5 cut model published in my K-5 preview article (accessible by my blog), then you'll see that the shutter opening is just as big as the sensor surface (which has a border too).

I think it is safe to assume that the shutter opening covers 16+2+2mm in the vertical direction (APS-C +2x SR landing area). Which is 20mm and not sufficient for FF even w/o SR. If you take my K-5 cut model image in full size, you should be able to gauge dimensions somewhere (e.g., by mount diameter) and determine the shutter opening size with sub-mm accuracy.

BTW, a FF shutter with SR would have to travel about 26mm. The 2mm SR landing zone on both sides is much more than actually required. I once did the math.
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